V. MAN f.l '11 Of.lf.AI MJH'/I f 3 3051 00004 0489 Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/shortfmpersongeo179fost j. 179 STATE OF ILLINOIS ADLAI E. STEVENSON, Governor DEPARTMENT OF REGISTRATION AND EDUCATION C. HOBART ENGLE, Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON, Chief CIRCULAR NO. 179 SHORT PAPERS ON GEOLOGIC SUBJECTS Major Aquifers in Glacial Drift near Mattoon, Illinois By John W. Foster Notes on the Illinois "Lafayette" Gravel By J. E. Lamar and R. R. Reynolds Cambrian and Lower Ordovician Exposures in Northern Illinois By H. B. Willman and J. S. Templeton An Occurrence of Barite in the LaSalle Limestone of Illinois By Raymond S. Shrode Revision of Croixan Dikelocephalidae By Gilbert O. Raasch REPRINTED FROM THE TRANSACTIONS OF THE ILLINOIS STATE ACADEMY OF SCIENCE VOL. 44, pp. S5-128, 137-151, 1951- PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1952 JUJNQSS GEOLOGICAL SURVEY LIBRARY JUL 16 1952 Illinois Academy of Science Transactions, Vol. 44, 1951 GEOLOGY MAJOR AQUIFERS IN GLACIAL DRIFT NEAR MATTOON, ILLINOIS* JOHN W. FOSTER State Geological Survey, Urbana Subsurface Pleistocene studies near Mattoon recently conducted by the Illinois State Geological Survey have revealed the probable origin and nature of occurrence of two gravel aquifers of major importance. The report describes a veneer of sand and gravel which rests on the Sangamon interglacial surface over a wide area north of the Shelbyville moraine which here marks the maxi- mum stage of Wisconsin glaciation. A second, more restricted but more favorable aquifer of Shelbyville age lies immediately south of the Shelby- ville terminus and extends from the ground surface to the top of Illi- noian drift. The relationships of these gravel aquifers to the Cerro Gordo and Shelbyville moraines, to the undulations of the Sangamon surface, and to the topography of the top of the bedrock are described. The investigation is expected to lead to the discovery of unknown aquif er deposits of similar genesis in areas of similar geologic conditions. Introduction. — Geological and geophysical studies of the gravel aquifers near Mattoon have been conducted by the State Geological Survey at intermittent periods for many years. The older surveys have been directed principally toward the location of the areas most favorable for well field construction. The pres- ent investigation, largely a subsur- face study by the records of 114 drift borings, has been designed toward not only descriptive geology but an insight into the physical develop- ment of the Mattoon aquifers. It is hoped that this insight will lead to- ward a better understanding of the groundwater available to Mattoon, its industries and hinterland farms, and lead also toward the discovery of other still unknown aquifer de- posits of similar rank lying in areas of similar Pleistocene history. General setting. — The one hun- dred and ninety-two square miles of the Mattoon investigation lie en- tirely in southwestern Coles county and include Townships 12 and 13 north, Ranges 7 and 8 east, and the north four section tiers of Township 11 north, Range 7 and 8 east. The terrain is largely rolling to nearly level upland prairie, shedding drain- age west into the Kaskaskia, south into the Little Wabash, and east into the Embarrass systems. Relief is only 165 feet, with elevations rang- ing from about 620 feet on the Kas- kaskia River in the northwest area and the Little Wabash River in the southwest area to about 785 feet on the highest crest of the Shelbyville moraine (fig. 2) near the southwest Illinois Academy of Science Transactions corner of T.12 N., R.8 B. Mattoon, with a population estimated at more than 18,000, lies on the gentle north or backslope of the Shelbyville mo- raine in the south-central area. As in most of east-central Illinois, the glacial drift near Mattoon rests on the shales, thin limestones, sand- stones, and coals of the Pennsylvan- ian system. Bedrock here is exposed at very restricted locations along the Kaskaskia A^alley north of the Cerro Gordo moraine (figs. 2 and 3). Everywhere else drift, with estimat- ed thicknesses up to approximately 220 feet in southwestern T.12 N., R. 7 E., buries the bedrock surface and completely obscures its topog- raphy. Drift of Illinoian age is capped with younger Wisconsin de- posits associated with the Shelby- ville and Cerro Gordo ice lobes (Leighton, Ekblaw, Horberg, 1948, p. 22) of Tazewell age. Each of these drift mantles shows surface expres- sion, the Illinoian as a nearly level till plain beyond the influence of Wisconsin ice, the Shelbyville as a prominent moraine, and the Cerro Gordo as a more gentle moraine and till plain in the far north portion of the Mattoon area. The bedrock surface. — The buried bedrock surface of the Mattoon area is principally interfluve between the pre-glacial Embarrass (Horberg, 1950, p. 84) and Middletown (Hor- berg, 1950, p. 73) drainage systems. The shallow sags in the bedrock topography in the northeast portion of the area were apparently courses tributary to the great Embarrass bedrock valley which trends south- erly in the eastern part of Coles County. The more prominent bed- rock valleys of the Mattoon area Fig. 1. — Index map showing location of the area of investigation near Mattoon, Illinois. appear to trend westward and north- westward as part of the buried Mid- dletown valley which in turn is in confluence with the ancient course of the Mississippi River in eastern ;' Mason County. Although Illinoian drift to a great degree obscured the subdued topography of the top of' the bedrock, the southernmost of the west-trending bedrock valleys ap- pears to have influenced the building of a Wisconsin gravel aquifer, and 1 probably influences the hydrologie characteristics of that aquifer. Illinoian drift. — For the most parf the bedrock near Mattoon is mantled Aquifers in Glacial Drift CONTOUR INTERVAL 50 FEET [9 | 3 ? MILES Fig. 2. — Surface contour and moraine configuration map. Illinois Academy of Science Transactions CONTOUR INTERVAL 25 FEET ^-60<\ APPROXIMATE ELEVATION OF TOP OF BEDROCK A = BEDROCK EXPOSURE IOI23 MILES Fig. 3. — Buried bedrock surface contour map. Aquifers in Glacial Drift with Illinoian drift. Two exceptions are noted: (1) Reliable samples from a 1944 Mattoon City test hole lo- cated NE. SE. NE. sec. 18, T.ll N., R. 7 E., indicate that the basal ma- terial is a fine to coarse silty sand with an abundance of humus and most likely represents a Yarmouth interglacial alluvial deposit. No con- vincing evidence of Kansan drift has been noted in this subsurface investigation. (2) Three borings in T.ll N., R. 7 E., penetrated to the bedrock without apparently cutting pre-Wisconsin material. These sug- gest that Sangamon erosion denuded the bedrock at numerous though probably restricted locations princi- pally along Sangamon drainage sys- tems and in localities where Illinoian drift was originally thin. Inasmuch as the top of Illinoian drift appears to have been a gently undulating plain, at an average elevation of about 625 feet, the greater Illinoian drift thicknesses occur generally over the depressions of the bedrock surface. The greatest Illinoian drift thickness in the Mattoon area is esti- mated to be about 90 feet where a low bedrock surface is overlain by a relatively high Sangamon surface in sec. 19, T.12 N., R.8 E. Illinoian drift over wide areas here is exclusively a grey or grey- green calcareous clay till which lacks the sandy character typical of Illi- noian drift in most of Champaign County. The upper 5 to 10 feet of Illinoian till shows clearly by repre- sentative samples the weathering during the Sangamon interglacial period. The general textural homo- geneity of the Illinoian drift lends ' no support to the possibility of mul- 1 tiple Illinoian glaciation, although this fact by no means precludes such a history. Aquifer material in Illinoian drift, though very uncommon near Mattoon, may be extremely impor- tant in its influence on the hydrology of Wisconsin gravels immediately south of the Shelbyville moraine. In sec. 18, T.ll N., R.7 E., pre-Wiscon- sin sand deposits are as thick as 45 feet and occupy there a well- developed valley of the bedrock sur- face, drift boring number 4, fig. 4. The nature of this pre-Wisconsin sand is unknown, but the deposit is very likely restricted to the bedrock valley in which it lies, and probably trends westward in conformity with the valley configuration. Samples from boring number 4 suggest that this sand is silty to very silty and may not in itself be an important direct source of groundwater. Sangamon soil. — Well drillers in the Mattoon area are familiar with the "peat" which rests largely on Illinoian till. It is this remarkably preserved soil section that has pro- vided an excellent horizon marker here. The excellence of the Sangamon interglacial soil development as a marker near Mattoon has enabled the use of driller's logs where they might otherwise be of little value in geological Pleistocene interpreta- tion. Unusual preservation of Sanga- mon loam here through the over- running of the later Wisconsin glacier is probably due to the equally unusual nature of the advance of the Shelbyville ice. Shelbyville ice progression and the spreading of gravel veneer. — The progression of Shelbyville ice into the Mattoon area early in Tazewell Illinois Academy of Science Transactions 5 • V- ~v= Aquifers in Glacial Drift 91 time, with a southwesterly lobate axis, began the key period of aquifer building. Subsurface data through- out the area show that a widespread veneer of gravel and sand was laid directly on the Sangamon soil by strong pro-glacial fluvial activity. Figure 4 illustrates the persistence with which these coarse Shelbyville elastics were spread, apparently without much regard for the hills and dales of the Illinoian (or Sanga- mon) surface. The adaptability of the gravel veneer to the topography indicates that the veneer was spread either subglacially under hydrostatic pressure or as ice-contact accumula- tion along the advancing zone of strong periglacial fluvial activity. Deposits of outwash origin distant from the glacier mass would con- centrate in the existing sags of the Illinoian drift surface with very thin deposition, if any, on the topo- graphic highs. The wide occurrence of the gravel veneer here apparently eliminates the possibility of sub- glacial origin, leaving the likelihood of ice-contact origin. Fan and outwash complex. — Shelbyville ice appears to have reached the north portion of T.ll N., R.7 E., about 5 miles south of the present city of Mattoon, without great mechanical change. Here, how- ever, ice-front progression ceased and deterioration of the front prob- ably paced the movement of ice into the terminus area. During this part of the Shelbyville time began the building of a complex fan and out- wash deposit immediately south of the glacier terminus near sees. 17, 18, 19, and 20, T.ll N., R.7 E., south of the present Lake Mattoon. Sands of this complex accumulated first in the sag in the Illinoian drift surface (which is apparently the re- flection of a partially obscured bed- rock valley). The present valley of the Little Wabash River cuts deeply into the Shelbyville moraine (fig. 2) directly behind the Shelbyville fan and out- wash complex and may represent the course of a subglacial or super- glacial torrent from which a portion of the complex would have been de- rived. Periglacial deposits here orig- inating from an ice-fed torrent would likely have spread as an al- luvial fan close to the stream 's emer- gence from the ice mass. Such fan- type gravels would combine with earlier and later outwash gravels to form an aquifer deposit similar to the complex found in sees. 17, 18, 19, and 20, T.ll N., R.7 E. Borings 7, 8 and 9 of fig. 4 show the occurrence of a till bed five to ten feet in thickness intercalated between gravels near the south edge of the Shelbyville moraine. This till zone may have been the result of a minor northward retreat of the ice front a distance of about two miles. The momentary ice front control by ablation may then have been lost by reinvasion to a position nearly equal to the former Shelbyville ter- minus, spreading as it progressed a second gravel veneer, shown on fig. 4. The second invasion renewed the building of the fan and outwash complex beyond the limit of Wis- consin glaciation. Subsequent till deposition resulted in the prominent Shelbyville moraine which is the landmark of the Mattoon region. Fluvial deposits of sand and gravel associated with the final recessional 92 Illinois Academy of Science Transactions stage of the Shelbyville ice front are very rare or absent within the till near Mattoon. Although the extent of the final retreat of the Shelbyville ice front is not known, ice of this stage of Tazewell glaciation must have ablated at least 50 miles, as Shelbyville drift in central Cham- paign County exhibits a young soil profile, but little or no leaching. Cerro Gordo lobate ice. — Post- Shelbyville glaciation in the Mattoon area is believed to include only the development of the more restricted Cerro Gordo lobe, with an axis simi- lar in direction to that of the earlier Shelbyville lobe. Perhaps, because of meager fluvial activity or because of lack of coarse clastic load, the Cerro Gordo glacier failed to spread before it the gravel veneer which Shelbyville ice spread with such uni- formity. Drilling above the Shelby- ville drift in and north of the Cerro Gordo moraine has revealed no more than widely scattered thin sands of limited groundwater resources. Cer- ro Gordo ice near Mattoon reached its maximum in extreme northern T.12 N., R.7 and 8 E., about one mile north of the city (fig. 2). The till sheet deposited by the Cerro Gordo ice rests on older Shelbyville till. The horizon between the two tills should be recognizable from sample study on the basis of weath- ering on the top of the buried Shelbyville till and on the basis of pre-Cerro Gordo soil development. The subsurface distinction of the Cerro Gordo and Shelbyville tills, however, has not been made in the area of this investigation for lack of suitable sample cuttings in the key area of Cerro Gordo overlap. Illinois State Geological Survey files contain sample study records which show that pre-Cerro Gordo soil has been identified at many well sites in Douglas and Champaign counties, north of the Mattoon area. Geo-hydrology of the aquifers. — Despite the uniformity of thickness of the Shelbyville gravel veneer north of the Shelbyville moraine, the textural composition of the aqui- fer shows great variation from place to place. The veneer exhibits over wide areas a three- to ten-foot bed of very fine sand at the base of the aquifer, directly above Sangamon soil. Where this fine sand composes the entire thickness of the Shelby- ville veneer, well construction even for domestic groundwater is difficult. Borings in the veneer not restricted to any given locality commonly ex- hibit a bed of clean, very coarse gravel with a thickness up to fifteen feet intercalated between beds of less well sorted silty sand and gravel. It is the very coarse gravel deposit which yields abundant ground- water at scattered sites through the Mattoon area, although even the poorly sorted sand and gravel, where the coarse material is missing, yields more than adequate domestic groundwater supplies. Far more favorable than the gravel veneer north of the Shelby- ville moraine is the gravel aquifer pf the fan and outwash complex in the area of sees. 17, 18, 19, and 20, j T.ll N., R.7 E., largely south of the | limit of "Wisconsin glaciation. The veneer and the complex gravel for- mations are distinct both geologi- cally and by hydrologic characteris- tics and are being considered here J as two aquifers. The fan and out-| wash complex has a number of, Aquifers in Glacial Drift 93 geologic features which encourage favorable groundwater yields : (1.) The sand and gravel com- plex of Shelbyville age extends from the ground surface to the top of the Illinoian drift with thicknesses up to 65 feet. These thicknesses are due to the well-defined sag in the Illinoian drift surface previously de- scribed. No such thicknesses of gravel are known in the Shelbyville veneer to the north. (2.) The fan and outwash com- plex in a number of test holes in T.ll N., R.7 E., has two or more zones of clean, very coarse gravel with com- bined thicknesses of twenty feet or more. (3.) The complex is in hydro- logic contact with the buried Shelby- ville gravel veneer on the north, thereby facilitating groundwater re- charge. The complex is probably also in hydrologic contact with Illinoian gravels occupying the bedrock valley which directly underlies part of the complex area. Borings here have encountered an upper Illinoian till deposit which, if continuous, might effectively seal the deep gravels from recharge of the shallower gravel complex. Where erosion of the upper Illinoian till may have exposed deep Illinoian gravels during Sangamon time, Illinoian and Wisconsin gravels may be in direct contact. Inasmuch as the deeper Illinoian gravels are believed to be generally silty and limited in permeability, wells in the complex area will prob- ably be constructed in the clean Shelbyville gravels, even though j these are relatively shallow and j: widely City of Mattoon well field con- struction. — Mattoon wells are con- centrated in two areas, the Dorans field near sec. 30, T.12 N., R.8 E., and the Southwest field near sec. 18, T.ll N., R.7 E. City wells constructed prior to 1944 were located largely in the Dorans area. These penetrate to depths of 40 to 70 feet and tap groundwater in the Shelbyville gravel veneer. The Dorans wells have never shown capacities adequate for the demands of a city the size of Mattoon. By 1935 Lake Mattoon Avas supplying the city with the major portion of its water needs. Limited surface inflow in the summer of 1944, caused by rainfall deficiencies recorded by the United States Weather Bureau in the months of May, June, and July of that year, lowered surface storage to an alarm- ing level. Geophysical exploration was conducted by the State Geologi- cal Survey for the purpose of dis- covering, if possible, groundwater aquifers more prolific than that at the old Dorans field. Electrical earth resistivity exploration delineated in 1944 the very favorable gravels in the area of sees. 17, 18, 19, and 20, T.ll N., R.9 E., described in this study as the Shelbyville fan and outwash complex. Lake Mattoon still furnishes the major portion of water requirements at Mattoon though limited pumping of the Dorans well field continues. However, the construction of wells in the Southwest area, since 1944, which penetrate the most prolific deposit near Mattoon, safeguards the city's supplies against future low water levels of Lake Mattoon. Fur- thermore, industries which can lo- 94 Illinois Academy of Science Transactions cate in the vicinity of the Shelbyville fan and outwash complex will be assured of a source of abundant groundwater available for indepen- dent development. This favorable situation exists at very few indus- trial sites in this region. Acknowledgment. — The author gratefully recognizes the many con- tributors to the present concepts of Illinois Pleistocene upon whose groundwork this investigation has been made. The author is indebted to M. M. Leighton, Chief, and George E. Ekblaw, Head of Engineering Geology and Topographic Mapping Division, of the Illinois State Geo- logical Survey, for helpful comments during the assembling and interpre- tation of data. REFERENCES Horberg, C. L. (1950), Bedrock Topo- graphy of Illinois: Illinois State Geo- logical Survey Bull. 73, p. 73. Horberg, C. L. idem. p. 84. Leighton, M. M., Ekblaw, George E., and Horberg, C. L. (1948), Physiographic Divisions of Illinois, Jour. Geol., vol. 56, no. 1, p. 22. Illinois Academy of Science Transactions, Vol. 44, 1951 NOTES ON THE ILLINOIS "LAFAYETTE" GRAVEL* J. E. LAMAR and R. R. REYNOLDS State Geological Survey, Vrbana Extreme southern Illinois contains many exposures of bro w n chert gravel and red sand which are loosely referred to as the "Lafay- ette" formation of Tertiary age. De- posits of Lafayette-type gravel oc- cur in other parts of Illinois but are mostly thin. 1 One of the northern- most sizable deposits in Illinois which closely resembles the southern Illinois gravels is found near Hamil- ton in Hancock County, about 250 miles northwest of the main body of "Lafayette" sediments in south- ern Illinois. The "Lafayette" gravel is com- monly regarded as stream deposited. The drainage patterns that must have been involved in most of this deposition have not been established, nor is it possible to say to what ex- tent the gravels may have been re- worked. Solution of these problems will involve, among other things, a study of the sedimentology and lith- ology of many samples carefully taken with regard to their topo- graphic occurrence and elevation. The preliminary work here described was undertaken to obtain better knowledge of some of the physical characteristics of the Lafayette gravel which might be important to broader investigations. The six samples used in this in- vestigation were taken from the following exposures (fig. 1) : LAFAYETTE GRAVEL • Depot Fig. 1. — Locations of deposits s and of other outcrops of "Lafayette" gravel as reported by Horberg, Leland, Preglacial gravels in Henry Co., Illinois, Trans. 111. Acad. Sci., vol. 43, p. 172, 1950. Sample No. 1. SE% SW*4 sec. 6, T. 4 N., R. 3 W., near Hamilton, I'ul .1 lr-.ll.-. I ],i |ienio>Moii .)(' 111.' Chirf. Illinois rll n „ „ , „ , . , State Geological Survey, Urbana. 111., 10 feet 01 "Tavel 111 gravel pit. 1 Horberg, Lelaml, I're-ghioial erosion surfaces in TJil„ TT „ + 4^^ „U„-,,+ fiOA 4? „4- Illinois: Jour. Ceo!., Vol. 1,1V, No. ;■:, mi.;, ,,. 184. LievatlOll about 620 feet. 96 Illinois Academy of Science Transactions Fig. 2. — Particle-size histograms. The vertical bars, reading from left to right, indicate percent retained on the following sieves: 2", 1.5", 1.05", .742", .525", .371", 3 mesh, 4, 8, 10, 14, 20, 28, 35, 48, 65, 100, 150, 200, and 270 mesh; passing 270 mesh plus 2 microns, and minus 2 microns. Sample No. 2. NE% SEVi see. 4, T. 44 N., E. 3 E., near Grover, Mo., 30 feet of gravel. This deposit is not far from Calhoun County, 111., and was sampled instead of Calhoun County deposits because the expo- sure was better and the thickness of the gravel greater. Elevation about 820 feet. Sample No. 3. SW% NEVi sec. 34, T. 15 S, R. 3 W. near Fayville, 111., 10 feet of gravel in small gravel pit. Elevation about 400 feet. Sample No. 4. NE% SWVi sec. 15, T. 16 S., R. 1 W. near Mounds, 111., 3% feet of gravel in road cut. Elevation about 400 feet. Sample No. 5. SWV 4 SWy 4 SEVi sec. 29, T. 14 S., R. 2 E. near Grand Chain, 111., 8 feet of gravel in rail- road cut. Elevation about 430 feet. Sample No. 6. SW% SWVi sec. I 28, T. 14 S, R. 5 E. near Renshaw, 111., 20 feet of gravel in road cut. Elevation about 520 feet. Mechanical Analyses Mechanical analyses of the six samples were made by standard pro- :< cedures. Results are shown in figure ;' '■'Lafayette" Gravel 97 PLUS 10 MESH FRACTION 12 3 4 5 6 MINUS 10 MESH FRACTION Fig. 3. — Histograms showing first quartile (left-hand bar), median (center bar) and third quartile (right-hand bar) of "Lafayette" gravel samples. 2. All the charts are bimoclal. Those for samples 2, 3, 4, 5, and 6 are roughly similar. The curve for sam- ple 1, however, differs in the shape and amplitude of its sand fraction peak and in the particle size dis- tribution of the gravel fraction. In figure 3 are shown certain sta- tistical values determined from the particle size analyses. The quartile and median data for the samples as a whole indicate that samples 2 and 3 are the coarsest and sample 6 is the finest. In the plus 10 mesh fraction, sample 1 is the coarsest and samples 4 and 6 the finest. In general a kinship is suggested between samples 1, 2, and 3 from Hamilton, Grover, and Fayville, respectively, and be- tween samples 4, 5, and 6. Pebble Counts In order to determine the lithology of the "Lafayette" gravel, pebble counts were made on 100 pebbles selected at random from each of four size fractions of each sample. All chert pebbles were classified accord- ing to color : brown, light buff, red, and black. The size fractions counted and the results of the counts are shown in figure 4. The following characteristics are indicated for each of the size frac- Samples 1, 2, and 3 con- tain black and red chert and sample 2 is rela- tively high in light brown chert. Sample 1 differs from the other samples in having a high content of red chert and vein quartz. Samples 1, 2, and 4 contained more light brown chert than the other samples. Sample 1 differs from the other samples in having a high content of vein quartz and of buff chert pebbles. Samples 1 and 3 are high in vein quartz peb- bles. Sample 2 is high in light brown chert pebbles and Sample 4 is high in red chert. Sample 1 is unlike all others because of its Illinois Academy of Science Transactions 1 1 ! 1. 05x.742" L LL_ II .iii 'I 8 ; ^ 1 i — 1 — iRH— k ■ m s^r&c I l . ' - 355** 1 .742 x. 52 5" 1 ■ 1 lL^lxL }\ .525x371 i_ ui 1 SAMPLE NUMBER Fig. 4. — Results of pebble counts in percent by number of pebbles. Percent scale changes at 20 percent. " Lafayette" Gravel SAMPLE NUMBER Fig. 5. — Degree of polish of pebbles. Left-hand bar indicates number of peb- bles having "good" polish, center bar "fair" polish, and right-hand bar "poor" polish. low chert content and high compensating vein quartz content. Sam- ples 1, 2, and 4 contain the most light brown chert pebbles and also the most red chert pebbles. To summarize the foregoing, sam- ple 1 differs from all other samples in its high vein quartz content. Sam- ples 1 and 2 are akin in several respects, as are samples 5 and 6. Samples 3 and 4 are of intermediate character. This is probably to be expected because samples 1 and 2 are from the Mississippi Valley and samples 5 and 6 from the Ohio Val- ley, whereas samples 3 and 4, near the junction of the two valleys, show affiliations with both valleys. The Mississippi Valley gravel samples commonly differ from the Ohio Val- ley gravel in the following ways: they contain more vein quartz, light brown chert, red chert, and black chert pebbles. The Ohio Valley sam- ples are characterized by a high con- tent of brown chert pebbles. Conclusions regarding the lith- ology of pebbles counted in relation to size are shown in table 1. Table 1. — Prevalence of Pebbles by Kind Kind of pebble Brown chert Light brown chert Red chert Black chert Vein quartz 1 . 05 x .742 inches .371 x .19 .371 x .19 inches .525 x .371 Illinois Academy of Science Transactions Table 2. — Shape of Pebbles (In percent by numbers of pebbles) 1. 05 "x. 742 "fraction .742 "x. 525 "fraction 1 .05 " x .525 ' ' fraction Sample No. I* II III IV I II III IV I II III IV 1 64 32 32 32 52 36 39 16 40 36 16 28 24 55 4 12 12 16 4 8 16 16 20 36 16 32 6 64 28 44 60 56 40 37 38 24 24 16 20 12 28 47 30 12 32 20 12 16 8 16 16 20 8 16 24 16 16 64 30 37 47 54 38 38 19 33 27 18 20 26 52 8 21 16 14 10 8 9 2 16 3 ?0 4. . . . 21 5 16 6. . 28 10 * I — Flattened or disc shaped. Ill — Bladed or lath-like. II — Spheres or equidimensional pebbles. IV- — Rod shaped. These data carry various impliea- pebbles from samples 2 and 3 were tions regarding source deposits. The measured to provide comparative black chert, and to a lesser extent data. the red chert, appears to have come Standard methods 2 were employed from deposits having a uniform par- in measuring the pebbles. Shape tide size distribution. In contrast designations were assigned by the the vein quartz pebbles came from Zingg shape classification 3 , which a source that yielded pebbles mostly provides for four classes. These are smaller than % inch. indicated in table 2 together with In addition to the kinds of pebbles results of the shape studies, mentioned, there were observed in Table 3 ._ Average Sphericitt Valtjes the gravel samples, in amounts prob- == ably less than one percent, other kinds of pebbles that may prove s valuable in determining the source of the gravel. These were quartzite, agate, and jasper pebbles. No ig- neous rock pebbles were observed, 1 - nor have they been seen in Illinois 2. outcrops. Shape 4 Shape studies were made on two 5 sizes of chert pebbles, namely, 1.05" x .742" and .742" x .525". Pebbles 6 - of these sizes were selected because = ,, . _ nn . 2 Krumbein, W. C, Measurement and geological' they are Common and easily handled. significance of shape and roundness, Jour. Sed. Petr. , In addition, the shapes of 25 quartz vol 3 - Krumbein,' w e a, 9 op' c&'./p. 67. Size of pebbles 1.05 "by .742" .742" by .525 'Lafayette" Gravel 101 The results indicate a similarity of shape between pebbles in samples 2 and 3 in the 1.05 x .742 inch frac- tion and a less striking similarity between samples 3 and 5 in the .742 x .525 inch fraction. Considering' both fractions together there is a likeness between samples 4 and 5. The shape data do not, therefore, show any consistent relationship be- tween the samples. Flattened or disc-shaped pebbles are most common ; they comprise 45 percent of the six samples investi- gated. Twenty-four percent of the pebbles are spherical or equidimen- sional, 13 percent bladed or lath-like, and 18 percent rod shaped. Shape determination on two sizes of quartz pebbles from sample 2 and one size from sample 3 yielded differ- ing results (table 2), but show a dominance of flattened or disc- shaped and spherical or equidimen- sional pebbles. Sample 2 contains no bladed or lath-like pebbles. Sphericity Sphericity was determined direct- ly from a modification of the Zingg diagram. 4 Table 3 gives the spheric- ity data. No distinctive differences are apparent between samples. Roundness The roundness of the pebbles pre- viously measured for shape was de- termined by comparing them with Krumbein's visual roundness chart. 5 The results are given in table 4 ; also comparable measurements of 25 vein quartz pebbles from samples 2 and 3. The chert pebbles of sample 1 are the least round. The other samples show no outstanding differences. The quartz pebbles are more round than the chert pebbles. The roundness values of the chert pebbles commonly result from a rounding of the edges and corners of the pebbles rather than major alteration of the general pebble shape. Well-rounded chert pebbles are infrequent in the sizes studied; such pebbles as are found may have come from a distant source, although some of them may have had a high initial roundness. Table 4. — Avkkaok Roundxess Values Size of pebbles Sample No. 1.05" by .74 .742" 2" by 525" 1 — chert pebbles . . . .44 40 2 50 48 3 48 46 4 48 48 5 50 51 6 52 48 2 — quartz pebbles.. 73 3 73 Since the quartz pebbles are more rounded than the chert pebbles, and also are probably more resistant to abrasion, it seems likely that they may have attained much of their present degree of rounding in a pre- " Lafayette" cycle of erosion. Con- ceivably most of them found in the "Lafayette" gravel in southern Illi- nois may have been derived from the Casey ville (Pennsylvanian) con- glomeratic sandstones which form prominent escarpments a few tens 102 Illinois Academy of Science Tr of miles north of the gravel area. This is suggested by the appearance of the pebbles and by a petrographic and sedimentologic study made of them by Reynolds." The source of the quartz pebbles in the (1 rover and Hamilton samples is a problem for further study. Polish An outstanding characteristic of most "Lafayette" gravel deposits of Illinois is the polish shown by the pebbles. Some pebbles are complete- ly or almost completely polished; Fig. 6. — Polished section of a brown "Lafayette" chert pebble. The marginal, brown-stained zone which gives the peb- ble its exterior color is evident except at the broken upper left corner. An area of brown discoloration produced by the infiltration of iron oxide along incipient fractures is shown by the dark-grey area at the left of the picture. Reflected light. X 7. Fig. 7. — The natural surface of one side of a chert pebble which is silicified oolite. It suggests the surface of a freshly broken oolitic limestone. X 4. others have rough corners and edges, with polish on only the larger flat or nearly flat surfaces; still others show polish only in re-entrants in the pebbles. The variations in the extent of the polish may be a func- tion of th« amount of wear to which a pebble has been subjected since the original development of the polish. The pebbles of sample 6, which have dull, lusterless surfaces, are an exception. The sampled deposit was partly cemented, and the pebbles were coated with a film of limonite. Attempted removal of the limonitic coating by rubbing resulted in a moderate degree of polish, but this may have been caused by the rub- bing. Solution of the limonitic film by hydrochloric acid revealed a dull unpolished surface on most pebbles, but in the re-entrants of a few peb- bles some polish was evident. This suggests that the gravel of sample 6 was once normally polished. Its loss of polish could be attributed to processes related to cementation; however, as the deposit from which , sample 1 was obtained was also part- j "Lafayette" Gravel 103 ly cemented, though less so than sample 6, and yet contained many highly polished pebbles, cementation cannot be regarded as always ad- versely affecting polish. It was evident from an examina- tion of 300 pebbles that in general most of the chert pebbles in the ' ' La- fayette ' ' gravel have a higher degree of polish than the other pebbles. The vein quartz pebbles and the few qnartzites as a rule have only a fair degree of polish. In an attempt to determine varia- tions in degree of polish between samples, the chert pebbles whose shapes were measured were arbitra- rily classified according to whether their polish was deemed good, fair, or poor. The results are shown in figure 5. Sample 1 from Hamilton, 111., has the highest degree of polish. Samples 2 and 3 have a moderate degree of polish, whereas samples 4 and 5 are the least well polished. The data at hand are not sufficient to permit an interpretation of these observations, but it is suggested that they probably relate to the history of the pebbles after polishing and per- haps to the varying susceptibilities of different varieties of chert to polishing. The means by which the "Lafay- ette" pebbles received their polish were not investigated. However, the dense surfaces of some of the porous chert pebbles and of some of the in- frequent sandstone pebbles observed suggest that they may have been im- pregnated with silica. The brown color and polish of the "Lafayette" chert pebbles may be in some way related. Thin sections of chert pebbles show that, although the interiors of the pebbles may be buff, the characteristic brown is com- monly restricted to a relatively thin zone at the surface and to bands or streaks along cracks and fractures in the pebble (fig. 6). Twelve peb- bles from sample 1 were boiled in hydrochloric acid. Six lost their dark-brown color and changed to an orange brown or yellow brown with cream-colored blotches. Three peb- Jbles became light buff to cream. Three others were little changed. It is noteworthy that though there was a marked color change, the polish of the pebbles was not appreciably altered ; the gloss may have been in- creased, thus suggesting that the polish is not necessarily now linked with the brown color. This inference is further suggested by the fact that the "glazed" pebbles, subsequently described, though white or cream colored, are also polished. It seems possible, therefore, that the polish of some pebbles is unrelated to color. Oolitic Pebbles A considerable number of the peb- bles in the samples studied are silici- fied oolite. The abundance of oolitic pebbles in the 50 pebbles studied for shape is shown in table 5. Some of the pebbles are obviously chert; others appear to be more coarsely crystalline quartz, although they may be varieties of chert. Many show pitted surfaces, but oolitic peb- bles without pits are also common. Numerous coarser-grained pebbles have on their flat or rounded sur- faces an intricate pattern composed of roughly crescentic indentations somewhat akin to chattermarks but not regularly distributed. These markings also occur on pebbles which contain fossil detritus but only 104 Illinois Academy of Science Transactions Fig. 8. — Thin sections of silicifled "Lafayette" pebbles. A. Originally probably a limestone composed of calcareous detritus and possibly some oolite grains. B. Oolite showing radial structure but little annular banding in grains; grains have dark centers. C. and D. Oolite showing well-developed annular and radial structure; the white centers are macrocrystalline quartz. X 18. scattered oolite grains. Many of the oolitic pebbles showing the markings are maroon colored. A few oolitic chert pebbles have one natural sur- face that resembles a freshly broken piece of oolitic limestone. On these surfaces the oolite grains resemble rounded sand grains, and the pebbles might be described as sandstone from casual examination (fig. 7). The oolitic pebbles are diverse (fig. 8). Some consist largely of roughly spherical oolite grains, gen- erally showing an annular struc- ture ; others are composed of numer- ous annular oolite grains, together with an abundance of granular ma- terial, probably detrital, which does not show notable concentric struc- ture or spherical shape. Small fossils ''Lafayette" Gravel Sample No. .742" x 1.05" .525" x .742" .525" x 1.05" 4 10 8 12 6 6 10 4 14 4 5 2 10 3 6 13 5 5 6 such as brachiopods and fragments of large fossils are common in some pebbles. The grain size of the oolites and other materials is variable, as is the ease with which the textural characteristics of the pebbles can be determined without thin-sectioning. The source of the oolitic pebbles is not known. The textures and character of many of the pebbles are approximated or essentially dupli- cated in the oolite of the Ste. Gene- vieve and Salem formations, as now exposed in Illinois, which may have been the source of many of these pebbles. It is of interest that no consider- able amount of oolitic chert is known in the Salem, Ste. Genevieve, or Chester formations as now exposed in southern or western Illinois. As previously noted, some of the oolite pebbles composed of relatively coarse-grained quartz show natural surfaces like those of a freshly broken piece of limestone oolite, a type of fracture which would prob- ably not be expected from the silici- fied pebbles as now found (fig. 8). This phenomenon can be accounted for in various ways, but the question is raised whether these pebbles may not be the result of some special set of silicification conditions rather fayette" pebble showing: white "glaze" over part of exterior, and a light band concentric with the glaze. Reflected light. X 8. than those which normally produce chert in limestones. "Glazed" Pebbles The samples studied, especially nos. 1 and 2, contain pebbles whose exterior consists partly or wholly of white, siliceous, often porcelain-like material resembling a "glaze" 7 or coating (fig. 9). On some pebbles the glaze is about % of an inch thick and covers most of the surface. Other pebbles show only small glazed areas. The glaze occurs alike on brown and black chert pebbles. Both the un- glazed and glazed parts of many of the pebbles are polished in normal because the t phei ■„.„,. that the glaz i describes the appearance of t lis usage is not meant to eonnc ■as necessarily an addition to t Illinois Academy of Science Transactions Fig. 10. — Chert from a bedrock out- crop showing the development of a white chalky material on the top of the speci- men. Similar material occurs on the bottom but is not visible in the photo- graph. The white chert in other parts of the specimen is not chalky. About nat- ural size. fashion. No glaze was observed on sandstone, vein quartz, or quartzite pebbles. In sample 1, 22 of the 50 pebbles examined had more or less glaze ; 4 of the 50 pebbles in sample 2 had glaze. The origin of the glaze cannot be certainly determined from the mate- rial at hand. It is suggested, how- ever, that a porous, white, weathered zone, such as commonly develops on chert (fig. 10), was later impregnat- ed with silica to produce the hard, dense exterior material, or glaze. Different degrees of erosion of the weathered zone, possibly before im- pregnation, would account for the differences in the amount of glaze now evident. Agate Pebbles Samples 1, 2 and 3, from the Mis- sissippi Valley, contain pebbles of agate, mostly in shades of gray, white, and brown. These pebbles are not numerous, but they are signifi- cant because they are rare or absent in the other samples. They were ob- served to be also relatively common in the "Lafayette" gravel in Mis- souri across the Mississippi River from the deposit in Alexander County, 111., from which sample 2 was obtained. The agate pebbles are rounded to irregular. In sample 1, two brown chert pebbles were partly coated with banded quartz similar to parts of the agate pebbles. The agate pebbles are of different types. A common variety is of crys- talline quartz, showing comb struc- ture in some specimens, which is interspersed between irregular mas- ses of banded chalcedony and within which round agates as much as y 2 inch in diameter are distributed. In one pebble studied in detail, the con- tact between the comb quartz and the banded chalcedony is transition- al and consists of a series of thin alternating bands of crystalline quartz and chalcedony. The crystal size of the macro-crystalline quartz bands decreases away from the comb quartz and ultimately becomes micro-crystalline . The general structure and char- acter of the pebbles suggest that they were originally cavity linings or fill- ings. The chert coated with banded silica presumably represents part of a cavity lining and cavity wall. No deposits from which the agate pebbles could have come are known in Illinois. Their presence along the Mississippi Valley suggests that they may have come from Missouri or other areas adjacent to the valley. "Lafayette" Gravel Table 6- -Fauna List* Sample No. Location Name Age Grover Mo Fayville, 111. Mississippian Coliininaria alveolata? . . . . Upper Ordovician Mississippian 4 Cya.lhophyllum Devonian and Mississippian Mississippian Mississippian to Permian Silurian to Pennsylvanian Mississippian Mississippian Silurian to Permian Grand Chain, 111 Triplophyllum 5 6 Fenestellid bryozoa * Identification and age by W. H. Fossils Silicified fossils, commonly as par- tial specimens of individuals, are relatively abundant in some deposits of "Lafayette" gravel. Corals are most frequent. Some chert frag- ments contain a great many crinoid stems, and molds or partial molds of brachiopods, bryozoa, and gastro- pods. Table 6 lists the genera which were obtained as a result of limited collecting. Undoubtedly a systematic collec- tion from each locality would yield more detailed lists of fauna. The presence of fossil corals, crinoids, and brachiopods of the Lower and Middle Silurian is reported 8 in a deposit of Lafayette-type gravel in Henry County in northwestern Illi- nois. The data in table s State Geological Survey, 1942. that there is no need to assume a distant source for the fossiliferous chert in any of the southern Illinois "Lafayette" deposits studied, as southern Illinois or areas adjacent to it contain outcrops of limestone, some of it cherty, of Ordovician, Silurian, and Mississippian age and also extensive deposits of Devonian chert formations, especially near the localities from which samples 3 and 4 were taken. The Grover deposit is similarly situated with reference to outcrops of Ordovician, Silurian, and Mississippian rocks. Conclusions It appears from the foregoing that a study, including particle size dis- tribution, particle shape, pebble counts, and paleontology of fossil molds, of a greater number of sam- ples taken with cognizance of the relation of the deposits to topo- 108 Illinois Academy of Science Transactions graphic levels might yield informa- special features, such as polish, tion which would add materially to glazed pebbles, agate pebbles, and the knowledge of the source or silicified oolite pebbles, are in them- sources of the "Lafayette" gravel selves problems that merit further in Illinois and adjacent states, and investigation and that may aid in which would also broaden the factual a better understanding of the broad- basis for interpreting the history of er problems of the "Lafayette" the formation of the gravel. Various gravel. Illinois Academy of Science Transactions, Vol. 44, 1951 CAMBRIAN AND LOWER ORDOVICIAN EXPOSURES IN NORTHERN ILLINOIS* H. B. WILLMAN AND J. S. TEMPLETON Illinois Geological Surrey, TJroana Drilling in central northern Illi- nois has revealed the presence of Cambrian and lower Ordovician strata beneath glacial drift in a large area south of the Sandwich fault zone, as shown on the state geologic map (Weller and others, 1945), and one exposure of Cambrian strata at Oregon, Ogle County, 111., has long- been known (Bevan, 1929, 1935, 1939). No other Cambrian expo- sures and no exposures of lowermost Ordovician have been recognized previously in Illinois. The present study has shown that exposures west of the area mapped as Cambrian, formerly believed to be of Shakopee, Platteville, or Gal- ena age, actually belong to the Cam- brian Trempealeau formation and to the lower Ordovician Gunter and Oneota formations. The area under- lain by Cambrian and lower Ordo- vician rocks has been extended west- ward from Rochelle nearly to Eock Eiver, and the Sandwich fault zone has been traced west of Rock River (fig. 1). Outcrops near Millbrook, Kendall County, previously assigned to the Galena formation, now are known to be Oneota. The exposures show that at least the lower part of the Trempealeau formation in this region is a reef -type dolomite. Identification of the formations has been based mainly upon lith- ology as revealed by samples from wells, but partly upon fauna. The exposure at Oregon lies near the crest of the Oregon anticline, but all of the remaining exposures are near the crest of a major uplift on the south side of the Sandwich fault zone, herein named the Ashton Arch (figs. 1, 4). The exposures all are located on preglacial bedrock up- lands (Horberg, 1950, plate 1, sheet 1 ) . The absence of outcrops between the Fox River and Rochelle is ex- plained by the presence of two major southward-trending pregla- cial valleys and deposition of a thick Wisconsin morainic belt over the site of the valleys. Stratigraphy The exposed pre-St. Peter forma- tions of central northern Illinois are shown in columnar section in figure 2, and are described below. Franconia formation. — The Franconia formation, the oldest for- mation exposed in Illinois, is known to crop out only in the lower part of an abandoned quarry 3,700 feet from the north line and 1,100 feet from the east line of sec. 3 (elon- gate), T. 23 N., R. 10 E., Ogle County (Oregon quadrangle), one- half mile east of Oregon (fig. 1). The exposure is poor and has a total thickness of about 30 feet. It con- sists mainly of argillaceous, silty, glauconitic and muscovite-bearing, partly dolomitic, greenish-gray sand- stone which ranges from very fine grained to coarse grained, but is Illinois Academy of Science Transactions Fig. 1. — Location of Cambrian, Gunter, and Oneota exposures in northern Illinois. mostly fine grained. The grains are clear and mainly angular, although the medium and coarse grades are rounded. Abundant scattered fine pellets of dark green glauconite give the rock a "salt-and-pepper" aspect. Green glauconitic clay occurs as partings between the beds and as streaks and mottlings within the beds. The heavy mineral suite con- sists of abundant garnet, with sub- ordinate amounts of tourmaline and zircon. The sandstone is laminated and friable. It is in lenticular to regular beds from a fraction of an inch to 9 inches in thickness. A one- foot bed of impure, buff, chalky dolomite in thin irregular layers is present 20 feet below the top of the formation (fig. 3), and scattered dolomite lenses occur elsewhere. Partings of silty, partly dolomitic, olive-gray, laminated, friable shale are common. The following fossils from the sandstone, identified by G. 0. Raasch, show that the strata belong to the Bad Axe or highest member of the Franconia formation : Dikel- locephalns n. sp., Illaenurus n. sp., Saukiella minor Ulrich and Resser, and Trilobita n. gen. and sp. In ad- dition, small fragments of oboloid brachiopod shells are common, and ropy excremental worm-castings cov- er many bedding surfaces. Subsurface data indicate that the Franconia formation, exclusive of Ironton strata, ranges from 65 to 120 feet thick in north-central Illi- nois. The middle portion of the for- mation consists of sandy, glauconitic dolomite and limestone, but the lower part is composed of sandstone and sandy dolomite like the higher, exposed beds. A thin, dolomitic, coarsely glauconitic, coarse-grained sandstone, formerly regarded as the Cambrian and Lower Ordovician Exposures St. Peter LOWER ORDOVICIAN Shokopee New Richmond l6 l -80' 116-180' UPPER inter 17-55' CAMBRIAN Trempealeau 145-175' /v,/y,/,. h /•;//•/■ ZT2Z 7W X77s TM- \!Jj.:./$ Sandstone, pure, white, fine- to medium- grained, thick-bedded, wit h red to green clay at base. Dolomite, variable, mostly argillaceous, finely cryslalline and medium -bedded, with layers of sandstone, siltstone, shale, and oolitic chert. Sandstone, dolomitic, fine-grained, cross- bedded, with layers of sandy litho graphic dolomite and chert. Dolomite, coarsely crystalline, partly finely crystalline at top and bottom, sandy towards base, mostly thick- bedded, with oolitic chert. Dolomite, glauconitic, finely crystalline; upper portion (unexposed) is cherty and sandy; lower portion is pure, Fig. 2. — Pre-St. Peter formations exposed in northern Illir Illinois Academy of Science Transactions Cumbrian and Lower Ordovician Ex '** 'Up— ri^-\. f h*m ' "' * ''*$$ ' ^V /: . ''<& basal member of the Franconia, now is considered the top unit of the Ironton formation. The Franconia formation is correlated tentatively with the Davis, Derby, and Doerun formations of Missouri (Workman and Bell, 1948, p. 2053). Trempealeau formation. — Twen- ty-three feet of basal Trempealeau dolomite overlies Franconia sand- stone in the quarry at Oregon. An extension of this exposure is found in a low ridge littered with Trem- pealeau float % mile northwest of the quarry, 500 feet from the north line of sec. 3 and 1,700 feet from the west line. The major Trempealeau exposures found in this study are as follows (fig. 4) : (1) Quarry at Prairie Star School, 7 miles southeast of Oregon, SW14 SW14 SE% sec. 5, T. 22N., R. 11 E., Ogle County (Dixon quad- rangle), where 18 feet is exposed (fig. 5). Fig. 6. — Trempealeau and Gunter dolo- mites in quarry south of Rochelle, N. line NE14 SE1/4 NE1/4 Sec. 26, T. 39 N., R. 1 E., Lee County, 111. Illinois Academy of Science Transactions y ° n ' —A, Breccio \ Oon SCALE IN FEET Fig. 7. — Diagrammatic sketch of pre-St. Peter exposures in hill on north side of State Highway No. 2, NE% SE% SE14 Sec. 17, T.23 N., R.10 E., Ogle County, 111. Ct, Trempealeau; Oon, Oneota; Onr, New Richmond; Os, Shakopee; Osp, St. Peter, mainly filling pre-St. Peter channels. (2) Three quarries 1% miles northeast of Ashton, NWy 4 NWy 4 NE*4 sec. 22, SE% NW*4 NW% sec. 23, and NE corner sec. 23, T. 22 N., R. 11 E., Lee County (Eochelle quadrangle), where 14 to 22 feet of dolomite is exposed. (3) Quarry 6 miles south of Rochelle, N. line NB% SE^ NE%* sec. 26, T. 39 N., R. 1 E., Lee County (Rochelle quadrangle), where 6 feet of dolomite is exposed, with 5 addi- tional feet below water level (fig. 6). (4) Bank on north side of State Highway 2, 2% miles southwest of Oregon, NE% SE^ SB 14 sec. 17, T. 23 N, R. 10 E., Ogle County (Dixon quadrangle), where 4 feet of dolomite is exposed at road level and 3 feet is exposed 15 feet above road level (fig. 7). A few small outcrops of Trempea- leau dolomite occur in the vicinity of the principal exposures (fig. 4). The area has not been mapped in detail, and other outcrops may be present. The Trempealeau formation is composed of light buff, finely crystal- line to medium crystalline, reef -type dolomite, which is finely porous to vuggy, and occurs mainly in irregu- lar beds from 1 to 4 feet thick. The basal beds at Oregon are glauconitic and silty, but the dolomite in the remaining and stratigraphieally higher exposures is pure. Many vugs are lined with white to pink crys- talline quartz which readily differ- entiates the Trempealeau from other formations in this region. In places the bedding disappears into or is draped over structureless columnar Cambrian and Lower Ordovician Exposures 115 masses of dolomite more than 10 feet thick, which are interpreted as reef- cores of algal origin. Weathered bedding surfaces commonly are cov- ered with a network of low, narrow, vertically-sided ridges, and weath- ered faces are marked by narrow, irregular, vertical flutings; these phenomena also may be of algal origin. The rock contains algal domes and rare trilobite and gastro- pod remains. G. 0. Raasch has iden- tified the following forms: Crypto- zoon sp. Hypseloconus sp. nov., and Saukiella sp., cf. indenta Ulrich and In the subsurface of central north- ern Illinois the Trempealeau forma- tion ranges from 145 to 175 feet thick where not reduced by pre-St. Peter erosion. It consists of (1) a lower, partly glauconitic dolomite containing quartz-lined vugs, with- out disseminated sand except in places at the base, and (2) an upper, cherty, mostly sandy dolomite which lacks quartz-lined vugs and com- monly is non-glauconitic. All of the exposed Trempealeau belongs to the lower unit, which is correlated with the Potosi formation of Missouri (Workman and Bell, 1948, p. 2053) and with the St. Lawrence and Lodi members of the Trempealeau forma- tion in Wisconsin. South of the area of Trempealeau exposures glauconite in the lower unit occurs in three discontinuous zones of irregular thickness (Will- man and Payne, 1942, pp. 56-57). The upper unit apparently is not exposed. Although it is encountered in wells at Ashton and Dixon, Lee County, it lenses out northward and is absent at Rochelle and elsewhere in Ogle County. The upper unit is correlated with the Eminence forma- tion of Missouri (Workman and Bell, 1948, p. 2053) and with the Jordan and Madison members of the Trempealeau formation in Wiscon- sin. Continuous subsurface tracing of the Illinois Trempealeau units into the Wisconsin outcrop area is prevented by the presence of a ma- jor westward-trending pre-St. Peter valley just north of the Cambrian exposures, along which much or all of the Trempealeau formation has been removed (Meyer, 1948). In Wisconsin the Franconia-Trem- pealeau contact is marked by a diastem (Twenhofel, Raasch and Thwaites, 1935, pp. 1705-1706, 1717), but in the Illinois subsurface, Pranconia-Trempealeau relations are conformable and nearly grada- tional. The contact at Oregon is concealed by a short covered inter- val. The Trempealeau-Oneota 1 con- tact in Wisconsin seems unconform- able (Twenhofel, Raasch and Thwaites, 1935, pp. 1712, 1718). In the Illinois subsurface the Trem- pealeau-Gunter contact is abrupt, and in the quarry at Rochelle it is sharp and undulatory, with a relief of six inches. These features, to- gether with the absence of upper Trempealeau (Eminence) strata in Ogle County, suggest that a diastem or unconformity also separates the Trempealeau and Gunter formations in northern Illinois. Except at Rochelle the exposed Trempealeau is overlain unconform- ably by St. Peter sandstone or by glacial drift. At Oregon the upper Trempealeau beds in places have ^Although 1he Gunter formation is present in 116 Illinois Academy of Science Transactions been leached and altered to siltstone to a depth of at least 4 feet by pre- St. Peter weathering, and just south of the quarry the Cambrian strata abut against a channel-filling of St. Peter sandstone. Post-St. Peter thrust-faulting has altered the sand- stone overlying the Trempealeau formation to a quartzite-breeeia (Bevan, 1935, p. 384). Gunter formation. — In wells in central northern Illinois, 17 to 55 feet of impure dolomite and sand- stone lies between the Trempealeau and Oneota dolomites. In the past these beds have been variously as- signed to the Jordan, Gunter-Jor- dan, or Oneota formations. Recent stirdies have shown that they are very similar lil hologically to the type Gunter formation of Missouri, and can be traced into that formation ; they also can be traced to southern Wisconsin, where they overlie the latest Cambrian Madison sandstone and are included in the Oneota for- mation. Differentiation of Gunter strata thus involves a slight redefini- tion of the term Oneota as used in Iowa and Wisconsin. In Minnesota the Kasota and Blue Earth forma- tions (Powell, 1935) probably are equivalent to all or part of the Gunter formation. Known Gunter exposures in Illi- nois are confined to the quarry 6 miles south of Rochelle, described above under Trempealeau formation, and to a small exposure in a pros- pect pit and ravine, NW 1 /! NW 1 ^ SW14 sec. 31, T. 23 N., R. 11 E., Ogle County (Dixon quadrangle), 5 miles southeast of Oregon (fig. 4). In the Rochelle quarry (figs. 6, 8) the Gunter formation is 10 feet 3 inches thick, and consists principally Fig. 8. — Gunter and Oneota dolomites in quarry south of Rochelle, N. line NE14 SE% NE14 Sec. 26, T. 39 N., R. 1 E., Lee County, 111. of argillaceous, silty, finely glau- conitic, greenish-gray to cream, chalky dolomite which contains very fine muscovite flakes, and is slightly sandy in the lower 2 feet 9 inches. The dolomite is in irregular, lami- nated beds from 1 to 12 inches thick, separated by green clay partings. Some layers are algal and have high- ly undulatory bedding surfaces. Ir- regular masses of oolitic chert are common near the base and middle of the formation. Five feet below the top of the Gunter is a 30-inch bed of buff, medium crystalline, vuggy, cherty dolomite which closely resembles the overlying Oneota for- mation. The exposure 5 miles south- east of Oregon is very poor, and is composed of eight feet of impure, thin-bedded dolomite like that at Rochelle, but partly maroon. The Cambrian and Lower Ordovician Exposures 117 Gunter here is overlain by two feet of Oneota dolomite, but no Trem- pealeau strata are exposed. In subsurface (Workman and Bell, 1948, pp. 2054-2055) the Gun- ter formation consists of (1) very fine- to coarse-grained, poorly sorted to locally well sorted, incoherent sandstone, (2) varicolored dolomite which is chiefly chalky to litho- graphic and dense, and is partly argillaceous, silty or sandy, (3) vari- colored, oolitic to locally sandy chert which is principally white to orange, and (4) small amounts of slightly sandy, blue-green and dark-red, laminated, hard, smooth shale. The sandstone ordinarily is clean and white, but locally is argillaceous and gray-green. The fine sand is angular; the medium and coarse sand is mostly rounded, although some grains have been made angular by secondary crystalline quartz. The poorly sorted sandstone normally contains free, medium to coarse, sili- ceous oolites. A five-foot bed of ar- gillaceous, fine- and coarse-grained sandstone marks the top of the Gun- ter in wells between Dixon and Ore- gon, but lenses out eastward before reaching Ashton. Small Cryptozoon domes in the quarry south of Rochelle are the only fossils observed in Gunter strata. Oneota formation. — The principal exposures of the Oneota formation in northern Illinois are as follows (figs. 1, 4) : (1) On the west side of Fox River opposite Millbrook, Kendall County, where 12 feet crops out in a stream channel at a breached earth dam, SE14 NWiy4 SEy 4 sec. 8, T. 36 N., R. 6 E. (Sandwich quad- rangle) ; 10 feet is exposed in a small quarry, SE corner NE 1 /^ NE^i SE 1 /^ sec. 8, and 2 feet is exposed in an abandoned quarry, SW corner SWy 4 SWy 4 NWy 4 sec. 9. (2) In the quarry 6 miles south of Rochelle, where 31 feet is exposed (fig. 8), and in a shallow quarry Y^ mile eastward. (3) At Ashton, Lee County, in three quarries in the NW14 sec. 27, in two quarries on opposite sides of the road along the north line of the Ey 2 sec. 27, just east of State High- way 330, and in a shallow quarry in the center of the NE14 sec. 27, T. 22 N., R. 11 E. (Rochelle quadrangle). Here from 10 to 31 feet of rock is exposed in the individual quarries, and the total stratigraphic interval represented in the quarries exceeds 50 feet. (4) Shallow quarries 2 and 3% miles northwest of Ashton, W. line NEi/4 NEi/4 NWy 4 sec. 20 and NWy 4 NWi/4 SWy 4 sec. 18, T. 22 N., R. 11 E., Lee and Ogle counties, exposing 8 and 16 feet of dolomite, respectively. (5) Quarry 5 miles southeast of Oregon, S. line SEy 4 NWy 4 NEy 4 sec. 31, T. 23 N., R. 11 E., Ogle County, exposing 12 feet of deeply weathered rock. (6) Quarry and ravine 4 miles south of Oregon, SW^4 SW% sec. 26, T. 23 N., R. 10 E., Ogle County, exposing 45 feet of dolomite. (7) Cut on north side of State Highway 2, two miles south of Ore- gon, NE14 SEy 4 SEi/4 sec. 17, T. 23 N., R. 10 E., Ogle County. (8) Ravine 6 miles south of Ore- gon, SE14 SEi/4 SEi/4 sec. 4, T. 22 N„ R. 10 E., Ogle County, where 35 feet of dolomite crops out. 118 Illinois Academy of Science Transactions Localities 4 through 8 are located in the Dixon quadrangle. A few other small, scattered exposures less than 5 feet thick also occur near the principal localities listed above (fig. 4). Most of the Oneota formation con- sists of light-gray to blue-gray, uni- formly coarsely crystalline, dense, cherty dolomite in massive beds as much as 15 feet thick. Most indi- vidual crystals range from 1 to 2 mm. in diameter, and make the rock the most coarsely crystalline dolo- mite in the Illinois Paleozoic se- quence. Upon weathering the dolo- mite breaks down into regular, fri- able, 6- to 24-inch beds with dull brown-gray, coarsely "sandy" sur- faces, and the interior turns light buff with white flecks representing decayed cement between the dolo- mite crystals. Vertical faces gener- ally are regular, although occasional beds have a brecciated or pseudo- fucoidal structure and a rough face. Bedding surfaces are rough but have low relief. Except for occasional green clay films within the beds the rock is pure. At one locality (No. 8) thirty -five feet of upper Oneota dolomite is mainly finely crystalline and weathers to a coarsely pitted face; these features have not been noted elsewhere and apparently re- flect local facies-change. . Basal Oneota strata exhibit con- siderable differences from the main body of the formation. Much of the lowermost 9 to 10 feet is lithographic to finely crystalline, is in regular 2- to 8-ineh layers with smooth bed- ding surfaces, and weathers deep buff. In the quarry south of Rochelle some boulder-like, algal masses of lithographic limestone are present in this basal zone ; greenish-gray to buff shale partings are common, and the upper 6 inches of the zone are silty and laminated. Here the basal zone represents a transition from Gunter to typical Oneota sedimentation. Gray, white, and light yellow to pink or red chert, most of which is sandy, oolitic, conglomeratic, or strongly banded, occurs throughout the Oneota formation. In places the chert forms layers, lenses, or spher- oidal masses as much as 1 foot thick, but in others it consists of numerous small, irregularly shaped and orient- ed, angular fragments which locally form a disconnected meshwork with- in the beds. The lower 12 to 60 feet of the formation ordinarily are fine- ly glauconitic and somewhat sandy; a few sandy streaks are found at higher horizons. Cryptozoon is present to common in nearly all Oneota exposures and forms conspicuous small domes on the floor of the quarry at Ashton, center NE 1 /^ sec. 27 (part of Lo- cality 3). Gastropod molds pre- served mainly in chert from the quarry at Ashton, SW*4 SW% SE14 sec. 22 (part of Locality 3) have been identified as : Gasconadia putilla (Sardeson), Ophileta sp., Bhachopea sp., and Sinuopea sp. Identifiable fossils other than Cryp- tozoon have not been seen elsewhere. The fauna is typical of the Oneota formation of Wisconsin and the Gas- conade formation of Missouri (Ul- rich, Foerste and Bridge, 1930, pp. 186-222; Branson, 1944, pp. 35-47). Correlation of this exposure with cuttings from two Chicago and North Western Railway wells at Ashton (Illinois Geological Survey, sample sets 289 and 402), y 2 mile Cambrian and Lower Ordovician Exposures 119 south, show that the quarry floor is not less than 60 feet above the Gunter-Oneota contact. Part or all of this concealed interval is believed equivalent to the Van Buren forma- tion of Missouri (Workman and Bell, 1948, p. 2056). The total thickness of the Oneota dolomite in central northern Illinois ranges from 116 to 180 feet. The formation shows pronounced thin- ning along the crest of the western part of the Ashton arch. The contacts of the Oneota dolo- mite with the adjacent Gunter and New Richmond formations seem con- formable. Although Payne (Will- man and Payne, 1942, pp. 58-59) reports an unconformity between the Gunter ("Jordan") and Oneota formations to the southward, the presence of such an unconformity has not been confirmed by recent studies. New Richmond formation. — The New Richmond formation superfi- cially resembles the St. Peter sand- stone. However, it generally (1) has finer, more angular, less frosted grains, which more frequently are enlarged by secondary crystalline quartz, (2) is less well sorted, (3) contains free siliceous oolites and grains or nodules of chert, (4) has a much higher proportion of heavy minerals, with abundant accessory tourmaline, considerable ilmenite and leucoxene, and some garnet, (5) is much thinner-bedded and more strongly cross-bedded, and (6) is better cemented. It contains beds of lithographic dolomite and dolo- mite-cemented sandstone, has green argillaceous streaks in the upper part, and has gray, blue, or red shale layers in the lower portion (Willman and Payne, 1943, pp. 532- 533; Workman and Bell, 1948, pp. 2057-2058) . In central northern Illi- nois its thickness ranges from 16 to 80 feet, and increases rapidly to a known maximum of 190 feet at Starved Rock State Park, LaSalle County. It is correlated with the Roubidoux formation of Missouri, which carries the distinctive Lecano- spira fauna. Exposures of New Richmond in Illinois previously had been known only along Franklin Creek, Lee County, and Fox River, LaSalle County (Cady, 1920, p. 107; Knap- pen, 1926, pp. 40-43; Willman and Payne, 1943, pp. 532-533). The present study revealed poor expo- sures in (1) a small quarry 3% miles east of Grand Detour,' SE14 SE14 NWy 4 sec. 9, T. 22 N., R. 10 E., Ogle County (Dixon quadran- gle), where 3 feet of sandstone is ex- posed beneath Shakopee dolomite, and (2) a cut on the north side of State Highway 2, two miles south of Oregon, NE*4 SE*4 SE% sec. 17, T. 23 N., R. 10 E., Ogle County (Dixon quadrangle), where 11 feet of sand- stone, with covered intervals above and below, crops out between Oneota and Shakopee strata. Samples from both outcrops are slightly dolomitic and show the typical New Richmond heavy mineral suites (table 1). It is very probable that other outcrops of New Richmond exist in the north- east quarter of the Dixon quad- rangle, where they have not been differentiated from St. Peter sand- stone. Because the New Richmond offers little resistance to erosion, ex- posures not on major stream chan- nels are mostly veneered by soil or Shakopee float. Illinois Academy of Science Transactions Table 1. — Heavy Mineral Analyses of New Richmond Sandstone in Northern Illinois, in Percent Locality 1 Garnet 3.0 Ilmenite 17.0 Leucoxene 22.0 Magnetite 1.0 Tourmaline 42 . Zircon 12.0 Miscellaneous. . 3.0 Percent of heavy minerals by weight 0.006 2 3 17.0 3.0 17.0 20.0 10.0 18.0 6.0 2.0 35.0 36.0 10.0 15.0 5.0 6.0 0.003 0.006 Locality 1: 5' below top of sandstone in bluff on north side of Franklin Creek, N. line SE^ NW% SW% sec. 34, T. 22 N., R. 10 E., Lee County. Locality 2: 2' below top of sandstone in quarry, SEy 4 SE14 NW14 sec. 9, T. 22 N, R. 10 E., Ogle County. Locality 3: 15' below top of sandstone in bluff on west side of State Highway 2, NE% SE% SE14 sec. 17, T. 23 N., R. 10 E., Ogle County. locality ] t : Average of 19 samples from upper 23 feet of sandstone at ravine mouth, NW% SW14 SWy 4 sec. 8, T. 35 N., R. 5 E., LaSalle County. Analyses for localities 1, 2 and 3 by T. C. Busch- bach; analyses for locality 4 by Paul Herbert, Jr. Shakopee formation. — The Shako- pee formation consists of irregular, alternating, varicolored beds of dolo- mite, shale, siltstone, and sandstone, with dolomite greatly predominat- ing. The dolomite shows great varia- tion in purity, color, crystal size, porosity, and bedding. Most of it is argillaceous, silty, sandy, cherty, locally glauconitic, light gray to yel- low-buff, chalky to finely crystalline, dense, and thin-bedded. Brecciated and conglomeratic beds are very common. Bedding surfaces generally are ripple-marked and mud-cracked, and show occasional molds of cubic salt crystals. Beds of relatively pure, medium crystalline dolomite, as much as 18 inches thick, are com- mon in the lower part of the forma- tion. Although such beds locally re- semble Oneota dolomite they lack the distinctive chert layers, chert fragments, and coarsely "sandy" weathered surfaces of the latter for- mation, are thinner-bedded, and con- tain thin interbeds of typical Shako- pee argillaceous dolomite. The chert is mostly oolitic, but partly sandy or conglomeratic. The sandstone heavy mineral suites have a higher percentage of garnet and ferromag- nesian minerals than do those of the New Richmond sandstone (Willman and Payne, 1943, p. 535). Several layers of bentonite are present in a quarry in the SW^ SE^ NW% sec. 6, T. 23 N., R. 10 E., Ogle County (Oregon quadrangle), 2y 2 miles west of Oregon. The thickness of the Shakopee in central northern Illinois ranges from zero to about 165 feet, but in- creases rapidly to more than 600 feet in south-central Illinois. The Shako- pee is correlated with the Jefferson City and Cotter formations of Mis- souri (Workman and Bell, 1948, pp. 2058-2060), although equivalents of younger Missouri formations may be included locally. The Shakopee dolomite rests con- formably on New Richmond sand- stone, but is separated from the over- lying St. Peter sandstone by a major unconformity. Known exposures of the Shakopee formation in Illinois are found (1) along the Illinois River and its tribu- Cambrian and Lower Ordovician Exposures 121 taries from LaSalle east to Utica, LaSalle County (Cady, 1919, pp. 35-36), (2) along- Fox River near Sheridan and Millington, in LaSalle and Kendall counties (Willman and Payne, 1943, pp. 534-538), and (3) along Rock River and its tributaries between Franklin Grove, Lee County, and a point 2y 2 miles west of Oregon, Ogle County. Its dis- tribution in the latter area is shown in fig. 4. Structural Geology Ashton arch. — Most of the Cam- brian and lower Ordovician expo- sures described above lie on the crest or flanks of a major anticline which trends N. 60° W. across north- ern Illinois from central western Will County to central Ogle County, and which is here named the Ashton arch (figs. 1, 4). The name is de- rived from the town of Ashton, Lee County, which is located near the crest of the western portion of the arch, and near which Cambrian and lower Ordovician formations, brought to the surface by the arch, are well exposed. The Ashton arch first was rec- ognized by Cady under the name Ogle, Lee, and LaSalle counties anti- cline (Cady, 1920, pp. 90, 127-128, fig. 8), although later studies modi- fied the original concept of its loca- tion and extent. It formerly was considered to mark the crest of the Kankakee arch, a broad uplift which connects the Cincinnati and Wis- consin arches and separates the Illi- nois and Michigan basins (Pirtle, 1932, p. 149, fig. 1; Ekblaw, 1938). However, it has been found (1) that the pre-St. Peter axis of the Kanka- kee arch passes through the north- east corner of Illinois (Bays and others, 1945; Cohee, 1945, fig. 4; Meyer, 1948) and (2) that the post- St. Peter axis (Cohee, 1945, fig. 5; 1948, p. 1441, fig. 1) merges into the east flank of the Hersher anti- cline which is separated from the Ashton arch by a northward-trend- ing syncline (fig. 9). Accordingly it appears desirable to introduce a separate name, Ashton arch, for the relatively local uplift between Will and Ogle counties. The Ashton arch is bounded on the north throughout most or all of its length by the Sandwich fault zone. A graben and a syncline sepa- rates the western part of the arch from the smaller, parallel Oregon anticline on the north (fig. 4). A syncline also separates the eastern portion of the arch from the LaSalle anticline (fig. 9). The LaSalle anti- cline merges into the arch along the LaSalle-Lee County boundary, and cannot be distinguished farther north. However, a southward prong of the arch, which may represent a continuation of the LaSalle anti- clinal trend, extends northwestward from Dixon to the Savanna-Sabula anticline. The southwestward flank of the Ashton arch dips steeply into the Illinois basin. The western end plunges into the small but deep Polo basin, named herein from the town of Polo, Ogle County, which lies on the west side of the basin. The Ashton arch has a length of 80 miles and a width of 17 to 25 miles. The structural relief on the southwestern side is about 1,900 feet, and the maximum relief on the northern side at least 900 feet. The axis of the arch is at, or a short dis- tance south of, the Sandwich fault Illinois Academy of Science Transactions Cambrian and Lower Ordovician Exposures zone. The arch has a broad, nearly flat summit area, most of which dips very gently southward. The steepest dips on the structure, found on the southwest flank, are only from 1° to 3 C . Sandwich fault zone and Oregon anticline. — The Sandwich fault zone parallels the strike of the Ashton arch and bounds the northeastern side of the arch for most or all of its length. It has been traced from southern Will County to a point in northeastern Lee County 6 miles south of Rochelle, a distance of 68 miles, and probably continues at least 20 miles farther, to a point 2% miles southwest of Oregon, in central Ogle County (figs. 1, 4, 9). The fault zone has a maximum down- throw of at least 900 feet on the northeast side. Because the fracture appears to be compound rather than single the term fault zone is applied. Between Rochelle and Oregon the structure on the northeastern or downthrown side of the fault is com- plicated by a sharp uplift called the Oregon anticline (Bevan, 1935), the crest of which approximately paral- lels the Ashton arch and Sandwich fault zone and lies from 2 to 3 miles northward from the latter. The Cam- brian exposure at Oregon, a small Oneota exposure in the SE14 SE14 NBi/4 sec. 30, T. 22 N., R. 11 E., Ogle County (Dixon quadrangle), and a Shakopee exposure in the SWi/4 SE% NW14 sec. 6, T. 23 N., R. 10 E., Ogle County (Oregon quadrangle), are located on the anti- cline. The Oregon anticline contin- ues northwestward to join the Sa- vanna-Sabula anticline. A synclinal belt separates the Oregon-Savanna- Sabula anticlines and associated branch uplifts from the main body of the Wisconsin arch on the north. A narrow graben and a tightly compressed syncline, bounded on the south by the Sandwich fault zone, separate the Oregon anticline from the Ashton arch (fig. 4) . The graben is well exposed in the cut on the Chicago, Burlington, and Quincy Railroad, just south of the center of see, 7, T. 23 N., R. 10 E., where a dropped block of Galena dolomite is bounded by faults which bring the top of the St. Peter sandstone on the southwest and lower Platteville beds on the northeast to the same elevation as the Galena strata. The throw on the south side of the block is more than 165 feet and that on the north side is over 130 feet, The block is about one-half mile wide and is cut by numerous minor step- faults. Sharp synclinal folding with- in the graben is well developed in Platteville strata exposed at inter- vals from the NW% SW^ SE*4 see, 26, T. 23 N., R. 10 E., to the NEi/4 NWy 4 NEi/4 sec. 31, T. 23 N., R. 11 E., Ogle County (Dixon quadrangle), where northward dips range from 2° to 10° and southward dips from 14° to 30°. Structural history. — The major movement along the LaSalle anti- cline in northe r n Illinois was post-Mississippian, pre-Pennsylvan- ian, followed by lesser uplift in post- Pennsylvanian time (Payne, 1939). On the Ashton arch, Sandwich fault zone and Savanna-Sabula anticline, and probably on the Oregon anti- cline as well, the principal movement was at least post-Silurian, and may have taken place at about the same time as uplift on the LaSalle anti- cline. It is uncertain whether the 124 Illinois Academy of Science Transactions Ashton arch owes its present form solely to downthrow along the north- ern side of the Sandwich fault zone, or whether the arch formed prior to the faulting. The general northwestward to westward structural grain of north- ern Illinois (fig. 9) is affected by subordinate northeastward-trending cross folds of uncertain age. It is possible that the post-St. Peter Kan- kakee arch of northwestern Indiana, uplifted mainly in post-Pennsyl- vanian time, once was continuous with the Ashton arch, but was sepa- rated from it by later cross folding. There is considerable evidence for pre-St. Peter deformation in the area occupied by the Ashton arch, although generally it cannot be proved that the trend of the defor- mation paralleled that of the present structure. Upper Trempealeau (Eminence) strata thin or wedge out northward over the arch, but reappear in northernmost Illinois and southern Wisconsin, across a major pre-St. Peter valley. The Oneota formation thins conspicuous- ly over at least the western part of the arch, parallel to its general trend. The earlier movements cul- minated in uplift, folding, and ero- sion in post-Shakopee, pre-St. Peter time. At places in Lee and Ogle counties Shakopee strata are thrown into close folds which do not affect the overlying St. Peter sandstone (Knappen, 1926, pp. 83-84, 111) ; al- though formerly attributed to pre- St. Peter slumping, the folding is considered diastrophic by the writ- ers. Along Fox River in LaSalle and Kendall counties the Shakopee shows steep dips which do not ac- cord with the gentler post-St. Peter structure in amount or direction (Willman and Payne, 1943, fig. 1). At Oregon, Ogle County, a post- Shakopee, pre-St. Peter fault cut- ting Cambrian strata has a down- throw of 285 feet on the western side. Where adequate subsurface control is available the interval be- tween the top of the Glenwood for- mation and pre-St. Peter horizons in central northern Illinois shows sharp irregularities which cannot be explained by variation in the thick- ness of the formations, and which are attributed to post-Shakopee, pre- St. Peter deformation. Unfortunate- ly the amount or trend of such de- formation along most of the Ashton arch cannot be estimated because (1) the deeply incised pre-St. Peter drainage system makes the base of the St. Peter valueless for struc- tural control, and (2) erosion has stripped away the post-St. Peter formations. REFERENCES Bays. C. A. and Others (1945), Ground- water geology of northeastern Illinois: Illinois Geol. Survey, unpublished manuscript. Bevan, A. C. (1929), Fault block of Cambrian strata in northern Illinois (abstract): Geol. Soc. Amer. Bull. 40. p. 88. (1935), Cambrian inlier at Ore- gon, Illinois: Kansas Geol. Soc. Guide- book 9th Ann. Field Conf., pp. 383-385. (1939), Cambrian inlier in north- ern Illinois: Am. Assoc. Petr. Geol. Bull. 23, pp. 1561-1564. Branson, E. B. (1944), Geology of Mis- souri: Univ. of Missouri Studies, vol. 19, no. 3. Cady, G. H. (1919), Geology and min- eral resources of the Hennepin and LaSalle quadrangles: Illinois Geol. Survey Bull. 37. (1920), Structure of the LaSalle anticline: Illinois Geol. Survey Bull. 36, pp. 85-180. Cambrian and Lower Ordovician Exposures Cohee, G. V. (1945), Sections and maps of Cambrian and Lower Ordovician rocks in the Michigan basin, Michigan and adjoining areas: U. S. Geol. Sur- vey, Oil and Gas Inv., Prelim. Chart 9. ■ (1948), Cambrian and Ordovician rocks in Michigan basin and adjoin- ing areas: Am. Assoc. Petr. Geol. Bull. 32, pp. 1417-1448. Ekblaw, G. E. (1938), Kankakee arch in Illinois: Geol. Soc. Am. Bull. 49, pp. 1425-1430; Illinois Geol. Survey Cir. 40. Horberg, C. L., (1946), Pre-glacial ero- sion surfaces in Illinois: Illinois Geol. Survey Rept. Inv. 118, fig. 2. Knappen, R. S. (1926), Geology and mineral resources of the Dixon quad- rangle: Illinois Geol. Survey Bull. 49. Meyer, M. P. (1948), Pre-St. Peter struc- ture and topography of northern Illi- nois: Illinois Geol. Survey, unpub- lished manuscript. Payne, J. N. (1939), The age of the LaSalle anticline: Illinois Acad. Sci. Trans., vol. 31, pp. 182-183. Piktle, G. W., (1932), Michigan struc- tural basin and its relationship to sur- rounding areas: Am. Assoc. Petr. Geol. Bull. 16, pp. 145-152. Powell, L. H. (1935), A study of the Ozarkian faunas of southeastern Min- nesota: St. Paul Inst. Sci. Mus. Bull. 1. Twenhofel, W. H., Raasch, G. O., and Thwaites, F. T. (1935), Cambrian strata of Wisconsin: Geol. Soc. Amer. Bull. 46, pp. 1687-1744. Ulrich, E. O., Foerste, A. F., and Bridge. Josiah (1930), Systematic paleontol- ogy of late Cambrian and early Or- dovician formations of Ozark region, Missouri: Missouri Bur. Geol. and Mines, 2d ser. vol. 24, pp. 186-222. Weller, J. M. and Others (1945), Geo- logic map of Illinois: Illinois Geol. Survey. Willman, H. B. and Payne, J. N. (1942), Geology and mineral resources of the Marseilles, Ottawa, and Streator quad- rangles: Illinois Geol. Survey Bull. 66. (1943), Early Ordovician strata along Fox River in northern Illinois: Jour. Geol., vol. 51, pp. 531-541; Illi- nois Geol. Survey Circular 100. Workman, L. E. and Bell, A. H. (1948), Deep drilling and deeper oil possibili- ties in Illinois: Am. Assoc. Petr. Geol. Bull. 32, pp. 2041-2062; Illinois Geol. Survey Rept. Inv. 139. Illinois Academy of Science Transactions, Vol. 44, 1951 BARITE IN THE LASALLE LIMESTONE OF ILLINOIS* RAYMOND S. SHRODE Illinois State Geological Surrey, Urbana Many strata in the LaSalle lime- stone contain quartz, pyrite, and vugs of crystalline calcite in relative abundance. To date, however, the presence of barite has not been re- ported. This paper discusses and de- scribes an occurrence of this mineral. The barite was found in the upper part of the LaSalle Limestone near the top of the limestone exposed in the abandoned LaSalle Stone Com- pany quarry near LaSalle, in the SW14 SW14 NE% sec. 13, T.33 N. R.l E. and also in another quarry adjacent on the west. The mineral is easily distinguished in hand speci- mens of the limestone by its light pink color. It occurs in small masses of thin blades, or sheets, which are easily separated with the fingernail. The barite was identified by blow pipe and optical mineralogy tech- niques which were confirmed by x-ray analyses. 1 Indices of refrac- tion check well with those commonly reported for barite. A series of spe- cific gravity determinations with a Jolly balance averaged 4.151. This value is lower than the commonly re- ported gravity, but may be due to porosity resulting from the bladed character of the mineral. Figure 1 shows a broken surface through the interior of a calcite- and barite-filled cavity. The light area in the center is the barite. Sur- rounding the barite is a zone of coarsely crystalline dark calcite ♦Published with the perm: Illinois State Geological Surve; 1 Bradley, W. F., personal c which owes its color to a very thin coating, believed to be ferruginous. The triangular shape of many of the grains results from the exposure of only three faces of rhombic crystals. Some dark calcite grains are also evident adhering to the main body of the barite. Figure 2 is another cavity which has been sawed, smoothed and etched with hydrochloric acid to bring the barite into relief. The white-appear- ing barite in the center of the figure is partially surrounded by dark, coarsely crystalline calcite which blends into more finely crystalline clear calcite. The outline of the barite, however, is well defined and conforms to the outline of the coarse calcite crystals. The mottled area in the lower right-hand portion of the photograph is limestone matrix. Figure 3 is a thin section of a por- tion of a filled cavity. The cavity wall shown in the figure is part of a fossil or fossils. Just inside the cavity wall in the upper right a very thin zone of fine calcite is indis- tinctly visible. Radiating from the finely crystalline calcite is a layer of coarse calcite. A large calcite crystal is visible in the lower left of the photograph. The barite fills the space within the coarse calcite zone, and does not show crystal bounda- ries, but is in part characterized by a lack of evident crystallinity and faint striations running from upper left to lower right which result from its bladed structure. The conforming LaSaUc Limestone r iW^S^f^ * £ ■ n'fc* *2£ 'V and lined wit.li dark calcite. X 10. I Fig. 2. — Smoothed and etched surface of a barite- Illinois Academy of Science Transactions and fl. T i - - • ' of a cavity lined with coarse calcite contact of the barite and the coarse calcite crystals is marked by rela- tively dark lines caused by the coat- ing on the calcite previously men- tioned. The figures show two and possibly three stages of cavity filling. Dur- ing the first the clear calcite of rel- atively small crystal size was formed on the cavity walls, followed by coarser and more euhedral crystal- line calcite which appears dark. Lack of a clearly defined line be- tween the two calcites suggests con- tinuous deposition with conditions in the latter stage favoring the growth of larger and more perfect crystals. Subsequent to their com- plete growth the coarse calcite crys- tals have been coated with a thin brown to light-brown coating thought to be ferruginous. In some cavities this stage is absent, and the phenomenon is a local one. The barite postdates all other stages. This is indicated by the well-defined contact between the barite and the dark coarse calcite shown in figures 2 and 3, and is further borne out by the fact that some cavities contain the fine and coarsely crystalline cal- cite in varying amounts, but are devoid of any barite. The presence of a few calcite crystals attached to or within the barite, figures 1 and 3, might be interpreted to mean con- temporaneous or penecontemporan- eous deposition of the two minerals, but it is believed more likely that the calcite crystals projected from a portion of the cavity wall de- stroyed during preparation of the specimens for study. Illinois Academy of Science Transactions, Vol. 44, 1951 REVISION OF CROIXAN DIKELOCEPHALIDS* The trilobites under consideration are classic in that before 1914 all species then known were assigned to the genus Dikelocephalus, Owen, 1852, which genus was regarded as the principal index fossil for the Upper Cambrian of the Pacific realm. Today these same trilobites are consciously ignored by biostratig- raphers. This change dates from the early 1930 's, when Ulrich and Resser published their monographic revi- sion of the Upper 'Mississippi Valley Dikelocephalidae (Milwaukee Public Museum Bull., vol. 12, nos. 1 and 2, 1930 and 1933). Before 1914, less than a dozen species of Dikelocephalus were rec- ognized as occurring in Upper Mis- sissippi Valley strata. That year Walcott's paper reduced the scope of the genus by adding several new genera, as well as several new species to his "Dikelocephalinae." By 1933, Ulrich and Resser had multiplied this conservative figure to the astounding total of 123 species and varieties. They simultaneously succeeded in rendering the Dikelo- cephalidae useless for purposes either of biostratigraphy or phyl- ogeny, and this important fossil group has subsequently been shunned by paleontologists and stra- tigraphers. The primary intent of the present paper is the restoration of the [■ulilHn'i] wilh permit State Geological Survey, Urbana. Q of the Chief, Illin Dikelocephalidae to a useful status. This has involved a reduction of names from Ulrich and Resser 's 123 to 41, or by exactly two-thirds, thus eliminating 82 names. Such a re- vision is of course tentative, but is quite representative of the degree of species designation that the situa- tion merits. Consideration op Stratigraphic Assignments A secondary objective is assign- ment of the species to their proper places in the fauni-stratigraphic succession. This is necessitated be- cause Ulrich 's conception of the re- gional stratigraphic relations was at serious variance with the facts. Since these erroneous stratigraphic considerations seriously affected Ul- rich and Resser 's taxonomic conclu- sions, the results were unfortunate. For example, what is now con- sidered a single sequence, the Franc- onia formation, Ulrich formerly interpreted as three successive for- mations. He applied the name Franconia to the greensand facies occurring in the region of the Missis- sippi. To the equivalent non-glau- conitic sandstone facies in central Wisconsin he applied the name Mazomanie. To a shore facies of the same, he applied the term Devils Lake formation. He interpreted the Mazomanie as successive to the Franconia and the Devils Lake as 138 Illinois Academy of Science Transactions successive to both, in fact post-Cam- brian. Thus the phylogenetic suc- cession of Franconian dikeloeepha- lids is thrice repeated. The Trempealeau formation, which succeeds the Franconia, he conceived as a simple lithologic succession of basal dolomite (his St. Lawrence), siltstone (his Lodi), fine sandstone (his Norwalk) . All fossils, therefore, that occurred in siltstone he con- sidered to be ' ' Lodi ' ' in age, and all in sandstone to be " Norwalk. ' ' The fact is that Lodi siltstone lith- ology occurs ;i1 eight different time- stratigraphie horizons and Norwalk sandstone Lithology at six different horizons. Not only does the same lithology occur at a number of differ- ent stratigraphie horizons, but the same stratigraphie horizon is com- monly represented by different lith- ologies in different areas. In these cases there is commonly a double set of names for each species, one for the supposed "Lodi" and one for the supposed "Norwalk." Some of the authors' stratigraphie misconceptions have interesting re- sults. For example, the rich faunas from leached calcareous fine sand- stones at Osceola, Wis., are con- sidered to be the latest of Trem- pealeau faunas; they are in fact, the earliest. Another unfortunate result of the stratigraphie confusion was the ap- plication of some singularly inap- propriate specific names. Dikelo- cephalus norwalkensis, Saukiella norwalkensis, and Tellerina norwalk- ensis do not occur in the Norwalk, nor does Osceolia lodensis occur in the Lodi. Fortunately, all drop into the synonymy. Consideration of Taxonomic Criteria A third objective of the present paper is to present a concrete in- stance of the effect, on taxonomic results, of differing species concepts among different paleontologists. Once we leave the realm of living forms, the species concept is no longer subject to the restraint of genetic criteria. Inevitably, individ- ual paleontologists resort to their own methodology in grouping organ- isms into species, genera, and higher categories. The only test against which the quality of any worker's results may be applied is the prag- matic one of applicability of his bio- logic classification in the fields of biostratigraphy and of phylogeny. If the classification contributes to order it may be assumed to be valid ; if it contributes to confusion it may with equal justice be assumed to be invalid. The responsibility demand- ed of the paleontologist is accord- ingly great. From this viewpoint it follows logically that paleontologic studies must go hand in hand with strati- graphic studies. But erroneous stra- tigraphy renders confusion twice confounded. In the case of Ulrich and Resser's work in the Dikelocephalinae, the following practices and concepts have contributed to confused tax- onomy : (1) Basing of species on type individuals rather than on popula- tions. (2) Seeking for minute differ- ences rather than for likenesses. Croixan Dikelocephalids 139 (3) Misassoeiation of the vari- ous, separated component parts of the skeleton, commonly combining a cranidium from one locality, a pyg- idium from a second, and free cheeks from a third. (4) Failure to consider that variations may be a residt of succes- sive growth stages. (5) Rejection of any concept of individual variation. (6) The influence of strati- graphic misconceptions. (7) Rejection of trinomial clas- sification for varieties or subspecies. (8) Establishment of species on inadequate material. Basing of species on type individ- uals rather than on populations. — Under discussion of Calvinella spini- ger, on page 222, the authors state : However, we will admit at once that in studying the many slightly differing cranidia of Calvinella that we possess from the same bed and place [italics the writer's] on Trempealeau Mountain that provided Hall's original type of the species, we found it no easy task to determine precisely which particular kind is best entitled to the distinction of being recognized as the typical form of C. spiniger. Though our final selec- tion is not entirely satisfactory to us we are so nearly correct that the margin of error is practically negligible. . . . The remainder of the specimens of Calvinella that were found with C. spiniger, as here restricted, is divided into seven varieties and species. The series is clearly intergrading and in this writer's opinion, consti- tuted a single interbreeding species population of distinct zonal value. Seeking for minute differences rather than for likes. — An example of proliferation of species by Ulrich and Resser is provided by the Prosaukia occurrence at Table Rock in Adams County, Wis. The fauna here comes from a six-inch layer about a foot below the bare rock summit of the mesa, the area of which is less than an acre. From this layer, Ulrich and Resser describe eight species of Prosaukia, all but one of which appear to constitute a single species. The eighth, P. ? anomala, belongs to another genus, not of the Dikelocephalidae. Most of" the "species" the authors credit to the type locality only. Quoting from p. 146: All of the . . . species found at the Table Rock locality, except P.? anomala, seem very closely related. Their cranidia, in particular, though distinguishable by features that may seem of little import- ance, are much alike. The associated pygidia, on the contrary, indicate six readily discriminated forms, and on this account the species are based mainly on these. Cranidia, and in most cases also free cheeks, are assigned to them according to our best judgment. Actually none of their pygidia are complete, most are highly frag- mentary, and in the writer 's opinion show no significant differences. In connection with one of Ulrich and Resser 's species, "P. alternata," these authors frankly state : Again the material, which consists of a single cranidium, a good free cheek, and three imperfect but supplementing pygidia, leaves much to be desired. But as none of these specimens quite fits any of the previously described species and, despite their imperfections, permits ac- quiring . . . probably a true conception of the complete animal, we feel war- ranted ... in describing these and other Misassoeiation of the various com- ponent parts of the skeleton. — Many examples might be cited, but the fol- lowing should suffice : Dikelocephalus inaequalis is based on three types from the basal Jordan at Trempealeau, Wis., but the fotTrth type (an hypostoma) comes from the basal Trempealeau (five zonal 140 Illinois Academy of Science Transactions units lower in the section) at a dis- tant locality. Its proper association should have been with quite a differ- ent species, D. thwaitesi. The writers, of course, were of the im- pression that the enclosing strata at the two localities were of the same age. Failure to consider that variations may be a res nil of successive growth stages. — Ulrich and Resser's multi- plication of species of Dikelocepha- lus from the Locli Member is a con- sequence largely of failure to recog- nize morphological changes which accompany growth stages. Ulrich and Eesser cite (p. 31), but do not figure, evidence to refute this, but such evidence this writer has never observed although he collected most of the specimens used by them. He feels that at least four of their Lodi species are successive growth stages of a single species. Since most speci- mens are molts, it was quite possible for a single individual to produce four of Ulrich and Resser's species. Rejection of any concept of in- dividual variation. — This point is best illustrated by an oral communi- cation of the junior author to the writer. When the latter raised the question as to whether some of the differences among specimens of Dikelocephalus might not represent individual variation, Resser cate- gorically stated that "trilobites do not vary. If two specimens are different, they represent different The influence of si rati graphic mis- conceptions. — Instances are innu- merable and lead the authors repeatedly into frustrating phylo- genetic discussions, in an attempt to rationalize seeming anomalies. The degree of stratigraphic error might best be summed up by citing the fact that, of the 473 figures used to illustrate the Saukinae, at least 290 are referred to the wrong strati- graphic horizon. Rejection of trinomial classifica- tion. — The authors' proliferation of species is in part a result of the fact that they discriminated, according to their criteria, down to the variety or subspecies level and then usually omitted the middle or species name. This is borne out by such printed statements as the following: However, in general we are opposed to trinomial designations, [p. 206.] It is of little concern to us whether these forms are regarded as "species" or "varieties," or "mutations" or "hybrids." [p. 170.] Establishment of species on inade- quate material. — That many species were set up by the writers on a single fragmentary cranidium, pygidium, or free cheek is readily evident from even a casual inspection of the plates. Revision of the Family DlKELOCEPHALIDAE Inasmuch as the family Dikelo- cephalidae, as conceived by Miller (1889), Walcott (1914), and Ulrich and Resser (1930), is revealed by unpublished studies of the writer to be polyphyletic, it is proposed that Ulrich and Resser 's (1930) Saukinae be raised to a separate family, the Saukidae. Biostratigraphic studies show that the Saukinae arise from the mid-Franconian Ptychaspidae, which are descended in turn from \ early Franconian conaspid trilobites. : The latter appear to arise directly • from Old World Parabolina, present ji Croixan Dikcloct phalids 141 in the Upper Mississippi Valley sec- tion in the base of the Conaspis zone. The Dikelocephalinae (including the Osceolinae), on the other hand, ap- pear to have descended through the late mid-Franconian Briscoia, from the earlier Franconian genus Wil- bernia. The writer also questions the need for the subfamily Osceo- linae, comprising the genera Wal- cottaspis and Osceolia. Walcottaspis appears to be merely a descendant of Dikelocephalus, whereas Osceolia is an early Trempealeauan, probably terminal offshoot of Briscoia. Revision of the ' ' Dikelocephalinae ' ' Ulrich and Resser's, 1930, Dikelo- cephalinae, comprising 36 species of the genera Dikelocephalus, Briscoia, Osceolia, and Walcottaspis, are re- duced by the writer to a status of 12 species, as indicated in the follow- ing presentation, in ascending bio- stratigraphic order : Fraxconia Hudson member-Prosaukia curvico- stata faunal unit Briscoia schucherti U & R Hudson member-Briscoia sinclairensis faunal unit Briscoia sinclairensis Walcott Note: referred by Ulrich and Resser (1930— p. 59) to the "Lower Ozark- ian, Devils Lake Formation," but actually occurring late in the Pro- saukia subzone of the Franconia, Hudson member. Bad Axe member-Saukiella minor faunal unit Dikelocephalus postrectus U & R •' Trempealeatt Arcadia member-Osceoh'a osceola faunal unit Osceolia osceola (Hall) Synonyms: 0. obsoleta U & R, 0. arguta U & R, 0. lodensis reflexa U & R, 0. praecipta U & R Dikelocephalus thwaitesi U & R Synonyms: D. weidmani U & R, D. halli U & R, D. inaequalis U & R, pars (pi. 19, fig. 6) Note: D. thwaitesi of pi. 21, figs. 7-10 is D. oweni U & R. Lodi member-.s'«»?L-m suhrecta faunal unit Dikelocephalus (oweni var.?) bar- retti U & R Synonyms: D. brevis U & R (except pi. 14, fig. 2), D. edwardsi U & R, D. subplanus pars (pi. 14, figs. 3, 5), "D. cf. gracilis and retrorus" (pi. 14, fig. 9) D. norwalkensis U & R (except pi. 21, figs. 19, 20) Lodi member-Saukia suhlonga faunal unit Dikelocephalus oweni U & R Synonyms: D. gracilis U & R, D. ovatus U & R, D. ivisconsinensis U & R, D. subplanus U & R, D. retror- sus U & R, D. bcani U & R, D. raaschi U & R (pi. 10, fig. 1 may be a distinct variety), D. brevis pars (pi. 9, fig. 5?), D. marginatus pars (pi. 15, figs. 6, 7), D. thwaitesi pars (pi. 21, figs. 7 to 10). D. norwalk- ensis pars (pi. 21, figs. 19, 20) There is some doubt as to the proper reference of the specimen, fig. 6, pi. 9, designated as D. gracilis by Ulrich and Resser. Note: There is admittedly consid- erable variation evident among the various parts of the dorsal shields of Dikelocephalus occurring in the Lodi member. Much of this seems to be a matter of the age or stage of the molt, which doubtless most of the specimens represent. On this basis the Saukia sublonga zonal unit material seems to group itself as follows: 1. Small pustulose forms, with palpebral lobes close to parallel with longitudinal axis, and relatively pos- terior in position. Pygidia with five pairs of equal pleural ribs and long slender postlateral spines. 2. Medium-sized forms, generally not pustulose, with palpebral lobes intermediate between those of groups 1 and 3 in orientation and position. Pygidia averaging four pairs of subequal pleural ribs, with more triangular spines. Illinois Academy of Science Transactions 3. Large forms with palpebral lobes relatively far forward and ori- ented to anterior convergence. Py- gidia averaging 3% pairs of pleural ribs, alternatingly wide and narrow; pygidial spines short and basally broad. The Dikelocephalus of the under- lying Sa)ikiu subrecta unit is not greatly different from the 8. sub- lon'ga species except that the more juvenile characters of slender spines and of pustulation seem to persist in individuals of medium size. Lodi raemlwv- Sun kin lodensis faunal unit Dikelocephalus minnesotensis Owen Synonyms: D. Hotchkissi U & R, D. intermedins U & R, D. granosus U & R, D. wiltonensis U & R, "D. cf. orbicularis" U & R (pi. 17, fig. 1), D. brevis pars (pi. 14, fig. 2). Prob- ably also 1). orbiculatus U & R, and Briscoia? sp. (pi. 17, fig. 10) Note: As in the immediately an- cestral D. oweni group of forms, young individuals are granulose (cf. D. granosus U & R, above). The D. minnesotensis group differs from the D. oweni group particularly in pos- sessing a more expanded frontal limb and a narrower and more ellip- tical pygidium with subequal ribs even in the largest specimens. Jordan member-0 alvinella wisconsin- ensis faunal unit Dikelocephalus margi iiatus U & R (except pi. 15, figs. 6 and 7) Synonym: D. declivis U & R Jordan member-Calvinella pustulosa faunal unit Dikelocephalus marginatus U & R (as above) Dikelocephalus inaequalis U & R (except pi. 19, fig. 6) Note: D. juvenalis U & R of this zone is not recognized, being most probably the very young of the above. Jordan member — Saukiella pepinensis faunal unit Dikelocephalus marginatus U & R Jordan membev-Walcottaspis van- hornei faunal unit Walcottaspis vanhornei (Walcott). Note: The true position of this spe- cies in the Trempealeau faunal suc- cession was pointed out to the writer by W. C. Bell and R. Berg in con- nection with its occurrence near Reno, Minnesota. Dikelocephalid fragments occur- ring higher, in the Madison, may represent Walcottaspis rather than Dikelocephalus, but generically iden- tifiable material has not yet been obtained. Revision of the Saukinae Simple synonymic cross references do not suffice, in the case of the Saukinae revision, to indicate pro- posed revised nomenclature. This is because many of Ulrich and Resser's (1933) species not only appear to be synonyms, but the illustrated ma- terial which they figure as one spe- cies from one stratigraphic horizon commonly must be distributed among several species and several stratigraphic horizons. Accordingly, in many cases it has been necessary to make reassignments, both tax- onomic and stratigraphic, on the basis of individual plate figures. Stratigraphic references cited below are those of the writer, not of Ulrich and Resser. Plate 24. Figs. 1-9. Prosaukia misa (Hall) =P. misa (Hall) Franconia formation, Prosaukia sub- zone, misa — longicornis zonal unit (misa facies). Figs. 10-13. Prosaukia resupinata TJ & R=p. misa (Hall) Same occurrence as above. Plate 25 Figs. 1-7. Prosaukia curvicostata U & R Figs. 8-12. Prosaukia alternata U & R = p - curv Figs. 13-16. Prosaukia \- costata transversa U & R U & R Figs. IT-l&.Prosaukia demissa U & R Fig. 19. Prosaukia subconica U & R Franconia formation, Prosaukia sub- zone, curvicostata zonal unit. Croixan Dil-cloccph til ids Fig. 20. Saukiella transitu U & R= 8. conica U & R Franconia formation, Saukiella Min- or zonal unit, Prairie du Sac, Wis. (Not "Gibraltar Rock, Wis.") Plate 26. Fig. 1. Prosaukia concava U & R= P. misa U & R Franconia formation, Prosaukia sub- zone, misa — longicornis zonal unit (misa facies). Figs. 2-8. Prosaukia subrecta U & R Figs. 9-12. Prosaukia subaequalis U & R ' Franconia formation, Prosaukia sub- zone, curvicostata zonal unit. Figs. 13-17. Prosaukia clelecostata U & R=P. delecostata U & R Franconia formation, Prosaukia sub- zone, misa — longicornis zonal unit. Plate 27. Figs. 1-2. Prosaukia longa U & R= P. longa U & R Franconia formation, Prosaukia sub- zone, Briscoia sinclairensis zonal unit. Figs. 3-9. Prosaukia halli U & R= P. halli U & R Franconia formation, Prosaukia sub- zone, P. misa — longicornis zonal unit (P. misa facies). Figs. 10-11. Prosaukia brevisulcata U & R=P. longicornis var. brevisul- cata (U & R) Franconia formation, Prosaukia sub- zone, P. misa — longicornis zonal unit (longicornis facies) Figs. 12-21. Prosaukia longicornis U & R=P. longicornis U & R Occurrence same as preceding. Figs. 22-25. Prosaukia am.pla U & R= P. ampla U & R Franconia formation, Saukiella minor zonal unit. Plate 28 Figs. 1-2,4. Prosaukia magnicornuta U & R=P. longicornis U & R Franconia formation, Prosaukia sub- zone, misa — longicornis zonal unit (longicornis facies) Fig. 3. Prosaukia magnicornuta U &R =P. Z. var. brevisulcata (U & R) Occurrence same as preceding. Fig. 5. Prosaukia tiibercitlata U & R= P. tuberculata U & R Franconia formation, Prosaukia sub- zone, misa — longicornis zonal unit (longicornis facies). Figs. 6-7. Prosaukia longula U & R= P. tuberculata U & R Franconia formation, Prosaukia sub- zone, misa — longicornis zonal unit (longicornis facies). Fig. 8. Prosaukia valida U & R=P. valida U & R Franconia formation, Prosaukia zone, zonal unit undetermined. Fig. 9. Prosaukia lodensis U & R= Saukia cf. curvata U & R Trempealeau formation, Lodi mem- ber, $. subrecta zonal unit. Figs. 10-11. Prosaukia sp. undet.= Saukia subrecta U & R Trempealeau formation, Lodi mem- ber, S. subrecta zonal unit. Figs. 12-17. Prosaukia incerta U & R= Saukiella (?) incerta (U & R) Trempealeau formation, Lodi mem- ber, 8. lodensis zonal unit (sand- stone facies). Fig. 18. Prosaukia granosa U & R= P. beani U & R Franconia formation, Hudson mem- ber. Fig. 19. Prosaukia berlinensis U & R: validity undetermined Exact stratigraphic position unde- termined. Fig. 20. Prosaukia dubia U & R=P. beani U & R Franconia formation, Prosaukia sub- zone, misa- -longicornis zonal unit (longicornis facies). Fig. 21. Prosaukia beani U & R=P. beani U & R Occurrence same as preceding. Plate 29 Figs. 1-3. Prosaukia (?) anomala U & R: not a Prosaukia, but belongs to an undescribed, non-saukid genus Franconia formation, Prosaukia sub- zone, curvicostata zonal unit. Figs. 4-6. Saukia obtusa U & TL=8. acuta U & R Trempealeau formation, Lodi mem- ber, 8. lodensis zonal unit. Fig. 7. Saukia sublonga U & R=8. sublonga U & R Trempealeau formation, Lodi mem- ber, 8. sublonga zonal unit. Fig. 8. Saukia angusta U & H=S. lodensis (Whitfield) Trempealeau formation, Lodi mem- ber, 8. lodensis zonal unit. Figs. 9-10. Saukia modesta U & R= S. lodensis (Whitfield) Trempealeau formation, Lodi mem- ber, 8. lodensis zonal unit. Illinois Academy of Science Transactions Fig. 11. Saukia rudis hybrida U & R cf. 8. acuta U & R Trempealeau formation, Lodi mem- ber, S. lodensis zonal unit (sand- Figs. 12-13. Saukia whitfleldi U & R= 8. lodensis (Whitfield) Trempealeau formation, Lodi mem- ber, 8. lodensis zonal unit. Fig. 14. 8a itl- hi whit Heidi U & R=S. acuta U & R Occurrence same as preceding. Figs. 16-17. Saukia acuta U & R=S. acuta U & R Occurrence same as preceding. Fig. 18. San kid cf. irhitfleldi U & R= 8. subrecta U & R Trempealeau formation, Lodi mem- ber, 8. subrecta zonal unit. Fig. 19. Saukia subrecta U & R=£f. subrecta U & R Occurrence same as preceding. Fig. 20. Saukia subrecta U & R=un- determined trilobite, cf. Eurekia Occurrence same as preceding. Plate 30 Figs. 1-2. Saukia nitida U & R=8. subrecta U & R Trempealeau formation, Lodi mem- ber, 8. subrecta zonal unit. Fig. 3. Saukia ornata U & R lodensis (Whitfield) Trempealeau formation, Lodi m ber, S. lodensis zonal unit. Fig. 4. Saukia ornata U & R=unde- termined trilobite cf. Eurekia. Trempealeau formation, Lodi mem- ber. Fig. 5. Saukia curvata U & R=S'. cur- vata U & R Trempealeau formation, Lodi mem- ber, 8. subrecta zonal unit. Fig. 6. Saukia laerigevata U & R= S. lodensis (Whitfield) Trempealeau formation, Lodi mem- ber, 8. lodensis zonal unit. Fig. 7. Saukia subgranosa U & R=S. lodensis (Whitfield) Trempealeau formation, Lodi mem- ber, S. lodensis zonal unit. Figs. 8-10. Saukia separatoidea U & R =8. subrecta U & R Trempealeau formation, Lodi mem- ber, 8. subrecta zonal unit. Figs. 11-12. Sdiikia tumida U & R: not recognized; material inadequate. Trempealeau formation, Lodi mem- ber, S. lodensis zonal unit (sand- stone facies). Figs. 13-16. Saukia retusa U & R=£. acuta U & R Occurrence same as preceding. Figs. 17-25. Saukia rudis U & R=S. lodensis (Whitfield) Occurrence same as preceding. Fig. 26. Saukia parva U & R=S. acuta U & R Trempealeau formation; exact zonal occurrence not known. Plate 31. Figs. 1-5. Prosaukia acclivis U & R= P. halli var. acclivis (U & R) Franconia formation, Prosaukia sub- zone, misa — longicornis zonal unit (misa facies). Figs. 6-8. Prosaukia (?) ambigua U & R=undescribed genus aff. Taenice- plialus Franconia formation, Prosaukia sub- zone, misa — longicornis zonal unit (misa facies, late stage). Figs. 9-10a. Saukia prima U & R= Prosaukia (?) dilata U & R Stratigraphic occurrence uncertain. Fig. 11. Saukia gran.ilineata U & R: not recognized Trempealeau formation, Lodi mem- ber, Saukia subrecta zonal unit. Figs. 12-13. Prosaukia minuscula U & R=P. halli var. acclivis (U & R) Franconia formation, Prosaukia sub- zone, misa — longicornis zonal unit (misa facies, late stage). Figs. 14-20. Saukia separata IT & R= S. lodensis (Whitfield) Trempealeau formation, Lodi mem- ber, 8. lodensis zonal unit (sand- stone facies). Figs. 21-25. Saukia imperatrix U & R =8. imperatrix U & R Trempealeau formation, exact strati- graphic occurrence undetermined. Fig. 26. Saukia lodensis (Whitfield): relation undetermined; preservation inadequate Trempealeau formation, Lodi mem- ber, S. sublonga zonal unit. Fig. 27. Saukia lodensis (Whitfield ) = 8. lodensis (Whitfield) Trempealeau formation, Lodi mem- ber, 8. lodensis zonal unit. Figs. 28-31. Prosaukia dilata U & R== P. dilata U & R Stratigraphic occurrence uncertain. Plates 32 and 33. All specimens figured are conspecific with Saukiella pepinensis Owen and accordingly the following names may be considered synonyms: Saukiella typicalis Saukiella typicalis convexa Saukiella typicalis subrecta Saukiella subgracilis Croixan Dikelocephalids Saukiella subgraeilis hybrida Saukiella subgraeilis parallela Saukiella ampla The occurrence is not from the Lodi shale, as the authors state, but from the Saukiella pepinensis zonal unit of the overlying Jordan member of the Trempealeau formation. Plate 34 Saukiella pyrene ( Walcott )=&att- kiella pyrene (Walcott) Trempealeau formation, Arcadia member, Osceolia osceola zonal unit. Plate 35. Figs. 1-8. Saukiella pyrene (Walcott) =S. pyrene (Walcott) Trempealeau formation, Arcadia member, Osceolia osceola zonal unit. Fig. 9. Saukiella cf. pyrene=S. pyrene (Walcott) Trempealeau formation, Lodi mem- ber, sandstone facies. Fig. 10. Saukiella cf. pyrene=S. minor U& R Franconia formation, Bad Axe mem- ber, Saukiella minor zonal unit. Fig. 11. Saukiella cf. pyrene=S. in- denta U & R Trempealeau formation, Lodi mem- ber, Sdukia lodensis zonal unit (sandstone facies). Figs. 12-14. Saukiella pyrene limbata U & R=S. indenta U & R Trempealeau formation, Lodi mem- ber, Saukia sublonga zonal unit (sandstone facies). Figs. 15-21. Saukiella signata U & R= S. pyrene (Walcott) Trempealeau formation, Arcadia member, Osceolia osceola zonal unit. Fig. 22. Saukiella frontalis U & R= Saukiella frontalis U & R Trempealeau formation, Jordan member, Norwalk sandstone, Sau- kiella frontalis zonal unit. Figs. 23-25. Saukiella indenta U & R =S. indenta U & R Trempealeau formation, Lodi mem- ber, Saukia sublonga zonal unit (sandstone facies). Figs. 26-30. Saukiella indenta U & R ■=Saukiella frontalis U & R Trempealeau formation, Jordan member, Norwalk sandstone, Sau- kiella frontalis zonal unit. Plate 36. Figs. 1-3. Saukiella indenta U & R= S. indenta U & R Trempealeau formation, Lodi mem- ber, Saukia lodensis zonal unit (sandstone facies). Fig. 4. Saukiella indenta intermedia U & R=S. indenta U & R Trempealeau formation, Lodi mem- ber, sandstone facies. Figs. 5-11. Saukiella noriralkensis U & R=Saukiella pyrene (Walcott) Trempealeau formation, Arcadia member, Osceolia osceola zonal unit. Figs. 12-14. Saukiella norivalkensis U & R=S. indenta U & R Trempealeau formation, Lodi mem- ber, S. sublonga zonal unit (sand- stone facies). Figs. 15-25. Saukiella norivalkensis U & R=S. pyrene (Walcott) Trempealeau formation, Arcadia member, Osceolia osceola zonal unit. Figs. 26-27. Saukiella norivalkensis U & R—8. indenta U & R Trempealeau formation, Lodi mem- ber, S. sublonga zonal unit (sand- stone facies). Figs. 28-30. Saukiella simplex U & R= S. minor U & R Franconia formation, Bad Axe mem- ber, Saukiella mi nor zonal unit. Figs. 31-32. Saukiella (?) weidmani U & R—Tellerinn .' leucosia (Wal- cott) Trempealeau formation, Ar* cadia member, Osceolia osceola zonal unit. Plate 37. Figs. 1-5. Saukiella conica U & R= S. minor U & R Franconia formation, Bad Axe mem- ber, Saukiella minor zonal unit. Figs. 6-17. Saukiella minor U & R= 8. minor U & R Same occurrence as preceding. Figs. 18-29. Calvinella spiniger (Hall) =C. spiniger (Hall) Trempealeau formation, Jordan member, C. spiniger zonal unit. Figs. 30-35. Tellerina granistriata U & R=T. granistriata U & R Trempealeau formation, Lodi mem- ber, exact zonal position undeter- mined. Plate 38. All specimens figured are conspecific with Calvinella spiniger (Hall), and, coming all from the same stratum at the same locality, represent the range of variation of a single species popula- tion. Accordingly, the following names, appearing on the explanation of plate 38, may be considered as synonyms: Calvinella spiniger altimuralis U & R Calvinella spiniger communis U & R Calicinella spiniger communis muta- tion U & R Illinois Academy of Science Transactions Calvinella spiniger postlevata U & R Calvinella clivula U & R Calvinella spiniger (Hall) ? (of U & R) Occurrence: Trempealeau forma- tion, Jordan member, Calvinella spiniger zonal unit, which the writer believes to lie at the base of the Jordan, below siltstone bearing the Calvinella wisconsinensis fauna. Plate 39. Figs. 1-10. Calvinella pustulosa U & R =Calvinella pustulosa U & R Trempealeau formation, Jordan member, C. pustulosa zonal unit. Figs. 11-16. Calvinella sparsinodata U & R=cf. Calvinella spiniger (Hall) Trempealeau formation, Jordan member, C. spiniger zonal unit. Figs. 17-18. Calvinella pustulosa ver- nonensis U & R=Calvinella pustu- losa U & R Trempealeau formation, Jordan member, C. pustulosa zonal unit. Figs. 19-24. Calvinella notata U & R= Calvinella walcotti U & R Trempealeau formation, Jordan member, zonal unit undetermined. Figs. 25-30. Calvinella lata U & R= Calvinella walcotti U & R Occurrence same as preceding. Figs. 31-34. Calvinella walcotti nor- walkensis U & H=Calvinella spin- iger (Hall) Trempealeau formation, Jordan member, C. spiniger zonal unit. Figs. 35-36. Calvinella wisconsinensis junior U & R= Calvinella spiniger (Hall) Occurrence same as preceding. Plate 40. Figs. 1-14. Calvinella walcotti U & R= C. walcotti U & R Trempealeau formation, Jordan member, exact zonal position unde- termined. Figs. 15-22. Calvinella iralcotti planu- lata U & R=C. walcotti U & R Occurrence same as above. Figs. 23-33. Calvinella wisconsinensis U & R=C. wisconsinensis U & R Trempealeau formation, Jordan member, Calvinella wisconsinensis zonal unit. Fig. 34. Calvinella walcotti ? U & R: cf. C. walcotti U & R Fig. 35. Calvinella notata ? U & R: cf. C. wisconsinensis U & R Figs. 36-39. Calvinella walcotti ? U & R=C walcotti U & R Trempealeau formation, Jordan member, exact zonal position unde- termined. Plate 41 Figs. 1-9. Tellerina crassimarginata (Whitfield) =T. crassimarginata (W.) Trempealeau formation, Lodi mem- ber, Saukia sublonga zonal unit. Plate 42. Fig. 1. Teller 'ma crassimarginata ( Whitfield ) =T. crassimarginata (W.) Occurrence same as above. Figs. 2-3. Tellerina curta U & R=T. crassimarginata ( W. ) Occurrence same as preceding. Figs. 4-5. Tellerina gothamensis U & R =T. gothamensis U & R Trempealeau formation, Lodi mem- ber, Saukia subreeta zonal unit. Fig. 6. Tellerina bigeneris U & R: cf. Saukia acuta U & R Trempealeau fromation, Lodi mem- ber, Saukia lodensis zonal unit. Fig. 7. Tellerina strigosa U & R=T. crassimarginata (W.) Trempealeau formation, Lodi mem- ber, Saukia sublonga zonal unit. Plate 43. Figs. 1-5. Tellerina strigosa U & R= T. crassimarginata (W.) Occurrence same as preceding. Fig. 6. Tellerina lata U & R: cf. T, crassimarginata (W.) Occurrence undetermined. Fig. 7. Tellerina recurva U & R=T. crassimarginata (W.) Trempealeau formation, Lodi mem- ber, Saukia sublonga zonal unit. Plate 44. All specimens figured are conspecific with Tellerina ? leucosia (Walcott), and the following names may be re- garded as synonyms: Tellerina leucosia (Walcott) Tellerina leucosia variety Tellerina leucosia parallela U & R The occurrence is not the Norwalk sandstone (a member of the Jordan formation), but the basal Trem- pealeau, Arcadia member. Plate 45. Figs. 1-10. Tellerina extrema U & R= Tellerina ? leucosia (Walcott) Occurrence same as preceding plate. ; Figs. 11-13. Tellerina norivalkensis i U & R=T. crassimarginata (W.) Croixan Dikelocephalids 147 Trempealeau formation, Lodi mem- ber, Saukia sublonga zonal unit (sandstone facies). Fig. 14. Telleriiia nonralkensis U & R: not identifiable. Figs. 15-16. Tellerina norwalkensis U & R=r. crassimarginata (W.) Trempealeau formation, Lodi mem- ber, Saukia sublonga zonal unit (sandstone facies). Stbatigraphic Note arcadia member Since the latest published account of Trempealeau stratigraphy (Raasch, 1939), an additional mem- ber of that formation has been dis- criminated. This member, for which the term Arcadia is proposed, lies beneath the basal conglomerate of the St. Lawrence member, and rests with a strong basal conglomerate upon the Franconian, Bad Axe greensands. Areally the member is evidently continuously extensive north of the Black River valley, in Wisconsin and Minnesota, but to the southward is only locally present where small remnants have escaped pre-St. Lawrence erosion. The mem- ber bears the Osceolia osceola faunal The type locality selected is a road cut and small quarry on State High- way No. 93, 1 mile south of its junc- tion with Highway No. 95, east of Arcadia, Trempealeau County, Wis- consin. Here the persistent green- sands of the Franconia, Bad Axe member, are unconformably over- lain by a spectacular 3-foot bed of edgewise conglomerate, flat pebbles of buff siltstone in a greensand matrix. This bed grades upward into 17.1 feet of somewhat lenticular | strata of varied lithology, dominant- ly dolomitic siltstone and fine, green- , ish gray to light brown, somewhat dolomitic sandstone. Osceolia osceola Hall and Dikelocephalus thwaitesi U & R were collected. The Arcadia succession is over- lain by the basal St. Lawrence con- glomerate (with dolomitic matrix and Arcadia sandstone pebbles), succeeded by 11.2 feet of sandy dolo- mite. St. Lawrence Member The fauna of the St. Lawrence member continues to be designated by the term Platycolpus, to which the specific term eatoni has been added, since the range of the genus greatly exceeds the span of the mem- ber. However, as the St. Lawrence member, which seldom exceeds 20 feet in the Wisconsin-Minnesota re- gion, attains a thickness of more than 100 feet in parts of northeastern Illinois, it may represent a complex of faunal units, in addition to the substantive one to which the term Platycolpus eatoni is properly ap- plied. Lodi Member The Lodi member is characterized faunally by a succession of at least three closely related faunal units. Of these, the middle or Saukia sub- longa faunal unit yielded most of the classic collections from the ' ' Dikelocephalus beds. ' ' The earlier Saukia subrecta faunal unit occurs in a lentil of limited thickness, as yet known only from the lower Wis- consin Valley. The Saukia lodensis faunal unit occurs in siltstone in the upper part of the Lodi member, largely in Sauk, Columbia, and Dane counties in southern Wisconsin, but also in similar lithology at Still- water, Minnesota. In the interven- ing area (in a direct line), the Lodi member passes wholly to sandstone, 148 Illinois Academy of Science Transactions Chart No. 1. — Zonal Succession of Dikelocephalid Zonal Unit Dikelocephalidae Associated Genera 14. "Madison Fauna" unconformity undescribed Tellerina, Cnlri in Hit, Dikelocephalus^) base of Madison formation / 1: -iht urn ■ii>i>us, K/ilimins/iis, Stenopilus, Plethopeltis 13. "Van Oser Fauna" undescribed Saukia, Saukiella, Tellerina Stenopilus, Euptychaspis 12. Saukiella frontalis So itkirlla frontalis U.& R 11. Walcottaspis vanhornei Walcottaspis vanhornei (Wal- cott) Stenopilus 10. Saukiella pepinensis Sankiiila pepinensis (Owen) Dikelocephalus marginatus U&R Plethometopus 9. Calvinella pustulosa unconformity Calvinella pustulosa U&R Dikelocephalus inaequalis U&R Dikelocephalus marginatus U&R Eurekia, Corbinia, Triarthropis, Entomaspis Sii iiii/>ilu:\, "Agnosias ilis- parilis" Hall O < 8. Calvinella wisconsi- ( 'nli'inillii irisn in sinensis U&R Dikelocephalus marginatus U&R Tellerina sp. nov. Prosaukia sp. nov. Eurekia, Corbinia p 02 7. Calvinella spiniger 1 unconformity Calvinella spiniger (Hall) base of Jordan member Eurekia, Corbinia 6. Saukia lodensis 2 Sun l:ia lodensis (Whitfield) Saukia acuta U&R Tellerina crassimarginata (Whitfield) Sniikirlla ? indenta U&R Saukiella ? incerta U&R Dikelocepha lus minnesotensis Illaenurus, Eurekia, Corbinia, Plethometopus, Triarthropsis, Entomaspis, '•'ii/ii .' SuuLiiila indenta U&R Tellerina crassimarginata (Whitfield) Dikelocephalus oweni U&R Illaenurus, Acheilops, Euptychaspis, "Agnostus disparilis" Hall, Plethometopus, Corbinia, Entomaspis, Eurekia, Triarthropsis, Stenopilus 4. Saukia subrecta U&R base of Lodi member Saukia subrecta U&R Saukia curvata U&R Tellerina gothamensis U&R Dikelocephalus oweni var. barretti (U & R) Illaenurus 3. Platycolpus eatoni (Whitfield) unconformity undescribed Tellerina, Dikelocephalus base of St. Lawrence member Illaenurus, Eurekia, Plethometopus, Stenopilus, Corbinia, Platycopus Croixan Dikelocephalids 2. Osceolia osceola Osceolia osceola (Hall) Illaenurus, Eurekia, / ')il;eloei ■phalus Hi innli si 1 A K < 'orlniiia, Triarllrropsis, Saukiella pi/rene (Walcott) Euptychaspis Tellerina ? leiicosm (Walcott) base of Arcadia member, o unconformity Trempealeau formation < 1. Saukiella minorV&H San kid la minor U & R Illaenurus, Mimocheilus M Prosaukia ampla U & R P Dikelorephatus pos/rcel us UifcR <»j Dikelocephalus sp. nov. Briscoia sp. nov. Calvinella sp. nov. base of Bad Axe member, unconformity? Franconia formation 4. Briscoia sinclairensis Briscoia sinclairensis Platycolpus Prosaukia longa U & R 3. Briscoia (unnamed) Briscoia sp. nov. Platycolpus, Prosaukia near Zonf/a U & R Monocheilus 2. Prosaukia curvicostata Prosaukia curi'icostata U & R Briscoia schucherti U & R 3 1. Prosaukia longicornis Prosaukia misa (Hall) 3 Dartonaspis* p Prosaukia halli U & R 3 Cliarioeephalus 3 Prosaukia halli a eel iris V it k' Idahoia o Prosaukia delecostata U & R 4 Ellipsncephaloides 3 « Prosaukia longicornis U & R Ptychaspis* Pmsaukia I. brevisulcata* Wilbernia 3 Prosaukia lubereulata U & R 4 Litagnostus 3 Briscoia sp. nov. 4 Ptychaspis subzone Wilbernia, ancestral to Dikelocephalus Moiiorheilus, PsalasptS, Ptychaspis, ancestral to lila hoi a, Kill psoce phuloitles, Prosaukia Lilai/iiosius, Pseuilagnostus base of Hudson member, local unconformity Franconia formation A. central and soi Greensand facies ; Non-greensand fac s faunal facies. 150 Illinois Academy of Science Transactions in which somewhat different faunal facies of both the Saukia suolonga and Saukia lodensis faunal units have tentatively been identified. Jordan Member In western Wisconsin, conglome- ratic and dolomitic strata marking the base of the Jordan member over- lie the Lodi siltstone. The limited thickness of basal Jordan strata, in- cluding intercalated dolomitic shales, contains a closely packed suc- cession of faunal units, each char- acterized by a distinct species of Calvinella. Thus there appears to have prevailed here, as in the ease of the lithologically rather similar St. Lawrence member, a condition where factors favoring deposition were closely balanced by those favor- ing limited submarine erosion and/or nondeposition. Faunal units thus tend to be closely packed ver- tically and in lenses in areal dis- tribution. Along the Mississippi, the basal Jordan Calvinella beds are succeed- ed by siltstone strata similar litho- logically to those of the Lodi mem- ber. These strata enclose fossils belonging to the Saukiella pepin- ensis faunal unit, and where the beds attain their greatest thickness, in Dakota County, Minnesota, the suc- ceeding Walcottaspis vanhornei fauna as well. The siltstone grades eastward into unfossiliferous sand- stone. The main body of the Jordan member above and lateral to the siltstones and conglomeratic dolo- mites just described consists of fine, friable, more or less dolomitic sand- stones which Ulrich (1924) em- braced in his term Norwalk. The higher Norwalk beds are sparingly fossiliferous and best characterized by Saukiella frontalis U & R. The Jordan member was deeply eroded previous to the deposition of the Madison formation in central and southern Wisconsin; but along the Mississippi and in the lower Minnesota Valley the member is essentially its original thickness, with terminal sands ("Van Oser Beds," Stauffer, 1940) interpreted as a regressive phase of Jordan deposition (Raasch, 1939). Stauffer (1940) figures, but does not de- scribe, a fauna from these beds. Sunset Point Formation At most places, south of a line through central Trempealeau County, an independent depositional cycle, dominantly sand with varied proportions of dolomite, intervenes between the Jordan member of the Trempealeau and the Ordovician Oneota formation. To this unit, the term Madison has been applied (ref. Raasch, 1935). Because pre-Madi- son erosion deeply truncated the Jordan member in central and southern Wisconsin, the Madison merits the status of an independent formation. However, the applica- tion of the term Madison to these strata has created much confusion, owing especially to the widespread use of the same name for a Missis- sippian limestone unit in Montana and Wyoming. The writer therefore proposes that, for the Cambrian unit, the term Sunset Point be sub- stituted, after the bluff of that name at the Madison sandstone type lo- cality. The unit is locally fossiliferous, but the fauna has not been specifi- cally described. Croixan Dikelocephalids 151 The Saukia Zone Because many trilobite genera range through much of the Trem- pealeau and in fact back into the Bad Axe member of the Franconia and into the Madison (or Sunset Point) formation, this inclusive suc- cession of faunal units is considered to represent a single faunal zone. The saukid genera, Saukia, Sauki- ella, Tellerina, and Calvinella, i.e. Saukia in the sense of Walcott's (1914) use of the term, together characterize this succession, but the later subdivision and restriction of the term Saukia by Ulrich and Ees- ser leaves no single genus whose range coincides completely with the full time and rock span. Accordingly the writer proposes to apply the term Saukia to the succession in- volved, using that term in the broad- er sense of Walcott. Thus the bio- chron of the Saukinae coincides with the time span of the Saukia zone, except for the genus Prosaukia, which is ancestral to the other mem- bers of the subfamily and charac- terizes the Prosaukia subzone of the preceding Ptychaspis-Prosaukia zone. REFERENCES Miller, S. A., North American geology and paleontology, 1889. Owen, D. D., Report of a geological sur- vey of Wisconsin, Iowa, and Minne- sota . . ., 1852. Raasch, G. O., Stratigraphy of the Cam- brian system of the Upper Mississippi Valley, Guidebook Ninth Annual Field Conference, Kansas Geol. Soc, 1935, pp. 302-315. , Cambrian Merostomata, Geol. Soc. Amer. Special Paper 19, 1939. Stattffer, C. R., Fauna of the Van Oser beds, Jour. Paleontology, vol. 14, no. 1, 1940, pp. 54-56. ulrich, E. 0., and C. E. Ressek, The Cambrian of the Upper Mississippi Valley; Part I, Trilobita, Dikeloce- phalinae and Osceolinae, Milwaukee Public Museum Bull., vol. 12, no. 1, 1930. Ulrich, E. 0., and C. E. Resseb, idem.; Part II, Trilobita, Saukinae. Milwau- kee Public Museum Bull., vol. 12, no. 2, 1933.