Th isbook has been D!G1T17 E d ^ and « available ONLINE THE UNIVERSITY OF ILLINOIS LIBRARY GEOLOGnC NOTICE: Return or renew all Library Materials! The Minimum Fee for each Lost Book is $50.00. The person charging this material is responsible for its return to the library from which it was withdrawn on or before the Latest Date stamped below. Theft, mutilation, and underlining of books are reasons for discipli- nary action and may result in dismissal from the University. To renew call Telephone Center, 333-8400 UNIVERSITY OF ILLINOIS LIBRARY AT URBANA-CHAMPAIGN L161 — O-1096 Digitized by the Internet Archive in 2014 https://archive.org/details/sandstonesofwisc2519thwa Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate I. B. Up-curve! in beds of the Chequamegon sandstone, east side of Outer Island. These phenomena may be . Situated in sec. 23, T. 48, R. 5 W. at elevation of about 750 ft. A. T. Samples sent by courtesy of U. S. Geological Sur- vey. Sample No. Thickness Depth Pleistocene: Feet Feet 1 Grayish-red, sandy, non-calcareous clay 70 2 Very calcareous reddish-gray "hard-pan" or sandy cemented till 20 90 Chequamegon sandstone: 3 Soft medium grained light reddish quartz sandstone ■. 18 4 The same, slightly darker and harder 27 5 Pinkish white coarser and subangular grained quartz sandstone 25 6 Soft ; finer grained reddish quartz sandstone 20 7 Soft, very coarse gritty or conglomeritic red sandstone mixed with some finer sandstone 20 ; BAYFIELD GROUP. 35 8 Soft, nearly pure quartz sandstone, grains subangular and of medium size 30 9 Sample missing. Said to be same as last... 15 10 Soft fine to medium grained pinkish quartz sandstone 15 11 Much the same as the last but with somewhat more rounded grains 20 12 Nearly pure white sandstone, otherwise same as last 20 13 Sample missing. Said to be soft red sandstone 45 14 Sample missing. Said to be white conglom- erate with much water 30 375 Depth of well 375 375 At the quarry just north of the city very similar rock is seen, (see Fig. 2, p. 29). Between the thicker beds occur thin-bedded and shaley partings which sometimes show mud cracks. These exposures show a dip of about one degree to the south. Only comparatively small exposures occur on Madeline Island, the best being on the northernmost point. These beds show a darker color, together with red shaley beds, and pebbly layers. They appear to be a portion of the quarry beds. The now virtually idle quarries on Houghton Point north of Washburn are in heavily-bedded brownstone. 1 Between the heavy layers is frequently found a foot or more of thin-bedded or softer sandstone, while lenses of red micaceous shale often occur. At other places the bedding planes are marked by coarse yellowish grits or layers of pebbles. Clay pockets also occur. Plate III, A, p. 26. shows a thin section from this locality. Pike's quarry south of Salmo shows the greatest vertical ex- tent of the formation so far as known in any single exposure. The following section, from top down, shows the downward grada- tion of the quarry beds (numbers 7 to 9) to the less regularly bedded and variable colored layers of the main beds : i Buckley, E. R., Building and Ornamental Stones of Wisconsin, Wis. Geol. & Nat. Hist. Surv., Bull. IV, 1898, pp. 187-192. 36 BAYFIELD GROUP. SECTION AT PIKE'S QUARKY, SEC. 33, T. 50, R. 4 W. Thickness 9 Weathered and broken brownstone. 23 feet 8 Tnin bedded and cross bedded red shaley sandstone with yellow streaks '. 6 7 Heavily bedded brownstone with very few clay pockets, the best quarry rock 33 6 Unexposed below level of railway track 15 5 Heavily bedded brownstone in lake cliff, the top much broken up 35 4 Thin bedded, red, shaley sandstone 8 3 Heavy layer of brownstone 8 2 Gray to deep red, yellow, and white sandstone, varies from shaley to pebbly with much cross bedding, the coarser phases having the lighter colors 12 1 Heavily bedded coarse pebbly brown sandstone 20 Total thickness, about 150 feet Tm mediately south of the docks at Bayfield is a good exposure of brown sandstone in the cliffs. The usual thin bedded and sandy shale partings occur between the beds, and in . one place is found a small lense of pure clay shale, a foot or two in thick- ness. This is of varying shades from deep red to gray and green, and grades into the usual sandy shale; the associated yellow sandstone layers are also seen. On the south end of Basswood Island is an abandoned quarry which shows about 25 feet of brownstone in heavy layers. 1 The bedding planes are curved both up and down from the normal. A shale layer is found between the brownstone beds in the quarry and the same material is said to have been reached by boring not far below the bottom of the quarry. Pebbles and clay pock- ets are abundant. Plate III, B, p. 26, shows a thin section of this rock. A small quarry was once operated on the northeast corner of Hermit or Wilson Island. 2 The neighboring shore cliff shows thin bedded and cross bedded yellow and white mottled layers be- 1 Ibid., pp. 178-182. 2 Ibid., p. 185. BAYFIELD GROUP. 37 tween the heavy beds of brownstone. A thin section shows fairly well rounded grains averaging nearly 0.4 mm in diameter. About a fourth is altered feldspar including much microcline, although orthoclase is predominant. Stockton Island shows extensive outcrops on its eastern end. There the waves from the open lake have worn some striking caves and deep coves along the joints. The sandstone is some- what redder in color than the quarry beds. A few shaley layers are seen but most of the bedding is heavy. Some long partings dipping gently to the south resemble huge cross bedding, but are probably inclined joints. Cross bedding is common. At the southeastern end of the exposures occurs hard, cross bedded, white sandstone. Similar rock is also seen on Presque Isle Point. Near the southwest corner of Stockton Island is a large aban- doned quarry which shows a thickness of 73 feet of brownstone in the main opening. 1 Above this another excavation discloses rock which is mottled with white. The upper layers all show clay pockets. The partings are often shaley and show cross bedding. They pinch out rapidly, never having any great hor- izontal extent. Sometimes they end on downward pitches of the bedding, but in other places the layers bend up and are cut off by the heavy layer above as cross bedded layers are. Outer Island exhibits a fine exposure along almost the en- tire length of its eastern side. The sandstone is much varied in color, ranging from the usual brownstone to pure white. The changes are very rapid and often take place along joints. In general the coarse grained cross bedded layers are lightest in tint. Plate I. B, frontispiece, represents a curve in the bed- ding seen near the southern end of the island ; others occur but are not so distinct. Some interesting exposures occur on the east side of Oak Is- land. At the northeastern corner, heavy beds of brownstone with interbedded dark shaley layers are seen. Coarse yellow grits also occur. Toward the south, lenses of thin bedded yel- low and white banded sandstone appear, and also some hard white and brown coarse pebbly layers. 2 Ibid., p. 183. 38 BAYFIELD GROUP. On Point Detour and the adjacent headlands 1 occur beds of brown sandstone with the usual shaley partings. The depth to which weathering* here extends is unusual, being often 20 feet or more. North Twin Island shows strata near the bottom of the Che- quamegon formation. "White to brownish heavy cross bedded layers with occasional large ripple marks occur. Bear Island shows on its eastern coast a very good section. The southeasterly dip is here more pronounced than it is to the southeast, so that some 400 feet of the stratigraphic thick- ness is apparently seen. However, the layers pinch out and vary so rapidly that an exact determination is impossible. As we go north we descend to successively older and older strata. At the south, brown sandstone with irregular cross bedding and lenses of thin bedded yellow and pink sandstone occur. But towards the north end the color becomes gradually lighter until hard cross bedded white beds take their place. These basal beds show a few ripple marks. At some points wide ter- races of rock have been washed clean by the storms so that an excellent opportunity is given to study the curious forms of the curved cress bedded layers (see p. 30). On York Island, at the northwestern cape, is a fine exposure of the basal layers in a clean wave-swept terrace. White and reddish sandstone, with grit and conglomerate, occur as well as small lenses of thin bedded red and pink layers. At the south end of Devils Island, which is entirely surrounded by a rock cliff, the light colored basal layers of the Chequamegon sandstone are seen. There are interbedded layers similar to the main body of underlying Devils Island sandstone, so that no very sharp line of demarkation can be drawn. Devils Island Sandstone. General Character and Extent. The formation of the Bay- field group, which conformably underlies the Chequamegon sandstone, is called the Devils Island standstone, from the ex- cellent exposures on the island of that name. It is distinguished i Owen, D. D., Chippewa Land District, p. 54; Geol. Survey of Wis., ■etc., pp. 268, 305. BAYFIELD GROUP. 39 from the other formations of this group by its thin bedding, well rounded, medium sized quartz grains, which are almost free from any coating of iron oxide, by ripple marks, and by its pink and white color. Banding and mottling of the colors is frequent. A few iron concretions occur. No shale beds, feldspar grains, clay pockets or pebbles are known in this formation. The form- ation is much more porous and friable than the others of the group. For that reason more striking and larger sea caves have been w r orn in the Devils Island sandstone than in any other hor- izon. The Devils Island sandstone outcrops along a narrow belt stretching for twenty-five miles southwest from the island from which it takes its name. Fine exposures occur in the cliffs of Sand Island and Squaw Bay (Plates V, B, XXIII, A, p. 94). 1 Lesser ones are seen on Siskowit River and Lost Creek near Cornucopia. Twenty-five miles to the southwest an isolated ex- posure on Brule River corresponds so closely in character, that there is little doubt that it is to be placed in this formation. According to the writer's interpretation of the structure of the district, this formation must lie immediately beneath the drift south of Washburn, but no exposures of it are there known. It may pinch out or otherwise disappear in the interval. Thickness. The thickness of the Devils Island sandstone cannot be directly measured, but a computation based upon the width of outcrop and average dip places the figure at about 300 feet. Ripple Marks. The only portion of the Bavfield group which shows ripplemarks in any abundance, is the Devils Is- land sandstone and some of the beds immediately above and below. The accompanying diagrams show the most common types. Most have symmetrical ridges, little if any sharper than the intervening troughs. The other type has one side of the ridge much steeper than the other, as shown in Fig. 4, c, and Fig. 5, p. 40. Few were observed which show the subordinate ridge in the hollow T , such as is often seen in modern ripple marks formed on the lake bottom (Fig. 4, b). This is, however, a i Owen, D. D., Chippewa Land District, p. 55; Geol. Survey of Wis., etc., p. 268. 40 BAYFIELD GROUP. feature which is readily overlooked. The ripple marks occur both on cross and normal bedding planes, and where marked by a cement of iron along the bedding the effect upon the outcrop is very striking (Fig. 5). In size they average one to two inches from crest to crest, but occasionally much larger ones are seen. A B C D Fig. 4. Types of ripple marks. It is believed that the symmetrical ripplemarks are the result of wave action in standing water; and that the unsymmetrical form (Fig. 4, c) was formed by currents, either of water or air. Cr^ss Bedding. The common cross bedding of the Devils Island formation is more regular than much of that in the other members of the Bayfield group. The curious irregular curved forms there noted are seldom seen. The direction of Fig. 5. Ripple marks marked by iron bandinjr in Devils Island sandstone (Specimen li.058). One-half size. dip of the layers is usually to the south or southwest but there are some exceptions, although on the whole it is conspicuous for being in a direction opposite to that prevailing in the other formations. This fact is thought to mean that the material of the Devils Island sandstone was brought from a different direc- tion. Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate V, B. Sea caves in Devils Island sandstone, near Squaw Bay, Bayfield County BAYFIELD GROUP. 41 O R I E NT A S A NDSTO N E General Character and Extent. Underlying the Devils Is land sandstone occurs the lowest formation of the Bayfield group, the Orienta sandstone, so called from its exposure in the quar- ries near the village of that name. In its upper portion this formation resemhles the Chequamegon sandstone very closely, so that the possibility of repetition of that formation by faulting was considered. It is seen, however, that in the vicinity of Cor- nucopia tire uppermost beds of the Orienta pass beneath the Devils Island sandstone, although the actual contact is not ex- posed so far as known. Moreover, there is no repetition, so far as could be discovered, of the Devils Island formation to the west. In its lower portion, and indeed to some extent through- out, the Orienta contains more feldspar, mica and magnetite grains than the younger formations, the relative amount in- creasing towards the bottom, as do beds of red shale. The Orienta sandstone underlies the Western Plain, extend- ing from the line of outcrop of the Devils Island formation, all along the south coast north of the Douglas Range and into Min- nesota. Folding brings it up near Ashland, where rock be- lieved to be a part of it outcrops on the South Fork of Fish Creek, and it is found beneath the thick drift in deep wells. Thickness. As with the Chequamegon, there is no direct means of measuring the thickness of the Orienta sandstone. A computation based on a uniform dip of but one degree places the thickness on Brule River at about 1000 feet. This section is not good enough to be correlated with that shown on Middle River, where the formation is turned up by the thrust fault of the Douglas Range, exposing some 2,700 feet. As the Devils Is- land formation does not occur there, it appears certain that the Middle River section is all Orienta sandstone, thus placing the total thickness of the formation at 3,000 to 3.500 feet; 2,495 feet of this formation is penetrated by the deep well at Ashland (see p. 65), and 1,110 feet is exposed on Fish Creek. General Section. The following section shows the subdivis- ions of the Orienta sandstone from the top down. In the future it is possible that some parts may be distinguished as separate formations : 42 BAYFIELD GROUP. Approximate Thickness Upper beds. Mainly white and red sandstone with a few ripple marks 300-400 feet Upper brownstone (of Port Wing) 500-700 Copper Creek beds. White and pink fine grained sandstone with magnetite and mica grains and a few ripple marks 75-100 Main beds. Brown, red, and white sandstones, becoming progressively more felspathic towards the bottom, with thin beds of red shale 1800-2000 Total thickness, about 2700-3200 feet LOCAL DETAILS Aside from the small exposure believed to belong to this forma- tion, on Fish Creek near Ashland Junction, which will be de- scribed in Chapter V, the easternmost exposure of the Orienta Sandstone is on Sand Island. Here and on Eagle Island 1 and Squaw Point (Plate V, A, p. 40) to the west, occurs white cross- bedded, ripple-marked sandstone much resembling the basal beds of the Chequamegon formation. A few layers of red sandstone occur and there is much banding of the colors. A considerable amount of decomposed feldspar is seen among the sand grains. To the west, on Siskowit Point, the downward gradation of this type to the brownstone beds may be seen. Lenses of red shale also appear (Fig. 1, p. 29). A small quarry formerly operated here produced red and white sandstone. Similar sand- stone with red, shaley, and yellow gritty layers occurs on Bark Point, across a bay by that name to the w^est. Just west of the little settlement at Herbster is a bold rocky point projecting out from the sandy coast cliffs. 2 Here the sandstone is pebbly and cross bedded. Plate IV, B, p. 30, shows an interesting trough in the bedding seen here. A quarry was once started at this locality. The quarries at Port Wing are still operated at times and produce some of the best brownstone in the region. This rock is h'eavify bedded, free from clay pockets, has few pebbles, and 1 Irving, R. D., Geol. of Wis., 1873-9, vol. Ill, p. 208. 2 Owen, D. D., Geol. Surv. of Wis., etc., 1852, p. 269. BAYFIELD GROUP. 43 is of a somewhat redder and more cheerful tint than the brown- stone of the Chequamegon sandstone. In the quarry some peb- bles were observed along the bedding planes, together with black magnetitic layers, and a little white sandstone. 1 Plate VI, A, p. 44, shows a thin section of this rock. About a fourth is feld- spar, mainly orthoclase, but including some microcline and plagio- clase. The locality is notable for the unusual fact that the rock is sound clear up to the top and shows glacial striae. Another good quarry was formerly operated on the east bank of Iron River near Orienta, in the western part of Bayfield County. 2 The southeasterly dip of the formation, combined with the fall of the river, brings one to successively higher and higher strata in ascending the stream. "We pass from the heav- ily bedded brownstone up to thin bedded white and red banded rock. Near the road crossing, occur white and yellow heavier beds with thin layers of green shale. 3 West of the mouth of Iron River no exposures occur on the 'Coast. Only two small outcrops occur on Brule River below those of the Devils Island sandstone. On Poplar River are good outcrops for several miles above the mouth, but as the dip here is generally less, no great thickness can be observed. The rock is thought to be part of the upper brownstone. Much is thin ^bedded and grades to coarse yellow sandstone. The magnificent section seen on Middle River shows the lower portion of the formation and will be described in detail in Chapter V. West of Middle River, exposures are much less satisfactory. 'The Copper Creek and main beds are the only ones found. On Amnicon River for about three-fourths of a mile below the falls, at the contact with the trap, occur good sandstone exposures. The dip is southeasterly, except close to the trap, and some 275 feet of beds is thus exposed. Of these the lower 200 feet is mainly heavily-bedded brownstone, which once was quarried to 1 Buckley, E. R., op. cited, pp. 201-205. 2 Ibid., p. 209. s Owing to bad weather the survey of Iron River was not completed. No exposures were seen or reported where we would expect to find the 'Devils Island sandstone '.see map). Sweet, E. T., Geol. of Wis., vol. Ill, 1873 9, p. 322. 44 BAYFIELD GROUP . a considerable depth. 1 Associated white and yellow pebbly phases and magnetitic bands occur. These features serve to distinguish the beds from the upper brownstone of the Port Wing quarries, although the possibility of lateral change in the formation must be taken into account. Above these layers oc- curs a gradual transition to the Copper Creek beds. These are mainly thin bedded pink or red and white fine grained sand- stone. Much cross bedding, sometimes curiously curved, occurs (Fig. 3, p. 30). No ripple marks were observed, and this fact, taken in connection with fine grain, magnetite grains, and small thickness lead the writer to regard this as a p a rt of the Orienta sandstone rather than a lateral change in the Devils Island forma- tion. It is also simpler on structural grounds to assume this correlation, for it is entirely possible that these beds occur un- der one of the unexposed intervals in the Middle River section to the east. Similar beds appear to occur at two small exposures which were not visited by the writer. One of these is in Sec. 30, T. 48, R. 10 W. and was not discovered by any of Prof. Grant's party. 2 Another outcrop in Sec. 7, T. 47, R, 13 AY., was exam- ined by one of Prof. Grant's assistants and a sample preserved, which shows the rock to be very like that at the falls of the Amnicon River. It is a pebbly reddish and white sandstone. On Copper Creek next to the contact with the traps, a small thickness of thin-bedded, ripple-marked, red and white spotted sandstone is exposed, which in many respects resembles the De- vils Island sandstone. It is, however, much darker in color and is therefore thought to belong to the same horizon as the rocks described above. Farther to the west, below the lower falls of Black River (Plate XIX, p. 80) occurs red and white spotted irregularly colored sandstone varying much in bedding and coarseness of grain. The dip may be sufficient to expose as much as 200 feet of strata, showing a somewhat similar scccession as at Amnicon River (see p. 43) . No true brownstone was observed and there is a com- parative abundance of magnetite, micas, and ferromagnesian 1 Buckley, E. R., op. cited, p. 208. 2 Sweet, E. T., Geol. of Wis., 1873-9, vol. Ill, p. 348. Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate VI. Photomicrographs in Ordinary Light, Magnified 12 Diameters. (Buckley, Bull. IV, Plate LXIV). A ^ B A. Orienta sandstone (specimen 4717) from Port Wing. The section shows more feldspar (mainly orthoclase, but including some microcline and plagioclase), iron oxide, and ferromagnesian minerals, than those from the Chequamegon sand- stone, although not notably different from Plate III, B, p. 26. The cement in this specimen is partly iron. B. Orienta sandstone (specimen 4718) from quarry on St. Louis River, in bed 1 of section on p. 70. Nearly one-fourth of the rock is feldspar, mainly ortho- clase, but including as usual some microcline and plagioclase. The cement is in part argillaceous, being composed of the alteration products of the feldspars. BAYF1EFLD GROUP. 45 minerals. Ripple marks occur but rarely and cross bedding is not very abundant, but many clay pockets and pebbles were observed. Some of the thicker beds are of yellowish to white quartz sand- stone. One such was observed to fill a hollow in the thin bedded red layers. The latter type of rock appears to be more abun- dant in the upper part of the section but often occurs as lenses in the heavier bedded sandstones. In Sec. 29, T. 47, R. 14 W., occur several small exposures of yellow and white fine grained hard sandstone, very much' like some of the beds exposed on Copper Creek. A well at Foxboro in See. 6, T. 46, R. 15 W., almost on the west line of the state, shows the same kind of rock, 1 as do some small exposures near Holyoke visited by Professor Grout. 2 A small exposure of sandstone is said to exist on the Nemadji River in the NE14 Sec, 27, T. 48, R. 14 W., but was not visited by the writer. 3 The exposures of the Orienta sandstone on St. Louis River are described on p. 69. Econamic Products. The only product of economic value derived from the Bayfield group is building stone. 4 This is obtained from the most ferruginous phase of the sandstone which is commpnly known as "brownstone." Apparently this rock is found only in certain more or less well defined horizons. It will be noted that the quarries, especially those in the Apostle Islands^ are arranged in a nearly straight line along the strike of the formation. Therefore but one horizon of good quarry rock is believed to exist in the Chequamegon formation, although good stone was seen at many scattered points. Less is known about the Orienta sandstone. The best brownstone is that found at Port Wing and Orienta, but it is most probable that the brownstone quarry on Amnicon River is at a lower horizon. 1 Drillings sent by C. N. Edin, Dedham, Wis. 2 Personal communication. s Sweet, E. T., Geol. of Wis., 1873-9, vol. Ill, p. 318. 4 A full description of the quarries, including a list of buildings, constructed with Lake Superior stone, is found in Bull. IV, Wis. Geol.. and Nat. Hist. Survey, Building and Ornamental Stones of Wisconsin,, by E. R. Buckley, 1898, pp. 167-219. 46 BAYFIELD GROUP. The quarry at Fond du Lac does not produce a true brownstone, the rock being firmer, more felspathic, and irregularly colored red and white. Condition of the Quarry business in 1910. When the writer visited the district, no quarry happened to be in operation. The Port Wing quarries, which alone are operated on an extensive scale, closed down about the first of July. A very little stone had recently been quarried at Houghton. The Fond du Lac quarry is operated whenever the height of water in the river permits shipments, but is not equipped to furnish anything but rubble and irregular blocks. Causes of Decline. The causes of the almost complete aban- donment of this once apparently flourishing industry do not lie wholly in the character of the stone itself. 1 A few decades ago brownstone was a very popular building material in the eastern part of this country, so that when a similar stone was discovered in Wisconsin it was at once exploited. Between 1868, when the first quarry was opened on Basswood Island, 2 and 1893 the bus- iness continued to grow. A few years before 1893, a boom took place at the head of the lakes. As in all other business enter- prises, speculators then rushed into the brownstone industry. Stone, often of inferior quality, was put out at ruinously low prices. The bad results from the use of this low grade material naturally reflected upon the reputation of the good. Then came the inevitable panic, resulting from the unnatural expansion of business. The demand for stone decreased greatly and never again reached its former extent. > In the meantime, the brownstone buildings of the east had be- gun to look shabby and a cheerful light stone was demanded. This was supplied by the Bedford limestone, which is nearer the best markets and possesses the great advantage of softness. Tools dull much more rapidly in cutting sandstone, a fact wit- nessed by the extensive blacksmith shops of the Lake Superior -quarries. Before the Lake Superior companies could recover from the results of their folly before the panic, this light stone had captured the market. By 1897 very little stone was being 1 Buckley, E. R., op. cited, p. 211. 2 Irving, R. D., Geol. of Wis., 1873-9,* vol. Ill, p. 209. BAYFIELD GROUP. 47 taken out and today only two quarries are producing first class building stone. Plants representing an investment between two and three million dollars are now idle ; some have been burned j in most the machinery is rusted and useless and the buildings are falling to pieces. At several quarries large piles of cut blocks ready to ship remain unsold. 1 Perhaps now that business at the head of the lakes is developing upon a more sane and whole- some basis than formerly, quarrying will be resumed; for the stone has a useful field for buildings in smoky cities, as it does not show dirt like the lighter toned materials now in fashion. i In 1912 some of these were being shipped. 48 OEONTO GROUP. CHAPTER IV THE ORONTO GROUP AND OLDER FORMA TIONS, AND THE EXTENSION OF THE SANDSTONE GROUPS IN MINNESOTA Nomenclature. Beneath the Bayfield sandstone group de- scribed in the last chapter, is a great thickness of arkose sand- stone and shales, nearly all red in color, and for the most part occupying a highly tilted position wherever exposed. On ac- count of these differences, these rocks were separated by the former Geological Survey of Wisconsin from the overlying hor- izontal quartzose sandstone (see p. 16). At its base this ark- ose group is interbedded with* the traps, thus showing that it is a part of the Keweenawan series; but the bottom of this up- per division of the series is assumed to be the base of a thick conglomerate called the Outer Conglomerate, above which hor- izon no igneous rocks occur. With the exception of the basal portion, this group has not heretofore been divided into formations. Professors Lane and Seaman have recently applied the name Freda to the sandstone immediately overlying the black shale beds (Nonesuch forma- tion) just above the Outer Conglomerate. 1 As this formation (sometimes known as the W T estern sandstone of Michigan] is only a small portion of the entire thickness of sediments below the Bayfield group which have not yet been studied in i Lane, A. C, and Seaman, A. E., Geological Section of Michigan, Rept. of State Board of Geol. Survey, 1908, p. 23; Jour, of Geology, vol. 15, 1907, p. 36. ORONTO GROUP. 49 detail, it seems best to give a new name to the entire group and retain the name Freda for this formation. It will therefore be termed the Oronto Group, from the excellent exposures near the bay of that name in the northern portion of Iron County. The limits of the group may be fixed at the bottom of the Outer Conglomerate as a base, and the top of the highest thick beds of red shales and well-marked arkose sandstones as the top. General Character. The Oronto Group is composed of con- glomerate, sandstone, and shale; the last probably predomin- ating, although not often exposed. In color it is usually some shade of red, but a characteristic feature is banding, streaking, and spotting with greenish white. The white beds are seldom more than a few inches thick. Often the light color also extends along the joints. In the material of the component grains it differs mainly from the Bayfield group in having a greater pro- portion of undecomposed minerals, feldspars, micas, and ferro- magnesian compounds, as well as more mag^tite and calcium car- bonate. Quartz grains are, however, present in considerable quan- tity ; but are never, so far as known, predominant throughout any considerable thickness of strata. The shale is usually sandy and micaceous, but there are considerable beds of fine clay shale. Nearly always the shale beds are more brightly colored than the coarser grained strata, Areal Extent. In areal extent the Oronto Group underlies all of the Eastern Plain (see p. 12), between Ashland and the Southern Trap Highland, but it is exposed at but few points. It is probable that only the harder beds are exposed, since these would form the ridges now buried beneath the drift, which is usually so thick that only a few very deep wells enter the rock. It here forms a series of open folds (see p. 92). Isolated areas of rocks believed to belong to the Oronto Group occur also on Middle and St. Louis rivers in Douglas County, and rock of the same type is penetrated by deep wells in Superior. The Oronto Group is believed to extend far into Minnesota, for rocks of the same type have been found by drilling to lie beneath the Paleozoic sediments. Thickness. The thickness of the Oronto Group can only be determined by computations based upon discontinuous exposures. The sum of the stratigraphic thicknesses actually exposed is 4 50 OROXTO GROUP. nearly 10,000 feet, but evidently the total must be much greater. On Montreal River it is possible that a thrust fault, dipping to the north, might cause a repetition; but no direct evidence in- dicating its existence was discovered. 1 Computation places the thickness of the Freda sandstone between the top of the None- such shales and Lake Superior at 12,600 feet. Should an in- itial dip of even 10 degrees be subtracted, (which is much too great), the result would be 11,700 feet. On Fish Creek to the west, where there is every probability that the strata are thinner, since they thin rapidly in that direction at all lower horizons, 2 a thickness of fully 8,000 feet is indicated. There the jointing and shearing (see p. 93) do not indicate that the thickness is expanded by faulting. Since the rocks of this locality are be- lieved to lie at a horizon above those of Montreal river, a total thickness of the Oronto Group of some 21,000 feet is indicated. This figure agrees with other computations based upon tire dip, and appears to be conservative — provided lateral variations in the character of the strata along the strike, and concealed fault- ing have not confused the true sequence to a greater extent than now seems probable. Subdivisions. The Oronto Group may tentatively be sub- divided into the following formations; the basis of the division is in part structural but is mainly lithologic, and therefore rather unsatisfactory, owing to the possibility of lateral variation as mentioned above : Thickness Amnicon Formation. Red and greenish shales, arkose sandstone, and some conglomerate (1050 feet exposed) 5,000 feet Eileen Sandstone. Red and white somewhat quartz- ose sandstone (1800 feet exposed) 2.000 Freda Sandstone. Fine-grained red and greenish arkcse sandstone with a little conglomerate at top and bottom (about 8000 feet exposed) 12.000 1 Lane, A. C, and Seaman, A. E„ Geol. Section of Mich., Report of State Board of Geol. Survey of Mich., 1908, pp. 33, 38. Personal communications. 2 Irving, R. D., Geol. of Wis., 1873-9, vol. Ill, p. 12. Copper Bearing Rocks, Mon. I T . S. G. S., vol. V, pp. 153-230. n O RON TO GROUP. 51 Nonesuch Formation. Black, gray and red arkppe and shale (maximum) 350 Outer Conglomerate, coarse conglomerate (maxi- mum) 1.200 Total maximum thickness, about 21,550 feet Composition. The rocks of the Oronto Group are mainly composed of angular to subangular fragments, derived from igneous rocks without much chemical decomposition. The only marked exceptions are some of the red clay shales. At the base in the Nonesuch formation, occur many layers of black sand- stone composed of nearly unaltered fragments of trap, so thoroughly cemented with calcite that they are readily mistaken for igneous rocks. Higher in the group there appears a greater proportion of debris from quartzose rocks. In the conglomer- ates there is little difference in the source of the pebbles from those found in the Bayfield group. They comprise traps of many varieties (porphyries probably predominating), quart- zite, iron formation, chert, vein quartz, slate, and granite. In the coarse arkose grits, fragments of igneous rocks of both basic and acid varieties are plainly seen, but these rocks were not studied in detail. In most of the^roup the grain is so fine that the microscope is needed to determine the material; and as no detailed study was made, information is available for only a few varieties of rock. This will be found under the head of local details, page 54. The component grains are mainly feldspars (both orthoclase and more basic varieties, the latter being often too much altered to determine), quartz, mica, magnetite, ferro- Tnagnesian minerals and fragments of fine-grained igneous rocks. On the whole, the grains are less rounded than those of the Bay- field group and although extremely variable, appear to average considerably smaller in size. Often a somewhat clayey reddish or greenish matrix is seen, perhaps derived from these fine- grained porphyries, from the decomposition of feldspar, or some- times mainly composed of iron oxide. The cement is often caL cium carbonate, but enlargements of the feldspar and quartz grains are probably the dominant cement. Red oxide of iron is also quite abundant. fa 52 ORONTO GROUP. An interesting rock which was examined is the greenish cal- careous sandy shale of the Amnicon formation, found on Middle River (see p. 66, Plate VII, p. 50, and Plate XVI, A, p. 72). In the thin section, calcium carbonate was found to separate the very fine angular quartz grains, but on analysis this proved to be less than 20% of the rock. These ripple-marked and mud-crack- ed shales are interbedded with and merge into red noncalcareous shales and sandstone. Red spots also occur within the greenish rock and greenish within the red. It cannot be determined whether or not the calcium carbonate is original or has been introduced into the rock by underground waters from the adja- cent traps. The following chemical analyses are the only ones available Tor Wisconsin ; unfortunately they mainly represent the None- such formation, which is wholly composed of basic debris; none are typical of the great body of the group : ANALYSES OF SANDSTONE OF THE OKONTO GROUP. 1 * NONESUCH FORMATION o' 53 O < • Q CaO O da s % rs s* ea o Total J Locality. Bad River 54.50 49.14 55.91 51.98 Bad River FREDA SANDSTONE 73.24 68.91 White River Falls AMNICON FORMATION Bad River Falls 69.78 72.14 15.4B 7.93 .49 1.17 2.64 2.42 99.86 10.71 .12 8.55* * Calculated. i Geol. of Wis., 1873-9, vol. Ill, p. 203. Last by Victor Lehner, 1911. Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate VIII. Eileen Formation, South Fork of Fish' Creek, Sec. 19, T. 47, R. 5 YY., Town op Eileen, Bayfield County. A. Vertical beds of red and white streakel feldspathic sandstone, showing horizontal joints (see Plate X, B, p. 56). The strike is nearly east and west. B. Vertical beds of red and white sandstone and sandy red shale. Looking southwest. OR ON TO GROUP. 53 Bedding. The bedding of the Oronto Group varies from shaley to layers several feet in thickness (Plates VII, p. 50 ; VIII, p. 52, XI, p. 58; XVII, A, p. 74) ; but in general it is thinner and more regular than that of the Bayfield sandstones. How- ever, the difference may in part be accounted for by the fact that the exposures of this group are smaller; and being of steep- ly-inclined layers, they expose to view less of the horizontal var- iability of the beds. A very striking feature of much of the group is tire extremely rapid variations from very coarse grit or conglomerate to fine shale. Shale is very often interbedded with conglomerate through a considerable thickness of strata, the sharpness of contacts and scarcity of sandstone being remark- able (see pp. 62, 106). Cross-bedding is abundant, but us- ually small; no prevailing direction of dip was discerned. Curved beds are often met with, some being shown in Figs. 6, lfoot Fig. 6. Magnetite sand bed in Eileen sandstone, Fish Creek, Bayfield County. and 7, p. 55. Channels in the top of shale beds, filled with sandstone, are very common ; an example is given in Fig. 9, p. 69. Ripple marks are very abundant, being mainly of the symmet- rical type and rather small in size (see Fig. 4, p. 40). These occur in the finer grained rocks, especially shales, the surfaces being often greenish or red shale, or decidedly ferruginous sand- stone. In some localities mud cracks are extremely abundant; but they are readily overlooked, for they do not show on weathered surfaces. This remark also applies to rain prints, which were observed at but one point. Frequently none of these features are of such character that they may be utilized to de- termine the top of the strata. In the case of mud cracks, the filling often adheres to the nearest hard sandy layer, whether it is above or below the original surface. When developed in typical form, cross-bedding is the most reliable guide to the structure (see Fig. 8, p. 63 and Plate XIc, B, p. 64). Clay pockets and scattered pebbles are not common. Most of the former are of the small flattened type and grade into small lenses of shale. Veins of calcite are abundant. 54 ORONTO GROUP. LOCAL DETAILS Amnicon Formation. The top of the Amnicon formation may be observed on Fish Creek, near Ashland, and on Middle and St. Louis rivers in Douglas County. These interesting ex- posures and their correlation will be described in the succeeding chapter. Aside from these the highest known beds are those seen at the lower falls of Bad River, in sec. 25, T. 47, R. 3 W. About 750 feet of greenish and red coarse arkose sandstone, with in- terbedded red shales, are there exposed. 1 Microscopic examina- tion shows that the rock is composed mainly of small fragments of porphyry, although the debris of basic rocks is also seen. This is confirmed by the chemical analysis on p. 52. In some beds considerable quartz is seen. The section is probably near the base of the formation. Eileen Sandstone. In sees. 20 and 21, T. 47, R, 5 W., Town of Eileen, Bayfield County, the valley of the South Fork of Fish Creek exposes about 1800 feet of sandstone in vertical beds. The bedding varies from heavy to shaley with a little clay shale as shown in plate VIII, p. 52. The colors range from brick red, or occasionally even deeper shades, to clear white and show much mottling and banding of red and white. Beds composed almost exclusively of magnetite are also seen. Some are as much as eighteen inches in thickness but have only a slight lateral extent. Ripple marks, cross-bedded and irregularly bedded or domed beds occur, as shown in Figs. 6 and 7. Pebbles are not very com- mon. Microscopic examination (Plate X, B, p. 56) shows that only a third of the rock is quartz. Feldspars (mainly ortho- clase with some microcline and plagioclase, more or less altered), magnetite, mica, iron oxides, and rarely altered ferromagnesian minerals make up the larger portion of the rock. Trie grams are decidedly angular, as is usual in all the sandstones of the region. Were it not for the structural relations, however (see i Trving, R. D., Geol. of Wis., 1873-9, vol. Ill, pp. 14, 202; Copper- Bearing Rocks, Mon. U. S. G. S., vol. V, p. 132. Irving states that 2000 feet of strata were then seen, but the writer was able to find but the one exposure, about three-eighths of a mile long. No map of the river is accurate. O RON TO GROUP. 55 section, p. 63, and structure sections on map) these rocks might be regarded as tire lower part of the Orienta formation. On the west side of Clinton (locally known as Marble, or" sometimes Graveyard) Point, are low ledges dipping at a moder- ate angle to the northwest, exposing some 1,300 feet of fine- grained and somewhat shaley to heavily bedded sandstone (Plate IX, p. 54). In general character, it most resembles the last described beds and so is correlated with them. It is, how- ever, not greatly different from the rocks exposed on Bad River (see p. 54). Ripple marks, cross-bedding and contorted ,4 feet Fig. 7. Bedding in Eileen sandstone, Fish Creek, Bayfield County. bedding, as well as indistinct mud cracks are observed. A thin section (Plate X, A, p. 56) from' a representative phase shows not over a quarter of the rock to be quartz; the re- mainder is feldspar, about half orthoclase and half plagioclase, as well as ferromagnesian minerals and micas. 1 The minerals show considerable secondary enlargement. The cement is quartz, feldspar, and calcite. These rocks were formerly correlated erro- neously with the Bayfield Group (see pp. 15, 18). At Mason, in eastern Bayfield County, the well at the mill of the White River Lumber Co. penetrates, beneath 145 feet of drift, 650 feet of sandstone with occasional beds of red sandy shale, presumably belonging to the Eileen formation. Freda Sandstone. Below the dam across White River, in sec. 6, T. 46, R. 4 W., is an exposure of some 300 feet of strata, as shown in Plate XI, B, p. 58. This rock consists of coarse, red and greenish arkose alternating in rather thin beds, with red i Irving, R. D., Geol. of Wis., 1873-9, vol. Ill, pp. 16, 209. Copper- Bearing Rocks, Mon. U. S. G. S., vol. V, p. 366. Lane, A. C, and Seaman, A. E., Geol. Section of Michigan, Report State Board of Geol Survey of Mich., 1908, p. 33. 56 ORONTO GROUP. and green micaceous shales. Some poorly developed ripple marks and mud cracks were observed. Lithologically, this sec- tion might be assigned either to the horizon of the Bad Eiver rocks, or to the top of the Amnicon formation, as seen on Fish Creek five miles to the north (see p. 62) • but on structural grounds (see section E-P on general map) it is simpler to as- sume that it lies at the top of the Ffeda sandstone, in the gap between the Oronto Bay and Fish Creek sections (see p. 50) 1 In the valleys of the streams that flow into Oronto Bay, in Iron County, occur numerous excellent exposures of the Freda sandstone (Plate XI, A, p. 58). The prevailing rock is red, fine grained arkose. Some thin greenish beds occur. There are also numerous beds, usually rather thin, of markedly micaceous red shale. Toward the base of the formation, the grain becomes coarser, and there is found a 100-foot layer of conglomerate. The exposures represent a thickness of over 12,000 feet, but they are separated into two groups, with an unexposed interval of some 4,000 to 5,000 feet of stratigraphic thickness. As as- sumed in the case of the other interval of no outcrops, it is most probable that the lack of exposures is due to the softness of the rock (see p. 21). Microscopic examination 2 shows that less than a fourth of the rock is quartz, the bulk being feldspars and fragments of porphyries and basic eruptives with reddish de- composition products, micas, ferromagnesian minerals, and mag- netite. The cement is usually red oxide of iron, 3 but in some cases it is calcium carbonate, and to some extent quartz. The quartzes and feldspars show secondary enlargements. The Freda sandstone is also exposed on Potato Kiver, within the area of the map, but this locality was not visited by the writer. 4 Nonesuch Formation. Exposures of the Nonesuch forma- tion of the Oronto group occur on the Montreal, Oronto. ami 1 Irving, R. D., Geol. of Wis., 1873-9, vol. Ill, pp. 14, 202. 2 Irving, R. D., and Chamberlin, T. C, Observations on the Junction between the Eastern Sandstone and the Keweenaw series on Keweenaw Point, Lake Superior, IT S. G. S. Bull. 23, 1885, p. 82. s Irving, R. D., Geol. of Wis., 1873-9, vol. Ill, pp. 12, 199. Copper "Bearing Rocks, Mon. IT. S. G. S., Vol. V, pp. 133, 226. 4 Irving, R. D., Geol. of Wis., 1873-9, vol. Ill, pp. 188, 202. Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate X. Photomicrographs in Ordinary Light, Magnified 12 Diameters. A. Fine-grained arkose sandstone from Clinton Point, Iron County (specimen 11.124). The grains run from .1 to .3 mm. in diameter. About 25 per cent are quartz. The remainder is about half orthoclase, the' other half being micro- cline, plagioclase, ferromagnesian minerals, mica, and iron oxide. Calcite is in some places seen forming a cement. Most of the cementation is, however, due to enlargements of the quartz and feldspar grains. This section should be compared with those of the Bayfield group shown in Plates III, p. 26, and VI, p. 44, the' rock having formerly been placed in that group. B. Medium-grained red sandstone from Eileen formation on South Fork of Fish Creek, Bayfield County (specimen 11,369). This rock differs mainly from A, in the size of grains, which average nearly .3 mm. About a third of the rock is quartz, the remainder being mainly orthoclase. Some plagiioclase, micas, and ferromagnesian minerals are found. The large' white areas are holes in the section. ORONTO GROUP. 57 Potato rivers. This member is composed of red, gray and black arkose and shale, often with calcareous cement. Microscopic ex- amination shows that the rocks are composed of virtually un- altered debris of basic eruptives. The thickness on Montreal River is about 350 feet, lessening rapidly to the west. 1 Outer Conglomerate. Beneath the Nonesuch shale is round the great Outer Conglomerate, which is 1,200 feet thick on Mon- treal River, but thins to only 800 feet on the Potato. On the former stream, it outcrops in magnificent cliffs over 200 feet in height. The conglomerate is composed of pebbles averaging four or five inches in diameter — some measure fifteen inches or more. They are from the diabase, porphyry, gabbro, and other rocks of the Middle Keweenawan series, with subordin- ate, amounts of Huronian quartzite, iron formation, slate, and chert, as well as granite and vein quartz, probably derived mainly from the Archean. There is practically no matrix of sand, and the bedding is only apparent where there are thin layers of coarse arkose sandstone. As in the higher beds, cal- eite veins are common. - OLDER FORMATIONS The Traps. Beneath the base of the Outer Conglomerate is found a great series of interbedded traps, conglomerate, sand- stone, and shale. This reaches a thickness of some 1,200 feet on the Montreal, but to the southwest thins out to nothing within a few miles. Below this occur the traps of the Middle Keween- awan, including both extrusive and intrusive rocks. These rocks also occur in the Douglas Range. 3 The Pre-Keweenawan Recks. Still farther to the southeast, the Huronian sediments are found in a position stratigraphically below the eastern traps; and then in turn the older Archean granites, schists, and greenstones (see p. 7). 1 Irving, R. D., Geol. of Wis., 1873-9, vol. Ill, p. 200. Copper Bearing Rocks, Mon. U. S. G. S., vol. V, 1883, 'pp. 133-226. 2 Irving, R. D., Geol. of Wis., 1873 9, vol. Ill, pp. 12, 202. s In S. E. % of N. E. \i sec. 19, T. 47, R. 4 W., near Ashland, a huge mass of trap 6u feet across rises from the clay plain. As no rock is found in neighboring wells, the writer believes that this is simply an enormous boulder, and not an outcrop as believed by some. 58 ORONTO GROUP. Economic Products. So tar as known, no products of econ- omic value are found in the Orcnto group. In contrast to the earlier sedimentary series of the Lake Superior region, it con- tains no iron formation. Thin lenses of nearly pure magnetite occur in places, especially along Fish Creek ; but they are hardly greater in amount than the magnetitic sands of the modern lake beaches, or the drift. Most of the rock, with the possible exception of parts of the Eileen sandstone on Fish Creek is entirely unsuited for building stone. Occasionally, however, some of the red shale has been used for paint rock. Traces of copper and silver have been found in the Nonesuch formation, 1 but no concentration of these minerals or the abundant ferric oxide is known to have taken place in Wisconsin. The rocks of the Oronto group carry little water and such little as is found is often salty (see pp. 66, 73, 102). The Extension of the Sandstone Groups into Minnesota. The Bayfield and underlying Oronto groups of sandstones ex- tend into Minnesota, as shown by Hall 2 , between the continua- tion of the Douglas Trap Range to the southeast and the Huron- ian and Archean rocks to the northwest. The drift is so thick along this narrow strip, however, that the actual continuity with the Paleozoic sediments to the south is somewhat doubtful. 3 Nevertheless, a brief discussion of this question is not out of place. Most of the. rock exposed in this belt is a weli indurated, rather fine to medium grained, quartz sandstone pinkish to yel- low or gray in color. This, the rock of the well known quarries of the Kettle River region, is much like the sandstones found near the falls of Black River in Douglas County 4 (see p. 4.1 . 1 Irving, R. D., Geol. of Wis., 1873-9, vol. Ill, p. 206. 2 Hall, C. W., Keweenawan Area of Eastern Minn., Bull. Geol. Soc America, vol. XII, 1908. p. 313. 3 Grout, F. F., personal communications. 4 Grout, F. F., personal communications. Weidman, S., personal communications. It may not be too great a stretch of the imagination to suggest that these light colored rocks may be equivalent to the Devils Island sand- stone of the Apostle Islands. Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate XI. A. Red and greenish arkose sandstone and shale of Freda sandstone, on east bank at mouth of Oronto River, Iron County. B. Red and green shale and arkose grit of Freda formation, below dam of White River electric plant. Sec. 6, T. 46. R. 4 W., Ashland County. The north- easterly strike' is plainly indicated by the outcropping ridges ; the dip is 25 de- grees to the southeast, away from the observer. ORONTO GROUP. 59 Still farther to the south and southeast, in the valley of the St. Croix River, appear flat-lying, yellowish or pinkish quartz sandstones, bearing fossils of Upper Cambrian age. These rocks (the Potsdam sandstone of Wisconsin) rest with pronounced un- conformity upon both the tilted traps of Keweenawan age and the older Pre-Cambrian rocks. The known Cambrian rocks are separated from those of the Kettle River district by a consider- able drift-covered interval, probably underlain largely by trap and granite, as w T ell as by the great fault of the Douglas Range which is known to extend far into Minnesota, forming the north- western border of the traps. 1 In many places deep drilling has shown that the Potsdam rests upon a great thickness of red clastic rocks. These red rocks fill a great depression in the surface of the Pre-Cambrian crystalline rocks. 2 The thickness of these red rocks varies from nearly 2,500 feet in the center of the basin down to nothing at both sides of the trough and to the south, where the Potsdam rests directly upon the Pre-Cambrian igneous and metamorphic rocks. But few wells have penetrated the entire thickness of the red clastic series near the center of the basin. At Minne- apolis they are found to rest upon granitic rock, while at Still- water, apparently at the center of the trough, Keweenawan dia- base lies beneath (see p. 60). The red series is studied with difficulty as it is not known to reach the surface. The only information is derived from churn drill records and is hence rather unsatisfactory. N. H. Win- chell 3 believed they were a conformable connection of the Cam- brian with the Keweenawan'. More recently C. W. Hall re- studied the question, reaching the same conclusion (see p. 20), 4 "Phe red clastic series consists of red shale and sandstone while some fragmental volcanic rocks are thought to exist. The sand" 1 Hall, C. W., Keweenawan Area of Eastern Minn., Bull. Geol. Soc. America, vol. XII, 1901, p. 313. 2 Hall, C. W., Meinzer, O. E. and Fuller, M. L., Geology and Under- ground Waters of Southern Minnesota, U. S. G. S. Water Supply Paper 256, 1911, pp. 32, 48. s Winchell, N. H., Geol. of Minn., vol. I, 1882, pp. 422, 424, 537. 4 Hall, C. W., The Red Sandstone Series of S. E. Minn. Unpublished manuscript not available. Abstract, Science, vol. 27, 1908, p. 722. 60 ORONTO GROUP. stone is mainly composed of quartz grains, but much of it con- tains more or less feldspar and is very frequently calcareous. It is not always red, for occasionally white sand is reported. The following record of the well at Stillwater gives a good idea of the lithologic character of the series : LOG OF STILL WELL DEEP WELT 1 Pleistocene Thickness Glacial drift 18 feet • Cambrian Mendota limestone 85 Potsdam sandstone; calcareous sandstone of gray to yellowish color and some gray shale 592 Upper Eeweenawan Dark red, sandy calcareous shale 13 Coarse quartz sandstone 5 Fine dark red calcareous shale 11 Fine dark red calcareous sandstone with some calcite and pink feldspar grains 2233 2262 Middle Eeweenawan Diabase 450 Depth- of Well 3447 feet In general, these rocks are exactly like those of the Upper Keweenawan series of the Lake Superior basin, whose continu- ation they must be ; while beyond doubt the basin in which they lie is part of the Lake Superior syncline which passes up Che- quamegon Bay. and thence southeasterly across Wisconsin (see p. 88) . The portion in southeastern Minnesota is probably every- where separated by the traps of the north side of the basin, from the belt northwest of the fault in which lies the lighter colored i Hall, C. W., Meinzer, 0. E. and Fuller, M. L., Geology and Under- ground Waters of Southern Minnesota, IT. S. G. S. Water Supply Paper 256, 1911, p. 366. Meads, A. D., The Stillwater Deep Weil. Bull. Minn. Acad. Nat. Sci., vol. Ill, No, 2, 1891, pp. 274-7. Winchell, N. H., Natural Gas in Minnesota. Minn. Geol. and Nat. Hist. Survey, Bull. V, 1889, p. 25. ORONTO GROUP. 61 sandstones of the Kettle River region. A connection west of the fault, through the heavily drift-covered region of Anoka County, Minnesota", is nevertheless possible. Whether or not these buried red rocks are conformable be- neath the Upper Cambrian marine sediments is a question be- yond the scope of the present report. On the whole, the ocur- rence of the red series in a basin, overlapped on both sides by the overlying horizontal sediments which rest with pronounced unconformity upon the tilted Keweenawan traps, favors the view that an unconformity also exists between the sandstones (see p. 103). Such an unconformity probably could not be recognized in the records of churn drill holes. 62 RELATION OF SANDSTONES. CHAPTER V THE RELATION OF THE BAYFIELD AND OROXTO SANDSTONE GROUPS General Statement. The study of the relation of the Oronto and Bayfield groups of sandstones was the principal object of the present investigation. Outcrops are so scarce that we can at no place trace the two sandstone groups to a point of contact where their relations may be absolutely determined. But it is possible to find exposures at points where we should expect to to find the contact of the Bayfield and Oronto groups. At all of these localities there is a conformable gradation from quartz sandstone of the general type of the Bayfield group downward into red shales, and arkose sandstone or conglom- erate of the same general type as the main body of the Oronto group. Sections showing this general stratigraphic succession are found on the south fork of Fish Creek, near Ashland, not far from the main area of the Oronto group, in the deep well at Ashland, on Middle and St. Louis rivers in Douglas County, and in deep wells in Superior. This widespread stratigraphic se- quence shows a gradation between the two general types of sand- stone formations and is believed to indicate that the Bayfield and Oronto groups are conformable. Fish Creek. Beginning at the east, there is found an expos- ure of the supposed base of the Bayfield group, in the town of Eileen, near Ashland Junction, in eastern Bayfield County. Here the valley of the South Fork of Fish Creek exposes rock in sec- tions 11, 14, and 15. T. 47, R. 5 W. These exposures have never been described previous to the present study. The beds are ver- Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate XIa. RELATION OF SAXDSTOXES. 63 tieal or inclined steeply to the north, decreasing from 90° at the south to 70° at the north end < f the section, while the strike is approximately east and west (sec p. 93).. (See Plates XI b, p. 62, and XI c, p. 64). The following section is based upon a chained Fig. 8. Diagram showing the use of cross-bedding, in determining top of vertical strata. The top is to the right. plane table survey made by the writer in 1912, as shown in Plate XI a. The thickness are therefore accurate. Although the exposures are not continuous, the rock is everywhere so near the surface that abundant fragments permit the general character of the strata to be determined. SECTION ON SOUTH FORK OF FISH CHEEK, BAYFIELD COUNTY Bayfield Group: Orienta Sandstone. Thickness Feet. 14. Heavy to thin-bedded medium grained brown quartz sandstone, often cross bedded. This rock is of the typical Bayfield type, being apparently nearly all quartz, but with some mica flakes and feldspar grains. Is pebbly in places ..... 50 13. Red and white spotted, thin to medium bedded and cross bedded quartzose sandstone. A thin section (Plate XII, p. 66) shows that three quarters of the rock is quartz, the remainder being mainly orthoclase . . . . . . . 75 12. Unexposed, but shows abundant fragments of red grit and coarse red and white sandstone . . . 150 11. Red grit in small ledge ...... 5 10. Unexposed, but shows fragments as before . . 65 64 RELATION OF SANDSTONES. 9. Red grit in place ........ 5 8. Unexposed, but shows fragments as before . . 130 7. Red grit and red and white sandstone ... 10 6. Unexposed, but shows fragments as before . . 190 5. Medium to heavily bedded coarse brownish red and white (often in spots) sandstone with red and green- ish shaley partings and some red grit. A thin section from the coarse sandstone, with minute layers and pockets of shale, shows that it is not greatly differ- ent from the higher strata. Mc'st of the feldspar is orthoclase but some microline, plagioclase, and mica are seen. Most of the section (Specimen 11401) is red shale composed of hue grains of quartz cemented by iron oxide and the decomposition products of feld- spars, etc. A thin section (Plate XII, B) from the coarse white sandstone shows that less than half the rock is quartz. The grains are angular and vary from 2. mm to below .25 mm in diameter. Mica is abundant . . . . . • • • 23 0 4. Unexposed, but shows fragments of rock like the last 100 3. Thin to medium bedded shaley to gritty red and white sandstone ........ 15 2. Unexposed, but shows fragments like the adjacent rocks • • . • 85 (Oronto Group: Amnicon Formation. 1. Coarse red and white spotted arkose grit. Conglom- erate with pebbles seldom over 3 inches in diameter of trap, vein quartz, quartzite, slate, iron formation, and chert. The beds vary from a foot to 15 feet in thickness with subordinate layers of coarse arkose sandstone and red and greenish shales. There is some cross bedding (see Plates Xlb, XIc) . . 1,050 Total thickness 2,160 An unexposed interval of nearly 5.000 feet separates this sec- tion from the outcrop of the Eileen sandstone in Sees. 20 and 21. This section is quite important, since it throws new light on the structural relations of the sandstone groups. It is but five miles from the area of rocks undoubtedly belonging to the Or- onto group. The lowermost beds of this section are as highly tilted as any of the recognzied Oronto group, and match it ex- Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate Xlb. A. General view, looking southeast, on South Fork of Fish Greek, XE y± of NE V±, Sec. 15, T. 47, R. 5 W., Bayfield County. The rocks are red conglom- erate and grit, with some red shale, and are correlated with the Amnicon formation. The dip is to the north and varies from 70° to 9-0°, while the strike averages N. 85° W. B Exposures upstream from A, looking east. The rocks are the same, hut with more layers of red shale. RELATION OF SANDSTONES. bo actly in lithologic character. The anticline of which they form part of the north limb involves at least 8,000 feet of strata. This fold must belong to the general system of Keweenawan folds (see p. 95), since there is no known subsequent period of de- formation with which it could be correlated. The correlation of the lower horizons of the Fish Creek section with the Oronto group seems therefore to be very well established. The correlation of the upper quartzose beds with the Bayfield group is much less definite. A drift-covered interval of about three and one-half miles exists between the most northerly ex- posure on Fish Creek and the outcrops of the Chequamegon sandstone on Wayman Point. This belt was carefully explored but no exposures could be discovered, and the inhabitants of the region unite in saying that none exist. The evidence on which the upper layers are correlated with the base of the Orienta sandstone depends on three factors: 1. The lithologic likeness is complete and can be better appre- ciated in the field than in a description. The bedding is also characteristic of the upper group, being frequently very heavy, but giving way rapidly to thin cross-bedded layers. 2. The dip of the beds decreases towards the north end of the section, thus indicating a probable flattening out of the beds to the north (see structure section E-F, on map). 3. The deep well at Ashland encounters a very similar suc- cession of beds to that shown in the section on Fish Creek, and indicates a pronounced flattening of the beds to the north. Ashland Well. At the plant of the Lake Superior Iron and €hemical Company in Ashland three miles to the northeast of Fish Creek, is a well 3,095 feet in depth. Although not situated in direct line between the most northerly exposure on Fish Creek and the nearest exposures on the northwest shore of Chequam- egon Bay, nevertheless if the position of the well is projected parallel to the probable strike of the rocks, it will be found to occupy a position well to the north of the Fish Creek exposures, thus serving very well to fill in the unexposed interval. If the red shales there encountered represent even approximately the horizon of those seen on Fish Creek, from which they differ only in lacking the conglomerate beds, a flattening out of the beds 5 < 66 RELATION OF SANDSTONES. toward the north is indicated (see structure section E-F). While it cannot be positively stated that any of the beds encountered in the well belong to the Bayfield group, it is nevertheless shown that the usual stratigraphic succession of sandstone grading down into red shales is here found. LOG OF "JUMBO" WELL AT ASHLAND Drilled in 1889 at works of Lake Superior Iron and Chemical Com- pany, near west y± post of Sec. 5, T. 47, R. 4 W. Elevation of curb^ 660 ft. A. T. Record from original log, furnished by Mr. J. E. John- son, Jr., manager, 1910. Thickness Depth Pleistocene: Feet Feet Red clay . . . . . . 40 Till with boulders 43 Red sand 92 175 Bayfield Group: Orienta Sandstone. Principally fine grainod sandstone of red color with few white IWs, and bands of red shale from one to ten *^et thick. A soft stratum 30 feet thick feet from the top, fur- nishes most ol the water. The flow did not increase below it. The water is said to be salty 2,495 2,670 Oron'o Group ; Amnicon Formation. Red shale 425 3,905 Depth of well 3,095 3,095 Middle River. On Middle River, in eastern Douglas County, m occurs perhaps the most interesting exposure in the entire dis- trict. Here over 3.000 feet of strata are exposed in a great over- turned fold. The structure of the locality is shown in Plate XIII, p. 66, which is based on a paced survey made by Grant in 1899. 1 (See also Plate VII, p. 50; Plate XVII, B. p. 74; and Plate XXI, 90). i Grant, U. S., Copper-Bearing Rocks of Douglas County, Wis. Geol. and Nat. Hist., Survey, Bull. VI, 1900, pp. 18, 20. Personal communications. Sweet, E. T., Geol. of Wis., 1873-9, vol. Ill, p. 347. Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate XII. Photomicrographs in Ordinary Light, Magnified 12 Diameters. A. Medium-grained feldspathic sandstone (specimen 11,404), from South Fork of Fish Creek, Sec. 11, T. 47, R. 5 W., Bayfield County. Average diameter of grains .35 mm. Seventy per cent of the rock is quartz, the remainder being feldspars (mainly orthoclase, but some microcline) and iron oxide. The cement is quartz. It should be noted that the rock does not differ greatly from those of the Bayfield group, Plates III, p. 26, and VI. p. 44. B. Coarse-grained white arkose sandstone (specimen 11,397), from a lower horizon than A. The average size of the grains is .5 mm, but the largest (of microcline) is 2.0 mm. long. Quartz makes up half of the rock ; the remainder is mainly orthoclase, but there is much plagioclase : little iron oxide is present. The coarseness and angularity of the grains, as well as the greater proportion of feldspar, should be compared with A. RELATION OF SANDSTONES. 67 SECTION ON MIDDLE RIVER, DOUGLAS COUNTY Thickness Orienta Sandstone. Feet 12. Horizontal thick-bedded light pinkish brown sand- stone; grains are of medium size and appear to be mainly quartz. The oanding with red and white (see p. 31) often gives the impression of folds. Estimated thickness ...... 225 11. Unexposed, but showing fragments of thick-bedded brown sandstone . . . . . 100 10. Thin-bedded reddish sandstone apparently contain- ing a larger proportion of feldspar. Dips 35° N. 50 9. Coarse white sandstone with pebbles of trap and quartz; some is very calcareous (Spec. 11243) Dips 60° N . . 50 8 Unexposed, probably contains the Copper Creek beds 300 7. Medium to coarse grained pebbly, brown sandstone; beds very heavy and hard to make out, but appear to be vertical Shows iron banding. It may cor- respond to the brownstone of Amnicon River (see p. 43). Grains of feldspar and mica are apparent to the unaided eye. Under the microscope (Speci- men 11240), it is seen that these constitute not over a quarter of the rock, the remainder being quartz. Micrccline and plagioclase are distinguishable, al- though most of the feldspar is orthoclase. Biotite is also seen . ... . . . . 250 6. Unexposed ........ 200 5. Fine grained red sandstone, apparently containing more feldspar and mica than higher beds; coarse, soft, brown sandstone; reddish, medium-grained, more feldspathic sandstone in vertical beds. The coarser rock was examined microscopically (Plate XIV, A, p. 68) and found to be over two-thirds quartz. The subangular grains average .35 mm. diameter. Some large microcline fragments were discovered. It is not notably different from the brownstone at Port Wing (see p. 42 and Plate VI, A, p. 44). The cement is alteration products derived from the feldspars ..... 250. 68 RELATION OF SANDSTONES. 4. Medium to fine-grained pinkish or brownish sandstone with frequent greenish bands and spots. Pebbles and clay pockets are not common. Some beds are cross-bedded or show curious curved forms; most are in heavy layers making the stratification diffi- cult to determine (see Plates XVII, B, p. 74 and XXI, p. 90). Towards the base a few thin layers of red shale occur. Two thin sections from the top and bottom of the horizon were examined (Speci- mens 11233 and 11235). The grains average less than .3 mm. in diameter and are slightly more angular than in higher strata. Quartz constitutes from a third to a half of the material. The re- mainder is almost whblly much altered feldspar, mainly orthoclase. Occasionally microcline is seen, as well as a few altered ferromagnesian minerals and mica. The cement appears to be quartz and feldspar enlargements, with some iron oxide . . 1,000 3. Red shale, dips about 70° S 10 2. Fine-grained thin-bedded hard red and greenish gray spotted sandstone. Dip varies from 45° S. to 80° S. Under the microscope this is seen to be little differ- ent from the lower part of the bed above (Plate XIV, B, opposite). Less than half is quartz. The grains average about .25 mm. diameter and are ce- mented by quartz and feldspar. Microcline and plagiocla.rs may be distinguished in the feldspars but most of them are much altered, generally to kaolin. Ferromagnesian minerals and mica can also be seen . . . . . . . . . 275 Total thickness of Orienta sandstone .... 2,735 Amnieon Formation. 1. Purplish red and light-greenish-gray very fine-grained sandstone to sandy shale, in beds seldom over an inch thick. Dip varies from 60° S. to vertical. Ripple marks and mud cracks are decidedly abund- ant. There is scarcely any cross bedding. In but one place were found ripple marks by which the inversion of the strata could be demonstrated! (see i Most of the ripple marks are of the symetrical indeterminate type^ Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate XIV. Photomicrographs in Ordinary Light, Magnified 12 Diameters. A. Coarse friable brownstone' (specimen 11,236) from bed 5. Middle River, Douglas County. Tbe grains are fairly well rounded as compared witb tbe older sandstones, and average about .35 mm. in diameter. The' rock is over two-thirds quarts. The remainder is, as usual, feldspar, in which some large microcline fragments are conspicuous. The cement is composed of alteration products from the feldspars. B. Pine-grained pink feldspathic sandstone (specimen 11,232) from bed 2. The grains are somewhat more angular than in A. and average .25 mm. in diameter. Less than half of the rock is quartz. Most of the feldspar is ortho- clase, but microcline and plagioclase are also abundant. Ferromagnesian min- erals, including micas, also occur. The cement is quartz and feldspar, stained with iron. This section should be compared with A, as well as with Plates X, p. 56, and XII, B, p. 66. RELATION OF SANDSTONES. p. 90). A thin section (Plate XVI, A, p. 72) from one of the greenish layers was examined. It consists of extremely fine angular grains of quartz (diameter about .01 mm.), with a few feldspars, im- bedded in a matrix of crystalline calcite which forms over a third of the slide, but analysis shows that it is less than 20% of the total (see p. 52). Mica flakes are seen in places, being concentrated on some bedding planes. The red noncalcareous phase was not examined microscopically . . 365 Total thickness 3,100 In the field the lithological likeness of the lower beds of this section to the upper part of the known Amnicon formation of the Oronto group is more striking than any description can make it, so that the writer has no reasonable doubt of the cor- relation as given. However, it must be remembered that the distance from undoubted outcrops of the Oronto group is con- siderable. St. Louis River. At the extreme northwest corner of the state, on St. Louis River, and extending for a short distance into Minnesota, is an exposure of about 730 feet of conglomer- ate, sandstone, and shale. These beds are prevailingly red in color but are spotted and streaked with white or greenish-gray. They contain much feldspar, decreasing in amount towards the top of the section (Plates VI, B, p. 44, and XVI, B, p. 72). Rip- ple marks, mud cracks, and rain prints are abundant (Fig.9). 5 feet Fig. 9. Redding in Amnicon formation, St. Louis River, Minn. In all their characters they are like the upper part of the Oronto group, and insensibly grade upward into rock like that of the lower part of the Orienta sandstone. 70 RELATION OF SANDSTONES. The following detailed section is based upon one given by N. H. Winchell of the Minnesota Survey, supplemented by the ob- servations of the writer. 1 The dip varies from 3° to 10° to the southeast (see Plate XV, opposite). "Winchell was inclined to assume the existence of an uncon- formity between the lowest conglomerate, which is mainly com- posed of pebbles of vein quartz, and that which overlies it and is composed of basic trap pebbles. He found a pebble in the latter which he supposed to have been derived from the pyritif- erous quartz conglomerate. The writer found no evidence for such a division. A large pebble of Keweenawan porphyry (Spe- cimen 11272) was found in the lower conglomerate, and there is a complete gradation between the two types (see p. 17). SECTION ON ST. LOUIS RIVER FROM FOND DU LAC TO THE POWER HOUSE Thickness Or'enta Sand stone. Feet 1. Upper Sandstone Beds. Reddish brown sandstone of medium grain (about .4 mm.) spotted with and grading into white. Contains pebbly bands, and micaceous shaley layers. The pebbles are trap, vein quartz, and quartzite. The bedding varies from thick to thin and cross-bedded. The quarries are on this bed. Under the microscope, the grains are seen to be subangular. About tivo-tJiirds of the rock is quartz, the remainder being more or less al- tered feldspar, mainly orthoclase (see Plate VI, B, p. 44). Microcline, plagioclase, mica, magnetite, and various ferromagnesian minerals appear. Specks of red iron oxide also appear, but the cement is quartz 200 2. Dark red shale, with greenish layers and spots . . 18 3. Lower Sandstone Beds. Sandstone much like 1. Is somewhat finer grained and contains more mica, shaley beds, red and green clay pockets, and beds i Winchell, N. H., Amer. Geol., vol. 16, 1895, pp. 75, 150; Minn. Geol. & Nat. Hist. Survey, 10th Ann. Kept., 1881, pp. 30, 91; Id., 23rd Ann. Report, 1894, p. 239; Geology of Minn., 1899, vol. IV, p. 15. RELATION OF SANDSTONES. 71 of magnetitic rock. Under the microscope (Speci- men 11328) shows little difference from 1, except that not much over half is quartz and there is more mica 70 Total Orienta sandstone ...... 288 Amnicon Formation. 4. Red shale, like 2 10 5. Alternating beds of sandstone and red shale. Thick- ness not accurately known. Under the microscope (Specimen 11326) shows medium-sized (about .3 mm. diameter) grains; all are subangular. Not much over half arc quartz, the remainder being mainly altered feldspars of all kinds, orthoclase being most abundant, but some microcline can be distinguished 75 6. Red shale . 10 7. Thin-bedded red and white sandstone, finer grained than the higher layers. Under the microscope. • (Plate XVI, B, p. 72) it is seen that less than half is quartz. The orthoclase feldspars are much altered, but some plagicclase was seen. There is consider- able mica. The cement is mainly calcite . . 3 8. Main Shale Bed. Red shale with thin partings of hard greenish sandstone, and some grit towards the base. Thickness not accurately known on account of faulting (see Plate XVII, A, p. 74) . . . 45 8Mi. Red and green" shale with ripplemarks . . 4.5 9. Grit and shale series. Grit ..... .5 10. Red shale 3.0 11. White grit 3.0 12. Red shale with greenish spots, ripple marks and rain prints (Specimen 11316) 14.0 13. Grit, mostly light colored ...... .75 14. Red shale . . . .25 15. Grit 2.0 16. Red shale .25 17. Grit .35 IS. Red shale .35 19. Grit 1.0 i Below this the numbering of the beds agrees with Winchell's sec- tion. 72 RELATION OF SANDSTONES. 20. Red shale, mud cracks . . . . . . 3.5 21. Grit, white, quite calcareous from decomposition of basic feldspars . . . . . . . 1.5 22. Red shale and shaley sandstone, with green layers and ripple marks . . . . . . . . 14. 0 23. Light red sandstone . . . . . . 3.0 24. Red grit, some white 4.5 25. Shaley red sandstone, very hard; some white . . 10.0 26. Red grit v\ . ; ; . . . . . v . 3.5 27. Dark red shale and shaley sandstone . . . 9.0 28. Lighter colored sandstone (white and red) . . 8.0 29. Green shale, ripple marks . . . . . . 5.0 30. Red shale and shaley sandstone, ripple marks . . 9.0 31. Grit with shale layers 4.0 32. Red and white mottled cross-bedded sandstone . . 5.0 33. Grit and conglomerate, coarsest in middle . . . 7.0 34. Shaley micaceous sandstone; pinches out . . . .75 35. Grit .85 36. Red shale .35 37. Grit ' 2.0 38. Red shale . . . . . . . . . .2 39. Greenish grit . . . . . . . . .5 40. Red and white sandstone, pinches out into conglom- erate. Under microscope (Specimen 11300) shows that over three-quarters of the rock is quartz. Re- mainder includes feldspars, mainly orthoclase, al- though both microcline and plagioclase were distin- guished, mica, and rarely ferromagnesian minerals. Average size of the grains is about .25 mm. but there is much variation. All are subangular . . . 1.2 41. Red grit passing below into coarse light colored sand- stone; cross-bedded . . . . . . . 3.5 42. Red conglomerate, coarser below; pebbles up to two inches ..... . - . . . 7.0 43. Red shale with green spots; some conglomerate . 1.5 44. Firm light-colored conglomerate with enclosed layers of cross-bedded sandstone . . . . . 3.0 45. Green and red shale, micaceous ■ . . . .7 46. Sandy shale 1.0 47. Fine red shale, ripple marks 15. 0 48. Quartz Conglomerate. Coarse hard pyritiferous con- glomerate. Color usually white to greenish. In- Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate XVI. Photomicrographs in Ordinary Light, Magnified 12 Diameters. A. Vei-y fine-grained greenish calcareous shaley sandstone from Middle River, bed 1, Amnicon formation (specimen 8611). The light-colored grains are fine angular quartzes, averaging .01 mm. in diameter. An occasional feldspar is seen, and mica is visible in the hard specimen. The matrix of the rock is crystalline calcite, which in one place forms a vein in which its characteristic cleavage may be seen. A chemical analysis shows CaO 10.7l9o, MgO 0.12%, which would require 8.55% CO, making a total of about 19% of carbonate (see p. 52). B. Fine-grained arkose sandstone from Amnicon formation, bed 7. St. Louis River (specimen 11.321). Quartte constitutes a little less than half of the rock. The remainder is feldspar (mainly orthoclase. but some more basic varieties), with iron oxide, a little mica, and calcite cement. RELATION OF SANDSTONES. 73 eludes layers of shaley sandstone, red conglomerate and red or green shale ...... 50 49. Conglomerate, somewhat coarser and very hard. The pebbles are of vein quartz, slate, quartzite, and iron formation. But one trap pebble was found. A foot of green shale is found in places next to the under- lying slate . . ..... . 100 Total thickness ........ 727 Total thickness computed from dip and allowing for faults 730 Huronian 50. Thomson slates. Gray slate Unknown A number of wells in Superior 1 disclose a similar succession. The result of the southeasterly dip of the sandstones is that while in East Superior solid brownstone is found, along' the St. Louis River to the west there is only discovered red sandy shale, carrying" a little salt water. The deepest well of which the record was obtained, is Patterson's, in N. E. % of N. E. % of Sec. 24, T. 49, R. 14 W. (see general map). This penetrates 185 feet of dry drift, below which was found over 700 feet of red sandy shale, undoubtedly belonging to the Amnicon formation. Summary. The exposures described above show that the stratigraphic sequence of a downward gradation through pro- gressively more and more arkose sandstone into red shales, some- times interbedded with conglomerate and grit, is found through- out the district. In the case of the exposures on Middle and St. Louis rivers it is practically certain that the upper beds belong to the Bayfield group. On the other hand, in the case of the Fish Creek section, farther east, the lower beds plainly be- long in the Oronto group, and are seen to grade upward into sandstones which are correlated with the Bayfield group. These facts indicate that the Bayfield and Oronto groups are conform- able and hence should both be placed in the Upper Keweenawan series. i Records furnished by J. A. Colwell, a well-driller, 1900, and Prof. J. A. Merrill, 1910. 74 RELATION OF SANDSTONES. The only alternative is to suppose that the Bayfield group rests unconformably upon the Oronto group, somewhere beneath the drift-covered interval at the head of Chequamegon Bay. While this cannot be disproved, there is nevertheless no evidence of any kind to indicate that such is the case and on such an hy- pothesis must rest the burden of proof. Categorically the evidence may be stated as follows: — 1. The same conformable downward gradation from more quartzose into more feldspathic sandstone and red shale is found at several widely separated localities within the district. 2. At most of these localities the identification of the upper beds with the Bayfield group is indisputable. 3. On Fish Creek the correlation of the lower beds with the Oronto group is almost equally definite, thus indicating that the lower beds at other places are presumably the top of the Oronto group. 4. No debris worn from the rocks of the Oronto group has been found in rocks of the Bayfield group. For the most part the younger sandstones are made up of grains which average somewhat larger in size than those of the Oronto group, thus showing that the materials of the Bayfield group must in any event have been largely derived from other sources than those of the Oronto group. 5. There is no universal structural difference between the two groups. The Bayfield group lies in the center of the Lake Superior synclinorium and hence is usually nearly horizontal ; but near Superior, all along the great fault of the Douglas trap range, and apparently on Fish Creek, it was involved in the extensive folding and faulting formerly supposed to be con- fined to the Oronto group. 6. If the Bayfield group is unconformable upon the Or- onto group, then its lowermost exposed m'ember is indistinguish- able on lithologic or structural grounds from the recognized Oronto group. There is, therefore, no reason to place the Bayfield and Oronto groups in different periods, and the evidence at hand drives us to the conclusion that the Bayfield group is a part of the Upper Keweenawan series. Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate XVII. A. North bank of St. Louis River, near line between St. Louis and Carlton counties, Minnesota. The cliff exposes red shale, with thin greenish sandstone partings. This is a part of the Amnicon formation. A normal fault of at least 40 feet displacement passes through the east end of the exposures (see p. 91). B. West bank of Middle River, Sec. 25, T. 18, R. 12 W., Douglas County, showing contact between the Middle Keweenawan traps at left of ravine, and overturued beds of red and greenish feldspathic sandstoue and shale of the Am- nicon formation to right (see fate XIII, p. 66). CONTACT WITH TRAPS. 75 CHAPTER VI THE CONTACT OF THE BAYFIELD GROUP WITH THE TRAPS General Statement. All along the south coast of the west end of Lake Superior the Bayfield group of sandstones comes into contact with the traps of Middle Keweenawan age, with dip to the southeast at angles varying from 10 to 70 degrees. At the head of the lake, on St. Louis River, near the western border of the state, the conformable downward extension of this group, the Amnicon formation, appears from beneath the younger sand- stones and is plainly in unconformable contact with the under- lying slates of Huronian age. Douglas Fault. It has long been recognized that the con- tact of the Bayfield group with the traps of the Douglas (or South) Range is marked by a fault. "With regard to the age, extent, and significance of this displacement there has been much diversity of opinion. As explained in Chapter III, Irving was of the opinion that the contact was an unconformity along a fault scarp complicated by slight recent movement on the same plane. Since that time, Grant recognized that an enormous displacement has taken place since the deposition of the sand- stone, and that the fault is of the thrust type (see p. 89). LOCAL DETAILS On Iron River, near the eastern boundary of Bayfield County, is the most easterly known exposure of the traps of the South or Douglas Range. Several miles west of this small out- crop, the crest of the Douglas trap range rises above the plain 76 CONTACT WITH TRAPS. of Pleistocene deposits (see p. 7, and Plate II, p. 8). Where Brule River crosses through a deep gap in this ridge, the contact of the sandstone and trap is not exposed ; it occurs beneath a cov- ered space of about a fourth of a mile. No distinct ev 1 lence of faulting or other disturbance appears at this point in either for- mation. A contact showing fine conglomerate was reported by Sweet 1 (see p. 44), in Sec. 30, T. 48, R, 10 W. This locality was visited by Grant's party, but the exposure was not found. Middle River Contact. Ten miles west of the Brule, Mid- dle River exposes the contact of the traps and the Amnicon formation, which conformably underlies the Bayfield group (see p. 66). Berkey, Van Hise, and Grant visited this locality in 1899 and made the map which is the basis of Plate XIII. They recognized the true bedding of the sandstone, which Sweet had not, 2 and assumed that the structure was anticlinal. (See also Plate XVII, B, p. 74, and Plate XXI, p. 90). The writer decided that this interpretation must be an error, since the strati graphic sequence observed in other localities is exactly opposite to that required by such an hypothesis (see sec- tion on p. 67). No evidence could be detected of a small syn- cline next to the fault at the eastern exposure, 3 such as would be expected below a thrust fault. The fault cuts the E-W strike' of the fold, the layers in which are somewhat broken near the contact. Furthermore, as explained on p. 68, ripple marks are found, which clearly demonstrate that the southward-dipping beds have been overturned. The structure deduced from these facts is that of an overturned and broken anticline, a normal structure in connection with a thrust fault ; while that described by Grant would be unusual. 1 Sweet, E. T., Geol. of Wis., 1873-9, vol. ill, p. 348. Irving, R. D., Copper-Bearing Rocks, Mon. U. S. G. S., vol. V, 1883, p. 257. 2 Sweet, E. T., Geol. of Wis., 1873-9, vol. Ill, p. 347. 3 Grant, U. S., Copper-Bearing Rocks of Douglas Co., Wis. Geol. & Nat., Hist. Survey, Bull. VI, 1801, structure section C-D on Plate IX, p. 31. Junction of Lake Superior Sandstone and Keweenawan Traps in Wis., Bull. Geol. Soc. of America, vol. 13, 1906, p. 8. Personal communications. Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate XVIII. A. East bank of Amnicon River, Sec. 29, T. 48, R. 12 W., Douglas County, showing thrust fault, which brings the traps into contact with the Orienta sand- stone. B. Key to the foregoing photograph. COy TACT WITH TRAPS. 77 Amnicon River Contact. Where Amnicon River crosses the fault, occur good exposures which bring to light the actual plane of contact. 1 Changes in the course of the stream have resulted in uncovering the formations for a considerable distance, as shown in Fig. 10. These exposures are particularly interesting, in that they show the direction of the fault to change abruptly from nearly E. and W. to N. 70° E. The eastern exposure, which doubtless was at one time a chan- nel of the river, is in a deep and narrow ravine following the j Orient a Jan ch tone ^ y Q 5 L ZQ FAULT Fig. 10. Sketch map showing contact of trap and sandstone on Amnicon River, Douglas County. contact for some distance. The actual junction is covered with but a few inches of soil. The south side of the ravine is an overhanging wall of brecciated trap. In the bottom, occasionally rising into ridges, and on the north wall, occur exposures of conglomerate with interbedded layers of sandstone. The dip of the beds varies from vertical to about 45° N. ; but the exact rela- tion of the horizontal sandstone farther to the north could not be determined because of fallen debris and the irregular shear- ing that conceals the true bedding. It is probably marked by- faulting, as will presently be explained. The conglomerate is composed of trap pebbles, usually much weathered, averaging an inch or tw T o in diameter, but with occasional specimens a foot, in diameter. All are well rounded, and, so far as was ascer- tained, composed mainly of porphyry and amygdaloids. The i Sweet, E. T., Geol. of Wis., 1873-9, vol. Ill, p. 346. Grant, U. S„ Copper-Bearing Rocks, p. ]9. 78 COy TACT WITH TRAPS. sandstone matrix and interbedded layers do not differ, except in coarseness of grain, from the rock exposed to the north (p. 43) ; some, however, appear to contain more small fragments of trap. At the west end of the ravine is exposed a thin la} T er of hard ferruginous sandstone. It forms the face of the north wall, dipping south at approximately the same angle as the fault (about 45°), thus cutting across the vertical layers of conglo- merate and sandstone beneath. On the upper surface of this layer are two sets of grooves; one is parallel to the dip, the other is inclined at an angle of about 30° in a NE-SW direction. The exposures are naturally much better on the present banks of the stream to the west (Plate XXII, B, p. 92). That this present course has not been occupied by the river for a great length of time, is shown by the fact that the fall of about 15 feet, just at the fault line, has receded but slightly into the traps. Plate XVIII shows the eastern wall of the gorge. Two faults are seen, of which the upper or southern one dips 38° S. with an E-W strike; the lower 46° S, strikes N. 72° E. They are separated by a zone of trap breccia, such as also occurs be- neath much of the bottom of the ravine to the east, as above described. It consists of angular and subangular fragments of weathered trap, some of them several feet across. North of the lower fault occurs a wedge of conglomerate, bounded on the north by an approximately vertical plane. In this conglomerate are two thin sandstone layers striking parallel to this fault, and dipping south at a somewhat steeper angle than the fault. The southern one is almost quartzite, and the other carries scratches like those noted to the east. The bedding of the conglomerate is otherwise indistinguishable. In places to the north it appears to be vertical, as shown by an interbedded sandstone layer; but there, with no appearance of fracture, the conglomerate gives -way above to horizontal thin-bedded quartz sandstone, and below -.to eross-bedded pebbly white sandstone, among the pebbles of which are some of quartz and quartzite. There an appearance of gradation is presented, but above the contact is sharp. Shear- in^makes the relations difficult to determine. Above the top of the conglomerate the foot wall of the fault is composed of much fractured sandstone. About 100 feet to the north a small thrust fault of undetermined throw is seen in the sandstone, as shown Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate XVIIIa. A. Lower Falls, Amnicon River, Douglas County (see Fig. 10, for location). Looking north, showing Copper Creek beds of Orienta sandstone. B. Diagram (drawn from a photograph) of part of west bank of Amnicon River, Douglas County, just north of contact of trap and sandstone (south of the falls shown in A) — for location, see Fig. 10. This exposure shows the dying out of the middle fault into a confused zone of fractures which separates the vertical conglomerate layers from the horizontal sandstone to the north — shown at the right. The shearing planes in the vertical layers are very complex, and are but partly represented. CONTACT WITH TRAPS. 79 in Plate XXII, A, p. 92; the dip is 38° S., the strike about N. 55° E. The west bank is not so well exposed, owing to a recent fall of the overhanging; wall of trap, thus concealing the main fault. Two other zones of displacement may be seen to the north. The southernmost of these is important since it shows the relation of the vertical sandstone and conglomerate to the horizontal sandstone beds. It is not clear just how the middle fault is related to that described on the other side of the river. The two there seen, appear to join beneath the water, so that no well-defined breccia is seen on the west bank, the actual contact being marked only by an inch or so of red shale. The middle fault of the west bank branches out from the others beneath the river. It is composed of several irregular planes of fracture, as is shown in the plate Between the two faults is found conglomerate with irregular interbedded sandstone layers, all much sheared, as is imperfectly shown in Plate XVIII a, B, p. 78. A remarkable feature is the sudden ending of one of the conglomerate layers against sand- stone below, with no appearance of fracture. Whether or not the alternation of conglomerate and sandstone represents bedding, is not known ; but it is worthy of note that no conglomerate is found in undisturbed sandstone beds, while most of the layers occupy a vertical or highly tilted attitude and are much sheared. This in- tense shearing is quite irregular ; no permanent system could be discovered, although many of the planes of fracture dip toward the fault at a low angle. In the western bank the northernmost fault is again seen, dipping to the south at an angle of 28°. There are but poor exposures in the wide valley that crosses the fault to the west of the river. The inclined conglomerate beds reappear, but their relation to the horizontal beds to the north is not clear on account of the intense shearing ; no pebbles extend beyond their limits. Fractures approximately parallel to the fault are most abundant, apparently representing the dying out of the middle fault. The strike of the contact is here N. 70° E. while that of the conglomerate is N. 50° E. 1 i About a quarter of a mile to the west is a valley which crosses the contact and is believed to show exposures; the writer was, however, unable to visit it. .80 CONTACT WITH TRAPS. Copper Creek. In sec 15, T. 47, R. 14 W., Copper Creek crosses the contact of the sandstones with the traps. The actual junction is covered by a few feet of debris, so that the attitude of the fault plane is difficult to determine. 1 Sweet's section shows it dipping to the north, but the writer was inclined to believe that the dip is about 60° to the south. The sandstone is sheared and sharply flexed for a distance of less than a hundred feet from the contact, thus indicating an upward movement of the igneous rocks. No conglomerates or other unusual features are found in the sandstone. Palls of Black River. What is probably the best known exposure of the sandstone-trap contact is that in the S. E. y± sec 21, T. 47, R. 14 W., at the picturesque falls of Black River. The locality is, however, more attractive for its scenery than for its rocks, for the latter are not everywhere well exposed. 2 Plate XIX is a map of this locality made by Grant in 1899, and shows the complex relations of the formations more clearly than could any description. Conglomerate is found in but two places. The pebbles are decayed acid lavas, vein quartz, and red quartzite, preserving about the same proportions as the pebbles in the main body of the sandstone. The sizes, however, average somew T hat larger, pebbles six inches in diameter being found. On the east bank, not far above the stream and beneath trap layers higher up the bank, is a ledge of conglomerate with two interbedded sandstone layers which dip 30-40° N. This is quite close to the main fault (which here dips some 50° S.) and to the supposed cross fault. Above the conglomerate, sandstone, bearing but occasional pebbles, approaches close to the fault. On the west bank of the stream is a larger ledge of conglomer- ate and sandstone. The clip here is reversed, being about 10 to 20 degrees to the southeast. This feature is shared, as may be seen in the diagram, by a number of sandstone exposures to the 1 Sweet, E. T., Geol. of Wis., vol. Ill, 1873-9, p. 344. Grant, U. S., Copper Bearing Rocks, p. 19. 2 Sweet, E. T., Geol. of Wis., vol. Ill, 1873-9, p. 340. Grant, U. S., Copper Bearing Rocks, p. 19. Wisconsin Geol. and Nat. Hist. Survey. Bulletin No. XXV, Plate XIX. Man., of vicinity of Lower Falls of Black River, Douglas County, showing contact of Middle Ke- wee^ Wan traps and Drienta sandstone. Map by U. S. Grant, 1899 ; sections by F. T. Thwaites, 1910. Contour interval, 20 feet ; elevations, 11 feet too high. CONTACT WITH TRAPS. 81 west. All of these rocks are quartzose, like those to the north, the only difference being that they are considerably harder. Above these exposures rises a cliff of trap breccia, in which are included several large pieces of red quartzite. The extent of the brecciation is much greater than noted elsewhere, extend- ing several hundred feet from the contact. The dip of the fault here cannot be measured, but appears to be not very steep. The sandstone north of a line bounding these south-dipping exposures, as usual dips to the north. These beds are described on p. 44. A study of the detailed map shows that the reentrant in the fault line must be the cause of the abnormal features here noted, just as the bend in the reverse directiuri at the Amnicon River is accompanied by peculiar phenomena. There must be a cross fault to account for this reentrant angle in the border of the trap ; the excessive brecciation is doubtless due to its presence. It is most probable that the south-dipping layers of sandstone are separated from the north-dipping by a continuation of the fault to the east of this break. This fault probably passes through the poor exposures on the west side of the valley, although they are so obscure that nothing can be stated positively; it may, however, join the main or southern fault beneath a covered space farther to the east. The writer could distinguish no definite evidence of unconformity, or of this reentrant being a sinuosity of an ancient shore line, as supposed by Irving. 1 In the S. E. *4 of the N. W. % of sec. 29 of the same town- ship occurs a small exposure of the contact. Irregularly-sheared sandstone occurs within two feet of trap breccia and dips about 40° N. for a short distance from the contact. Interpretation of Exposures. Irving believed that the phe- nomena above described represented an unconformity along a fault scarp, complicated by a slight amount of recent faulting on the same plane. Grant discovered the great amount of throw (at least 3,000 feet; see p. 66) which the fault possesses, and declared the evidence to be not conclusive in favor of uncon- i irving, R. D., Copper-Bearing Rocks, Mon. U. S. G. S., vol. V, 1883, p. 441. 6 82 CGXTACT WITH TRAPS. formity. All former investigators have assumed that the sand- stone was a marine deposit, a fact now regarded as decidedly doubtful. The following evidences favor the older view : 1. The presence of conglomerates, often of large pebbles main- ly, if not sometimes wholly, derived from the rocks of the adja- cent trap series. 2. The striking difference in the amount of folding and met- amorphism of the two groups of rocks, which implies that most of the deformation of the traps must have taken place before the deposition of the younger sandstones. 3. The enormous throw and subsequent erosion required by the fault hypothesis. 4. No strata are known from which the conglomerates might have been faulted up ; these is little to show definitely that they have been faulted up more than a few feet. 5. The dip of the fault plane to the south, giving it a large horizontal component of motion, would have cut away a large part of the coarse deposits adjacent to the fault scarp, thus ex- plaining the absence of conglomerate at some points. 6. As the fault is part of the Keweenawan deformation, rep- resenting a broken fold, it is quite probable that faulting began before the time of deposition of the adjacent sandstone, and per- haps continued through the time of its formation. 7. The Bayfield sandstones may be areally continuous with the horizontal light-colored sandstones of the Mississippi valley (cf. p. 58), which are separated from the tilted Middle Keweenawan by a considerable unconformity. The following facts support the Idea that the faulting is in large part, if not wholly, later than the deposition of the sand- stone : 1. Well defined conglomerates occur at but two localities out of the eight known exposures (less definite ones occur at two others) . 2. They are never found over a few rods from the contact. 3. They do not clearly grade into the adjacent sandstone. 4. They occur only at points of unusual minor faulting in the sandstone. CONTACT WITH TRAPS. 83 5. No patches of sandstone are found resting uncomformably upon the trap. 6. The amount of conglomerate is not sufficient, nor are the pebbles large or angular enough, to be regarded as either a subaerial or marine deposit along a fault scarp. 7. In many cases the pebbles are one-third vein quartz and quartzite ; these could only have been supplied from a distant source, and would not be likely to occur along a fault-scarp. The trap pebbles were not usually derived from the immediately ad- jacent rocks. 8. The inclination of the fault plane involves a large horizon- tal component, approximately equal to the vertical one. If faulting took place during or just prior to the deposition of the sandstone, this would involve a shifting northward of the scarp line, thus supplying a large amount of fresh unweathered sed- iment. No such amount of sediment of that character is found in the upper beds of the sandstone. 9. The throw of the fault since the deposition of the sand- stone is certainly great ; it must be at least 3,000 feet, since that thickness of strata is involved on Middle River. 10. The total displacement involved, even if the traps are as thick as was formerly supposed (see p. 89), is not greater than that of many well-known thrust faults. The uplifted rock may readily have been eroded during the long time that has since elapsed. 11. The conglomerates that occur are so intensely sheared that they may represent material from lower conglomerates which has been dragged up along the fault and worked into the sandstone, like the layers of sandstone that have been found in- cluded in the trap breccia (see p. 81). This hypothesis would explain some of the peculiar features of the conglomerates, found only at points of intense faulting. We have no knowledge of the stratigraphy of the lower beds of the sandstone in this part of the area, but conglomerates almost undoubtedly occur at lower horizons. 12. At several points, cross-bedded layers in the sandstone dip toward the fault. This would be an unusual occurrence if the rocks occupied the same relative positions at the time of the deposition of the sandstone, as they do today. 84 CONTACT WITH TRAPS. 13. The areal connection of the Bayfield group with the Cam- brian sandstones of Minnesota is not proved ; the age and struc- tural relations of the red sandstones beneath the light-colored Potsdam of Minnesota are unknown (cf. p. 58). 14. If the supposed fault scarp was being eroded during the deposition of the sandstones, then the sandstones must have been formed very soon after, if not during a period of regional deformation. This contrasts them sharply with those of the recognized Cambrian of southern "Wisconsin, which were evi- dently formed under quiescent conditions (cf. p. 101). 15. The sandstones adjacent to the fault (Bayfield group) are apparently conformable upon those which overlie the traps (Oronto group) and elsewhere are folded with them. They all belong to the same period of deposition. Therefore it is very improbable that 'the exposures described above represent an un- conformity involving any great lapse of time. If there is any unconformity, it is rather a local matter formed by slight move- ments of the fault during the deposition of the sandstones. The writer concludes that inasmuch as deformation appears to have been more or less continuous throughout Keweenawan time, the probabilities are, that while most of the movement on the fault may have been accomplished before the deposition of the sandstones now exposed, that slight movements may have occurred during their deposition, while a considerable amount has demonstrably occurred since. This hypothesis agrees best with all that is known of the phenomena (cf. p. 108). North Coast Contact. Near Duluth, several supposed con- tacts of the sandstone with the trap came to the notice of the writer. Upon investigation they proved to be either red syenite, cemented drift, or interbedded Middle Keweenawan sediments. But as explained on p. 107, it is entirely possible that an uncon- formable contact may yet be found. St. Louis River Contact. Although on St. Louis "River the ■Amnicon formation of the underlying Oronto group is seen in contact only with the Huronian slates, the presence of basic trap pebbles in such abundance implies a nearby exposure of Keweenawan igneous rocks during deposition, and hence a pos- sible unconformity. Less than a mile from the exposures on Mission Creek, near Fond du Lac, hills of intrusive gabbro rise CONTACT WITH TRAPS. 85 to a level some 500 feet above the exposures. About the middle of the interval between these exposures, some real estate specu- lators at one time drilled a deep hole in search of natural gas, the log of which is here given : LOG OF CHURN DRILL HOLE NEAR SHORT LINE PARK, MINN. Situated on S. line Sec. 32, T. 49, R. 15 W., in a ravine at elevation 900 feet A. T. (After N. H. Winchell, Natural Gas in Minnesota, Minn. Geoi. and Nat. Hist. Survey, Bull. V, 1889, p. 31; Geology of Minnesota, Vol. IV, 1899, p. 567.) Pleistocene. Thickness, feet Drift (clay?) no samples 100 Ketveenawan 2. Rock, said to be soft, no samples .... 131 3. Brownish red shale (called tuff by Winchell) 12 4. "Decayed trap", perhaps conglomerate 33 5. No samples, presumably same as above 104 6. "Surface eruptives", perhaps conglomerate . 37 7. The same, samples appeared to be in part fragmental, hence is probably a conglomerate .... 31 8. Gray slate or shale, said to be misplaced in record, but is possibly one of the greenish siliceous shales of the Amnicon formation . . . 15 9. Pink to purple quartz sand with some trap fragments 5 10. The same but coarser and pyritiferous with pebbles of trap .......... 5 11. "Gray slate", probably shale rather than slate misplaced in the record ........ 17 12. "Gabbro" . .16 13. Conglomerate of soft trap pebbles .... 2 14. Pink and gray quartzose, pyritiferous conglomerate and quartzite containing pebbles of slate, and probably layers of shale ........ 20 15. "Surface lavas like those at Duluth." These rocks are clearly igneous ....... 91 Huronian-Thomson Slate 16. Gray and black quartzose slate 930% Depth of hole 1,517% A little salt water was found, but no gas. 86 CONTACT WITH TRAPS. The interpretation of the foregoing record is difficult. Win- chell 1 correlated No. 14 with the similar white quartzose, pyritif- erous conglomerate which is exposed on the banks of St. Louis River, to the west. He believed that the conglomerate seen in the river valley was older than the Keweenawan lava flows, since it contains no trap pebbles. (One large one was found by the writer, see p. 70). The red conglomerates and shales of the St. Louis River section, which at that point immediately overlie the pyritiferous conglomerate, he held to be either separated by an unconformity from the pyritiferous conglomerate or to have been deposited contemporaneously with the traps. The inter- bedding of traps and conglomerates, which he believed was shown in the above drill record, fits with either hypothesis. 2 A single pebble of pyritiferous conglomerate found by him in the red trap conglomerate, was believed to come from the underlying conglomerate and therefore to indicate an hiatus between the deposition of the two conglomerates, the upper one of which he believed to be of pyroclastic origin. This view is illustrated in Plate XX, A. WincheH's views were not borne out by the writer's investi- gations. It seems clear that there is no unconformity between the two types of conglomerate on St. Louis River. The pebble of pyritiferous conglomerate found in the upper conglomerate, may well have been derived from some other similar formation. Furthermore, no evidence of contemporaneous volcanic erup- tions was discovered in the Amnicon beds of St. Louis River and Mission Creek. These sediments do not differ essentially in character from the other parts of the Oronto group. It would therefore appear probable that the pyritiferous conglomerate found in the drill hole is not the same as that on the river, but is a part of the Middle Keweenawan trap series with which it is interbedded. The scarcity of coarse trap debris, especially of the gabbro, in the conglomerates of St. Louis River, suggests that faulting since the formation of the sediments has brought the traps into their present position, so that they rise 500 feet or more above the river level. 1 Winchell, N. H„ Geol. of Minn., vol. IV, 1899. p. 15. 2 Hall, C. W., Keweenawan Area of E. Minn., Bull. Geol. Soc. of America,* vol. 12, 1901, p. 342. 3 (I 4 K,^ P 3^ 0 fi cd g cd 2 as P o-w i P^. cd rt . » ? r+ CD B P & P p (0 t3-p ©