or 
 
 THE UNIVERSITY OF 
 
 au of Economic Geology and Technology 
 HANDBOOK SERIES NO. 1 
 
 AIDS TO IDENTIFICATION OF 
 GEOLOGICAL FORMATIONS 
 
 BY 
 I, A. UDDEN 
 
 IRLF 
 
 105 
 
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 in 
 3) 
 
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 ublished by 
 
 Univcr-'.v of :. xa; 
 
EXCHANGE 
 
THE UNIVERSITY OF TEXAS 
 
 Bureau of Economic Geology and Technology 
 
 HANDBOOK SERIES NO. 1 
 
 AIDS TO IDENTIFICATION OF 
 GEOLOGICAL FORMATIONS 
 
 BY 
 J. A. UDDEN 
 
 Published by 
 
 The University of Texas 
 
 Austin 
 
CONTENTS 
 
 Page 
 
 Introduction 7 
 
 Minerals and rocks 9 
 
 Minerals 9 
 
 Quartz ., 11 
 
 Mica 12 
 
 Feldspar 12 
 
 Hornblende 13 
 
 Augite and pyroxene 13 
 
 Serpentine and chlorite 13 
 
 Glauconite 13 
 
 Calcite 14 
 
 Dolomite 15 
 
 Gypsum and anhydrite 15 
 
 Barite 16 
 
 Pyrite and marcasite 16 
 
 Hematite and limonite 17 
 
 Siderite 18 
 
 Salt 18 
 
 Asphalt, coal, and lignite 18 
 
 Rocks 19 
 
 Igneous rocks 19 
 
 Sedimentary rocks 20 
 
 Mechanical sediments 21 
 
 Chemical sediments 21 
 
 Organic sediments 21 
 
 Gravels, conglomerates, and puddingstone. . 23 
 
 Sands, sandstones, and quartzites 24 
 
 Packsand 24 
 
 Micaceous sandstone 24 
 
 Ferruginous sandstone 25 
 
 Bituminous sandstone 25 
 
 Cemented sandstone 25 
 
 Quartzite 25 
 
 Clay, shale, and slate (Argillites) 25 
 
 Marl . . 27 
 
 430114 
 
4 Contents 
 
 Page 
 
 Fire-clay 27 
 
 Fuller's earth 28 
 
 Micaceous clay and shale 28 
 
 Ferruginous clay and shale 28 
 
 Bituminous clay and shale 28 
 
 Carbonaceous clay and shale . 29 
 
 Soapstone 29 
 
 Gumbo 29 
 
 Limestone 29 
 
 Oolitic limestone 31 
 
 Shell breccia . . . : V. 32 
 
 Chalk 32 
 
 Lithographic limestone 33 
 
 Dolomite 33 
 
 Caliche 34 
 
 Travertine , 34 
 
 Bituminous limestone 34 
 
 Gypsum and anhydrite rock 35 
 
 Rock salt 36 
 
 Flint and chert i 36 
 
 Clay-iron-stone 37 
 
 The principal formations in Texas 39 
 
 Introductory 39 
 
 Pleistocene and recent formations 41 
 
 The Tertiary formations 42 
 
 The Upper Tertiary 42 
 
 The Middle Tertiary 43 
 
 The Dewitt Formation 43 
 
 The Fleming clay 43 
 
 The Catahoula sandstone 43 
 
 The Lower Tertiary 43 
 
 The Jackson formation 43 
 
 The Yegua clay 44 
 
 The Cook Mountain 44 
 
 The Mount Selman. 44 
 
 The Wilcox 44 
 
 The Midway 44 
 
 The Upper Cretaceous 45 
 
 The Tornillo . .45 
 
Contents 5 
 
 Page 
 
 The San Carlos beds 45 
 
 The Rattlesnake beds 45 
 
 The Escondido beds 45 
 
 The Navarro beds 46 
 
 The Anacacho limestone 46 
 
 The Taylor marl 46 
 
 The Austin chalk 46 
 
 The Eagle Ford 47 
 
 The Woodbine 48 
 
 The Lower or Comanchean Cretaceous 
 
 formations 48 
 
 The Buda limestone 48 
 
 The Del Rio clay 48 
 
 The Georgetown 49 
 
 The Edwards limestone 49 
 
 The Paluxy sands 49 
 
 The Glen Rose 49 
 
 The Trinity 49 
 
 The Jurassic formations 50 
 
 The Triassic formations 50 
 
 The Permian formations. 50 
 
 The Pennsylvanian formations 51 
 
 m The Bend formation 53 
 
 The Mississippian, the Devonian, and the 
 
 Silurian formations 54 
 
 The Ordovician formations 55 
 
 The Cambrian formations . 55 
 
 Archean and igneous rocks 56 
 
 Tables for identifying minerals and rocks 57 
 
 1. General appearance under a magnifying 
 
 glass 60 
 
 2. Examination of texture 62 
 
 3. Observation of color 63 
 
 4. Observation of streak 64 
 
 5. Test for hardness 65 
 
 6. Acid test 66 
 
 . 7. Test for fumes 68 
 
 8. Fire test . . 69 
 
INTRODUCTION 
 
 By long experience many drillers learn to dis- 
 tinguish, generally quite correctly, the three prin- 
 cipal classes of sediments in which most of their 
 work is done, such as shale or clay, sandstone and 
 limestone. The most experienced drillers in any 
 locality have, also, expert knowledge of what we 
 may call the "key rocks" of the field in which 
 they work. They know the rocks that . determine 
 the depth at which casing is to be set, or the depth 
 at which the tests for oil or gas must be made. 
 This knowledge of rocks is mostly taught from 
 mouth to mouth, by the head driller to his helpers, 
 entirely without any other guidance than the limited 
 experience of each driller. No aid, so far as I 
 know, has ever been prepared by geologists to help 
 drillers in procuring the information both are eager- 
 ly seeking. 
 
 With the extension of drilling operations to re- 
 gions of widely different geological formations and 
 conditions, it has become a matter of some urgency 
 that drillers' logs be made with greater accuracy 
 and uniformity than before. The only way to en- 
 courage improvement in this direction is to place in 
 the hands of the men who make the "logs," what 
 aid they need. By the use of a few very simple 
 devices, available to all, great improvements in re- 
 sults can be attained without the least loss of time.. 
 The present paper is an attempt to present some 
 information, in compact form, that will aid prac- 
 tical and intelligent men to correctly identify ma- 
 terials explored in drilling for oil, gas, or water. 
 It will be found that much of the information here 
 set forth consists of elementary methods 'and facts 
 drawn from mineralogy and geology. Most drillers 
 are eager to acquire all knowledge they can in these 
 sciences. They have exceptional opportunities in 
 
8 Handbook of Aids to 
 
 their occupation to return to the geological profes- 
 sion manyfold any little help that can be offered 
 them of the kind here attempted. 
 
Identification of Geological Formations 
 
 MINERALS AND ROCKS 
 
 Minerals, when pure, have a more or less constant 
 chemical composition, and with some exceptions they 
 have definite crystalline form. In these respects 
 they differ from rock which may or may not be 
 constant in its chemical composition. By rock we 
 mean any considerable quantity or large body of 
 material which has been brought together into a 
 deposit by some natural process. A mass of ice 
 resulting from the accumulation of snow, is a rock. 
 Salt or calcareous mud precipitated from the wa- 
 ters of the sea and laid down in beds, forms rock. 
 Sand and gravel are rock. When indurated, or 
 hardened, we call these two latter deposits sand- 
 stone and conglomerate, respectively. Many rocks 
 consist of mixtures of different kinds of minerals. 
 Many sandstones are mixtures of grains of quartz 
 sand and scales of mica, and many limestones are 
 mixtures of calcite and dolomite. 
 
 Minerals are classified according to their chemical 
 composition, while rocks are classified according to 
 the processes by which they have been formed, or 
 the manner in which the materials of which they 
 consist have been brought together. 
 
 MINERALS 
 
 The number of minerals which occur in sufficient 
 frequency to be of interest in formations that are 
 extensively drilled are quite few. Though more than 
 a thousand different mineral species are known-, less 
 than a score ar$ Commonly found in the forma- 
 tions which aue encountered by drillers in this state, 
 may be enumerated and classified as follows; 
 
10 Handbook of Aids to 
 
 Siliceous minerals: 
 Quartz. 
 Mica. 
 Feldspar. 
 Hornblende. 
 
 Serpentine and Chlorite. 
 Augite and Pyroxene. 
 Glauconite. 
 
 Calcareous, lime-bearing, minerals: 
 Calcite. 
 Dolomite. 
 Gypsum and Anhydrite. 
 
 Iron-bearing minerals : 
 Pyrite and Marcasite. 
 Hematite and Limonite. 
 Siderite, or clay-iron-stone. 
 
 Sodium chloride: 
 Salt. 
 
 Carbon compounds: 
 Asphalt. 
 Coal. 
 Lignite. 
 
 Barium compound: 
 Barite. 
 
 If these minerals are classified on the basis of 
 the acid elements entering into their composition 
 they arrange themselves in the following groups: 
 
 Oxides : 
 Quartz. 
 Hematite. 
 Limonite. 
 
 Sulphides : 
 Pyrite. 
 Marcasite. 
 
 Chloride: 
 Salt. 
 
 Silicates : 
 Mica. 
 Hornblende. 
 
Identification of Geological Formations 11 
 
 Feldspar. 
 Pyroxene. 
 Serpentine. 
 Glaucdnite. 
 
 Carbonates : 
 Calcite. 
 Dolomite. 
 Siderite. 
 
 Sulphates : 
 Gypsum. 
 Anhydrite. 
 Barite. 
 
 Hydrocarbons : 
 Asphalt. 
 Coal. 
 Lignite. 
 
 QUARTZ 
 
 Quartz is the oxide of an element called silicon. 
 It is the most common of all minerals and occurs 
 in many varities, such as rock crystal, which is its 
 purest but not its most common form; chert and 
 flint; agate; jasper; chalcedony; and a number of 
 other forms. It is a little harder than glass, which 
 it easily scratches. It may have, like most other 
 minerals, any color. The purest varieties are either 
 colorless and transparent, or white. Crystals of 
 quartz are usually six-sided like a pencil, and term- 
 inate in six-sided pyramids, or points, at one or 
 both ends. 
 
 Quartz is not affected by ordinary acids, and is 
 not much changed by heating. To drillers the most 
 familiar form of quartz is that seen in quartz sand 
 and in layers of flint and chert. The latter occur 
 in some limestones and greatly retard drilling on 
 account of the hardness of this mineral. The chert 
 layers in the Edwards limestone are from a few 
 inches to two and three feet in thickness. There 
 are also some chert layers in the Pennsylvanian 
 limestones and in the so called Ellenburger lime- 
 
12 Handbook of Aids to 
 
 stone. Small quartz crystals of perfect shape are 
 characteristic of the Redbeds of the Permian in the 
 northwest part of Texas. Sand grains of chalce- 
 dony are characteristic of some of the middle Ter- 
 tiary formations, and have come to be spoken of as 
 "rice sand," from a resemblance of the larger grains 
 to polished rice. 
 
 MICA 
 
 Mica is a silicate of magnesia. It splits into ex- 
 ceedingly thin transparent flakes, which are elas- 
 tic, and will spring back when bent. It will not 
 crumble when heated in a flame, as gypsum will. 
 It is not affected by acid. Some mica is black and 
 some has a golden color. It is generally present in 
 several different kinds of crystalline rocks, from 
 which it has been washed and mingled with sand 
 and silt, afterward often hardened into sandstone 
 and shale. Sandstones in the Cretaceous, and also 
 in the Pennsylvanian and the Permian, seldom have 
 much mica; while the sandstones in the Triassic of 
 West Texas and the sands and silts in the Eocene 
 Tertiary are usually very rich in this mineral. 
 
 FELDSPAR 
 
 Feldspar is a silicate of potash, soda and lime. 
 There are several kinds of feldspar. It has straight 
 cleavage planes that often show a pearly lustre. 
 Orthoclase feldspar has a pinkish color. It is not 
 quite as hard as quartz, but decidedly harder than 
 calcite. It can be scratched by hard steel, and 
 does not slack after heating to redness in fire. This 
 mineral is the principal ingredient in granite and 
 some other igneous rocks. In sedimentary rocks it 
 is sometimes found as pebbles in some conglomer- 
 ates, such as those occurring in the Pennsylvanian 
 in north Texas and in the Trinity formation in 
 central Texas, around the Llano uplift. 
 
Identification of Geological Formations 13 
 HORNBLENDE 
 
 This is a very dark greenish or almost black min- 
 eral with crystalline cleavage. .It is a silicate of 
 magnesium, iron, manganese, soda, and potash, and 
 has about the same hardness as feldspar. It occurs 
 in some intrusives, schists and gneisses, underlying 
 the sedimentary rocks in central Texas, and in the 
 Trans-Pecos country. 
 
 AUGITE (AND PYROXENE) 
 
 Augite is a greenish black mineral. It is a sili- 
 cate of lime, magnesia and iron. It has straight 
 cleavage in two directions, and is of a hardness 
 very slightly below that of feldspar. This mineral 
 is found in igneous rocks at a few points along the 
 Balcones Escarpment and in the mountainous coun- 
 try west of the Pecos. It does not occur in sedi- 
 mentary rocks. 
 
 SERPENTINE AND CHLORITE 
 
 Serpentine is a silicate of magnesia and iron 
 containing moisture. It has an oil green color and 
 a hardness varying from below to above that of 
 limestone. Chlorite is closely related to serpentine 
 but contains more iron and alumina. These miner-r 
 als are the hydrated decomposition products of such 
 minerals as augite and hornblende. They consti- 
 tuted the principal oil-bearing rock in the Thrall 
 oil field and are known to occur as altered intru- 
 sives at other points along the Balcones Escarp- 
 ment and in the Trans-Pecos country. 
 
 GLAUCONITE 
 
 This mineral is also known as greensand. It is 
 a silicate of iron and potassium, and is from light 
 green to very dark green in color. It will turn 
 
14 Handbook of Aids to 
 
 black on being heated to redness, and is then at- 
 tracted by a magnet. This mineral is always found 
 in small rounded grains usually mingled with sand 
 and clay. It occurs in some of the Cambrian rocks, 
 in the Taylor and the Navarro formations of the 
 Upper Cretaceous, and in the Eocene of the Ter- 
 tiary beds. A well-marked bed of glauconitic green- 
 sand is a good horizon marker in the oil fields 
 south of San Antonio. 
 
 CALCITE 
 
 Calcite, or spar, is identical in composition with 
 pure limestone. In most limestones this main ingre- 
 dient is mixed with some clayey material, and con- 
 sists of crystals of such small size as to be entirely 
 invisible to the unaided eye. The rock appears to 
 be a structureless mass. The mineral is carbonate 
 of lime, and hence consists of calcium in chemical 
 combination with oxygen and carbon. When burned 
 in a kiln it loses its carbon and forms quicklime. 
 Its hardness is less than that of soft iron and it is 
 easily scratched by a knife. Limestones in which 
 the calcite crystals are large enough to be readily 
 made out by the naked eye are often described as 
 "crystalline lime" by drillers. Most marble is of 
 this kind. The Edwards limestone in the Coman- 
 chean series of rocks is characterized by such a 
 coarse crystalline texture. In nearly all limestones 
 places are apt to be found where the original fine- 
 grained rock has been replaced by crystals of cal- 
 cite of large size which are spoken of as spar. Such 
 spar is nearly always present where underground 
 water courses traverse the formations, and quite 
 frequently they are associated with openings or 
 fissures and cavities in limestone. In the lower part 
 of the Comanchean limestones there are, especially 
 in the southwest part of the state, one or two layers, 
 some two feet thick, in which the crystalline cleavage 
 is continuous through the entire thickness of these 
 
Identification of Geological Formations 15 
 
 layers and cuttings made by the drill from these lay- 
 ers seem to be crushed crystalline calcite. 
 
 DOLOMITE 
 
 Dolomite is a mineral slightly harder than cal- 
 cite. It is a carbonate of lime and magnesia, and 
 is not as readily affected by acid as limestone, which 
 is a carbonate of lime. Dolomite is rare except as 
 it occurs in dolomitic rocks which are particularly 
 common in the Ordovician and the Permian series 
 of formations in Texas. 
 
 GYPSUM AND ANHYDRITE 
 
 Gysum and anhydrite both consist of sulphate of 
 lime. Gypsum contains in addition some water, 
 while anhydrite is free from combined water. When 
 gypsum is heated it loses its water and is said to 
 be calcined. In this state it has the same chemical 
 composition as anhydrite. "Neither one of these two 
 minerals effervesces when subjected to the action of 
 acids. Both are softer than calcite or limestone and 
 can be carved and cut with a knife. The crystals 
 of clear gypsum, known as selenite, have acute and 
 obtuse angles, and they have good cleavage in the 
 direction of their length and breadth. They will 
 split into thin leaves, which can readily be bent, 
 but which will not spring back to shape, as the 
 thin leaves of mica do. On account of its resem- 
 blance to mica, crystalline gypsum is often incor- 
 rectly called "isinglass." Anhydrite differs from 
 gypsum in its cleavage. It breaks straight in three 
 directions which are at right angles to each other. 
 Hence cuttings of anhydrite often have sharp, square 
 angles. 
 
 Gypsum occurs in many clays and shales as sep- 
 arate crystals. It frequently has formed from the 
 disintegration of pyrite in marls and other calca- 
 reous material. It is usually rare at depths below 
 
16 Handbook of Aids to 
 
 four or five hundred feet. Below this depth anhy- 
 drite mostly takes its place. This mineral occurs 
 in extensive beds in association with the cap rock 
 in our oil fields of the coast, and it underlies ex- 
 tensively the redbeds of northwest Texas. 
 
 BARITE 
 
 Barite is barium sulphate. It is white or color- 
 less and has about the same hardness as calcite, be- 
 ing easily scratched with soft iron. It is much 
 heavier than calcite. If some cuttings of barite 
 be thrown on the fire of a forge, they give a pale 
 yellowish-green color to the flame. Barite has 
 been found in considerable abundance in some of the 
 salt dome oil and sulphur fields on the Gulf Coast, 
 and it is occasionally encountered in the Permian 
 redbeds and in the Comanchean limestones. 
 
 PYRITE AND MARCASITE 
 
 Pyrite is an easily recognized mineral. It has a 
 yellow, metallic color*, frequently confused with that 
 of gold, is hard enough to scratch glass, and gives 
 sparks when hit with steel. It crystallizes mostly 
 in cubes and octahedrons. Frequently also its crys- 
 tals present five-sided flat surfaces. Consisting of 
 iron and sulphur, it is natural that this mineral 
 should give a sulphur odor when heated or when 
 violently struck with some substance of sufficient 
 hardness. One variety of pyrite often found in dark 
 clays is known as marcasite. This is usually almost 
 silver white. 
 
 Pyrite and marcasite are very common in clays, 
 shales, and slates of all colors except red and yel- 
 low. In fact, many red clays- have originally had a 
 dark color and they have become red by the ox- 
 idation of disseminated pyrite. Pyrite also fre- 
 quently occurs in limestone and in sandstone. When 
 evenly disseminated in such rocks and in micro- 
 
Identification of Geological Formations 17 
 
 scopic particles, and when present in sufficient quan- 
 tity, it gives them a dark color, or may cause them 
 to become even black. Water obtained from such 
 strata usually has the odor of hydrogen sulphide, 
 or of "rotten eggs." 
 
 This mineral has a very general distribution in 
 nearly all formations. It occurs in grains too small 
 to be seen with the naked eye and in grains and 
 crystals up to masses several feet in diameter. It 
 is even known to occur in extensive irregular lay- 
 ers several feet in thickness. It is especially apt to 
 occur at contacts between different formations. Lay- 
 ers of this kind have been reported from the base 
 of the Tertiary formations in south and east Texas. 
 They are very har t d and exceedingly resistant to 
 the drill. In coal-bearing and lignite-bearing beds, 
 pyrite frequently occurs in concretions or loaf-like 
 bodies, which lie flat with the strata. In the Aus- 
 tin Chalk this mineral often forms round balls from 
 one to two inches in diameter; the outer surface of 
 which everywhere exhibits crystalline faces. In the 
 Del Rio clay and in the Eagle Ford shale, pyrite 
 mostly occurs in irregular clusters of cubic crystals. 
 
 HEMATITE AND LIMONITE 
 
 Red and yellow ochre (hematite and limonite) 
 bear somewhat the same relation to each other* as 
 do gypsum and anhydrite. Both are oxides of iron. 
 In yellow ochre this oxide is combined with water; 
 while red ochre contains no water in combination. 
 Both are usually soft and are mostly found only 
 in the upper few hundred feet or more of deep ex- 
 plorations. They are for the most part derived 
 either from siderite or- from pyrite by oxidation. A 
 test which these minerals have in common with 
 pyrite and siderite is that they become magnetic 
 after being heated to redness. They form the col- 
 oring matter in most red and yellow rocks. In 
 east Texas extensive beds of hematite and limonite 
 
18 Handbook of Aids to 
 
 occur in the Tertiary sediments. Like pyrite these 
 minerals sometimes characterize zones of contact be- 
 tween formations of different ages. 
 
 SIDERITE 
 
 Siderite is a carbonate of iron. In the impure 
 state in which it mostly occurs it is called clay-iron- 
 stone. In hardness it slightly exceeds limestone. 
 It is also heavier than limestone. It effervesces 
 slowly in acid and becomes magnetic after ignition. 
 Clay-iron-stone has the texture and color of a dark 
 fine-grained limestone. It is the most easily smelted 
 iron ore. 
 
 This mineral occurs mostly in so called concre- 
 tions, which are loaf-like and range in size from 
 the smallest up to several feet in diameter. These 
 concretions are known as turtle-stones, and are most 
 frequent in association with beds of coal and lignite. 
 They are frequently found in marly formations such 
 as the Taylor Marl and the marls in the Midway 
 formation of the Eocene Tertiary. Drillers very 
 generally call them "boulders" when they are en- 
 countered in borings. 
 
 SALT 
 
 Salt is so generally known that it needs hardly 
 to be mentioned here. It crystallizes in cubes and 
 readily dissolves in water. Dissolved in the ground 
 water it occurs generally in the Pennsylvanian and 
 the Permian strata. On the Gulf Coast, it occurs 
 in enormous masses in association with the coastal 
 oil fields, and in the Permian of the west it under- 
 lies in several extensive beds the Panhandle and the 
 Llano Estacado. 
 
 ASPHALT, COAL AND LIGNITE 
 
 All these minerals are compounds of carbon and 
 are always black. They all burn. Asphalt readily 
 
Identification of Geological Formations 19 
 
 fuses when heated and gives off an easily recognized 
 odor. Coal and lignite burn, but do not fuse as 
 readily as asphalt. Particles of coal or lignite may 
 be identified by roasting them on a red hot tin 
 plate in a flame, when they glow for a while. Asphalt 
 occurs impregnating many limestones and filling cav- 
 ities and veins in limestones and shales. In some 
 sandstones asphalt occurs as an impregnation which 
 holds the grains together. Coal and lignite occur 
 in thin layers or thick seams and some kinds of 
 lignite are parts of trunks of trees imbedded in 
 shales and sandstones. 
 
 ROCKS 
 
 Rocks are classified into two principal groups: 
 igneous and sedimentary. Igneous rocks are formed 
 by the cooling and congelation of molten materials. 
 These may have welled out on the earth's surface 
 or may have been forced up into strata that form 
 the bedrock overlying the region where the fusion 
 occurred. Sedimentary rocks have been formed by 
 the natural and slow accumulation of sediments of 
 any and all kinds, either in the sea or on land. 
 Rocks of this latter kind underlie by far the larger 
 part of the State of Texas, and they are of chief 
 interest to drillers in the southwest. 
 
 IGNEOUS ROCKS 
 
 The only places where igneous rocks have been 
 encountered by drillers in this state are in the Cen- 
 tral Mineral Region in Llano, Burnet, Mason, Gil- 
 lespie, and Blanco counties; along the structure 
 called the Balcones Escarpment, in Kinney, Uvalde, 
 Medina, Travis, and Williamson counties; and in 
 the mountainous country west of the Pecos. No 
 igneous rocks are likely to be encountered by drill- 
 ing in any other part of the state. It has recently 
 been claimed that some igneous rock has been found 
 
20 Handbook of Aids to 
 
 in a deep boring on the Gulf Coast, but the full evi- 
 dence of this has not yet been furnished. The in- 
 trusives in the Balcones belt are practically all of 
 one and the same kind, a very tough, hard black 
 basalt, consisting mostly of augite or hornblende. 
 In many places this rock has been altered, so that 
 it is a very easily drilled green chlorite or serpentine 
 rock, softer than limestone. Such was the main 
 oil-bearing rock in the Thrall oil field. In the Cen- 
 tral Mineral Region, the chief igneous rock is gran- 
 ite. This consists of pink feldspar, with some quartz 
 and mica. There are here also extensive areas un- 
 derlain by gneiss and schist. Originally this gneiss 
 and schist was sedimentary rock, but this has been 
 changed by heat and pressure. It is now crystalline 
 and in part resembles granite. It usually contains 
 more mica than the granite and also contains horn- 
 blende. The igneous rocks of the mountainous coun- 
 try west of the Pecos are of many kinds, and can 
 not be briefly described. They belong for the most 
 part to the siliceous types, and range in texture from 
 granite through porphyries, basalts, and lavas to 
 obsidian, or natural glass. 
 
 SEDIMENTARY ROCKS 
 
 The sedimentary rocks may be classified into three 
 main groups: (1) mechanical sediments, which are 
 by far the most common; (2) chemical sediments, 
 which have been precipitated from solutions in the 
 ocean and in lakes; and (3) accumulations of the 
 remains of animals arid plants, usually called or- 
 ganic sediments, as animals and plants both are 
 organisms. 
 
 The following tables give a general view of the 
 classification of sedimentary rocks. 
 
Identification of Geological Formations 21 
 
 .- 
 
 PH 
 
 w o 
 
 II 
 
 2 
 
 60 
 
 V) 
 
 CO S ^ 
 
 EH 5? 03 
 
 38 
 
 4-> 
 ^ 
 
 03 
 O 
 
 0) 
 
 fl 
 
 o 
 
 -*-> 
 H 
 T3 
 
 
 0> 
 
 C 
 o 
 
 -4-3 
 
 1 
 
 H 
 
 
 
 
 c 
 
 
 in 
 
 m 
 
 S 
 
 .- 
 
 
 OJ 
 
 J3 
 
 s 
 
 
 0) 
 
 <u 
 
 s 
 
 *s 
 
 
 O 
 O 
 
 
 
 .2 
 
 S 
 
 'r^J 
 
 w 
 
 CO 
 
 o 
 
 .s? 
 
 jjT 
 
 -(J 
 
 15 
 X 
 
 O 
 
 03 
 
 !* 
 
 3 C) 
 
 8S 
 
 ^ ^H 
 03r^ 
 
 15 
 'O 
 
 'o 
 
 0) 
 
 .s 
 
 cC 
 
 g aft^ .s 
 
 
 
 <D 
 
 yj 
 
 
 
 
 
 o 
 
 ^ P-i <! Q O O OO 
 
22 Handbook of Aids to 
 
 In nature, the processes by which these different 
 rocks are made, seldom operate singly and the result 
 is that few if any sedimentary rocks are absolutely 
 pure. While it is very instructive for us to under- 
 stand these classifications, it is to be remembered 
 that few of these deposits are pure in nature; that 
 is, they are not always exclusively either one kind 
 or another. In practice it is misleading to regard 
 some rocks as belonging strictly to only one of these 
 groups. It is only exceptional conditions that have 
 caused nature to make anything but mixtures. Sand 
 is often mixed with some material that goes to make 
 up limestone. Rock salt is in places found mixed 
 with clay and fine sand. Fragments of shells or 
 crinoid stems may be mingled with other calcareous 
 material or with clay. Calcareous material may be 
 mixed with clay, and clay is often mixed with cal- 
 careous material. The latter mixture is so frequent 
 that it has a distinctive name and is known as marl. 
 Coaly material may be mixed with clay to such an 
 extent that it should be called coaly shale or "bone 
 coal." A sandstone may be impregnated with as- 
 phalt, in which case it is called asphaltic sandstone. 
 Mixtures of these kinds are so common that it be- 
 comes important to exercise caution in making cor- 
 rect determinations of sedimentary rocks, as when 
 two or three ingredients are mingled in nearly equal 
 quantities in the same rock. 
 
 It is also true that some sedimentary rocks can 
 be formed by either one of the three processes named. 
 Limestone, one of the most common of rocks, may 
 be entirely the result of mechanical action or silting 
 in water; it may be formed almost exclusively by 
 precipitation out of a solution, or it may result from 
 the growth of organic material. Thus some lime- 
 stones must be classified as mechanical sediments; 
 others as chemical sediments; and still others, as 
 organic sediments. 
 
Identification of Geological Formations 23 
 
 GRAVELS, CONGLOMERATES, AND PUDDINGSTONE 
 
 In running and turbulent waters, as in streams 
 and on shores, all finer sediments are carried away. 
 But such waters in many places have built up large 
 accumulations of materials of coarse grain, such 
 as gravel and sand. Deposits of this kind are in 
 time consolidated into conglomerate and sandstone, 
 
 Gravel rarely occurs in deposits of any great 
 thickness, such as to approach a hundred feet or 
 more. Much gravel is of comparatively recent or- 
 igin and occurs in the latest sediments which are 
 found in filled river channels in eastern and central 
 Texas. In the region of the Panhandle and the 
 Staked Plains there are quite extensive sheets of 
 gravel in the Pleistocene and Tertiary deposits. These 
 are frequently water-bearing. Some of our depos- 
 its of gravels are found on the uplands as well as 
 along stream channels in the central and northern 
 part of the state. Even the most 'recent gravels are 
 frequently cemented with lime so as to form quite 
 solid rock, or conglomerate. 
 
 Conglomerates, which are indurated gravels, oc- 
 cur sparingly in the Pleistocene and Tertiary on the 
 Gulf Coastal Plain. Sheets of conglomerates con- 
 sisting mostly of flint occur in the Pennsylvanian, 
 especially of northern Texas. A few of these con- 
 glomerates contain considerable feldspar in the form 
 of pebbles, and drillers have sometimes reported 
 these as granite, feldspar being present as the prin- 
 cipal ingredient in granite. Such rock should be 
 classified as conglomerate, whatever the composi- 
 tion of the pebbles happens to be. Conglomerates 
 are quite frequent in the Triassic formation in the 
 northwestern part of the state. These conglomer- 
 ates nearly always contain well-rounded pebbles of 
 quartz of a yellowish color, and they are in places 
 characterized by being impregnated with black oxide 
 of manganese. Most of the Triassic conglomerates 
 give good water. One conglomerate, quite like .the 
 
24 Handbook of Aids to 
 
 Triassic conglomerates, occurs in the Permian red- 
 beds in Tom Green, Coke, Runnels and Taylor coun- 
 ties. Conglomerates consisting mostly of small peb- 
 bles of white quartz mingled with sand and lime, oc- 
 cur in the Trinity sand. 
 
 Puddingstone is rare in the southwest, being found 
 only in some ancient schists and gneisses. 
 
 SANDS, SANDSTONES, AND QUARTZITES 
 
 Sand and sandstone present a great variety of 
 rocks. They consist of more or less rounded grains 
 which have been worn to their present size and form. 
 When these are entirely incoherent, we have sand. 
 In such case, the cuttings will consist of loose grains 
 all separated from each other. When the deposit 
 has become somewhat more indurated, the cuttings 
 will frequently show fragments consisting of grains 
 that adhere to each other, and we call the rock 
 sandstone. The grains are usually held together 
 either by a greater or less quantity of calcareous 
 material, or of oxide of iron, which are usually pres- 
 ent as cement. 
 
 In rare cases the sand grains are cemented to- 
 gether by pyrite. Such sandstones are usually black 
 and when ground up by the drill they cause the wa- 
 ter to become inky. 
 
 The fineness of the grain is spoken of as the tex- 
 ture of sandstones. The variation in this respect is 
 considerable. Some of the sandstones in the Trinity 
 formation consist of fairly fine grains and are soft. 
 These are known to drillers as pack sand. 
 
 In more than ninety per cent of all sandstones the 
 grains consist of quartz. In some sandstones the 
 grains are calcareous. Such is the Anacacho lime- 
 stone in the Upper Cretaceous. 
 
 Micaceous sandstones are such as contain mica. 
 Standstones of this kind are usually fine-grained. 
 The Triassic sandstones in northwest Texas are as 
 a rule highly micaceous. Sandstones occurring in 
 
Identification of Geological Formations 25 
 
 the Pennsylvanian and Permian are almost free 
 from mica in the central part cf the state. West of 
 the Pecos mica is more frequent in the Pennsylvan- 
 ian sandstones than farther east. Sandstones of 
 the Cretaceous in the southwest are almost devoid 
 of mica. 
 
 Ferruginous sandstone contains limonite or hem- 
 atite, which colors them red. Red sandstones of 
 this kind are most common in formations that con- 
 tain coal or lignite, and also in formations that 
 contain salt, as in the Permian in this state. The 
 ferruginous material has been in most cases in- 
 troduced by water flowing through the sands, long 
 after the sands were made. 
 
 Sandstones are usually more or less porous. When 
 these porosities have been filled with asphaltic ma- 
 terial they are called bituminous sandstones. When 
 the porosities are filled with oil they are spoken of 
 as oil sands, and when filled with gas, they are 
 called gas sands. When they are filled with water, 
 they are called water sands; when entirely empty, 
 they are called dry sands; and when filled with cal- 
 careous material we speak of them as cemented sand- 
 stone. 
 
 Borne sandstones have been so greatly hardened 
 by pressure or by infiltration of siliceous material 
 that they will break through the grains rather than 
 around them. This is the distinguishing feature of 
 quartzite. Reck of this kind occurs in association 
 with some schist and gneiss in the region of the an- 
 cient altered rocks in the central part of Tex?s. and 
 also locally in Tertiary sands on the Gulf Coast 
 Plain. The Trinity Sand has also been changed to 
 quartzite in a few widely separated areas of very 
 limited extent. 
 
 CLAY, SHALE AND SLATE (Argillites) 
 
 Argillites is a general name used to include a large 
 group of sedimentary deposits, which when first 
 
26 Handbook of Aids to 
 
 formed were mud-like or clay-like in nature. They 
 consist of the silt and clay brought into the sea and 
 other waters by rivers and smaller streams. These 
 materials finally settle on the bottom of the waters. 
 Rocks resulting from such deposits consist of min- 
 eral particles of small size, from the very finest sand 
 down to the smallest infinitesimal sizes. The coarser 
 argillites may be described as silty or even sandy 
 clays, or shales. In mineral composition there is 
 also considerable variation in this class of sediments. 
 They differ from sands and sandstones in containing 
 more aluminum-bearing material and less of silica. 
 Sands consist mostly of quartz while clays are large- 
 ly derived from the more easily crushed and softer 
 minerals, such as feldspar, and hornblende, and from 
 the insoluble residues of limestone and other disin- 
 tegrated rock. 
 
 The distinction between clay, shale, and slate is 
 based on hardness. Clay is soft and plastic. All 
 argillites, when first deposited, have been clay. In 
 time clay will harden under the burden of thousands 
 of feet of overlying formations. All argillites of 
 the latest age, the Quaternary, are yet soft clay. 
 Argillites of Tertiary age are in the condition of 
 clay, except in mountainous regions where they haVe 
 been subjected to heat and pressure. The same is 
 true to a lesser degree of the argillites of the Cre- 
 taceous, the Jurassic and the Triassic ages, which 
 are still older than the Tertiary. In the Permian 
 and the Pennsylvanian and other yet earlier Pal- 
 eozoic formations, most clays have been hardened to 
 a condition we find in shale, and in mountainous 
 regions this has acquired the yet greater hardness we 
 find in slate. In the crystalline rocks of the Archean 
 formations nearly all clay has been changed to the 
 condition we designated as slate or to some rock 
 still harder than this. 
 
 Of all rocks, clays and silts are most easily pen- 
 etrated by the drill. Three and even four hundred 
 feet of hole have sometimes been made in twenty- 
 
Identification of Geological Formations 27 
 
 four hours through such deposits. But at depth, 
 very fine-grained clays and sometimes silty clays 
 cause much trouble by "caving." Some clays which 
 cave are the so called "joint clays" or "slip clays," 
 which are cut by slippery joints that cause the 
 ground to give way and fall into the hole from its 
 walls. Some fire clay is of this kind. These clays 
 are specially apt to be found for some distance be- 
 low seams of coal and lignite and below layers' of 
 very black, laminated and resistant shale, often er- 
 roneously called "slate." Some caving shale breaks 
 into long and slender splinters and this has by some 
 been called "pencil cave" and "shoe peg" shale. The 
 best practical distinction between clay and shale is 
 to classify as ?lay only such material as is nearly 
 entirely soft and plastic and to regard as shale any 
 argillite which to some considerable . extent comes 
 up from the boring in solid broken fragments. 
 
 Owing to the presence or prevalence of certain 
 mineral ingredients, different kinds of clay, shale 
 and slate are to be noted. 
 
 Marl 
 
 A clay may be described as calcareous, when it 
 contains sufficient calcareous material to cause slow 
 effervescence on the application of acid. A clay 
 containing a considerable amount of calcareous ma- 
 terial is called marl, which may or may not also 
 contain some red or yellow oxide of iron. Nearly 
 all the clay formations of Cretaceous age in this 
 state are marly. Shale and clay in the Pennsylva- 
 nian and the Permian are quite commonly devoid 
 of a calcareous ingredient. In the Pleistocene de- 
 posits we find both marls and non-calcareous clays. 
 
 Fire-clay 
 
 Fire-clay is free from lime, as a rule. It occurs 
 in beds which were at one time exposed and leached 
 
28 Handbook of Aids to 
 
 at the surface and later, perhaps, buried under peat 
 which since has changed to lignite or coal. In tex- 
 ture, fire-clay is quite variable, from the finest clay 
 to silty and quite sandy clay. Fire-clays occur es- 
 pecially in coal and lignite-bearing formations, such 
 as the Pennsylvanian, parts of the Eocene Tertiary, 
 and the coal-bearing beds of the Cretaceous on the 
 Rio Grande. 
 
 Fuller's Earth 
 
 Strata of white kaolin-like clay occur in the lower 
 part of the Taylor marl and .at several levels in the 
 early and middle Tertiary. These strata are, for 
 the most part, deposits of volcanic dust, which has 
 been hydrated, and which are known in some cases 
 to be suitable for use as fuller's earth. Fuller's 
 earth is also found in the Tertiary. 
 
 Micaceous Clay and Shale 
 
 The shales and clays of relatively coarse texture, 
 such as silts, sometimes contain much mica and may 
 be called micaceous shales and clays. 
 
 Ferruginous Clay and Shale 
 
 In the Permian of northwest Texas much clay, 
 shale, and silt has a red color due to the presence 
 of oxide of iron. These deposits have long been 
 known as the "red beds," owing to their red color., 
 Red clays and shale occur also in the Pennsylvanian 
 as well as in deposits of other ages. 
 
 Bituminous Clay and Shale 
 
 This name is applied to argillaceous deposits con- 
 taining bituminous materials such as oil or asphalts. 
 They are nearly always of a dark color or even black, 
 and generally give "rainbow" colors and even strong- 
 er showings of oil, when explored in drilling. 
 
Identification of Geological Formations 29 
 Carbonaceous Clay and Shale 
 
 Shales containing coaly material in a finely com- 
 minuted state are dark or even black and grade into 
 impure coal, "bone coal." Shales of this kind are 
 common in the Pennsylvanian formations, and also 
 occur in association with lignite in the Eocene Ter- 
 tiary. 
 
 Soaps tone 
 
 This name is often incorrectly given to light-col- 
 ored shale. No soapstone is found in the sedimen- 
 tary rocks. The rock known by this name is a result 
 of metamorphism and occurs mostly only in asso- 
 ciation with crystalline rocks. 
 
 Gumbo 
 
 Any clay which will "gum" or adhere to the drill, 
 is called gumbo. The term is in general use mostly 
 among drillers on the Gulf Coast. 
 
 LIMESTONES 
 
 A limestone is a rock that is composed chiefly 
 of carbonate of lime. Rocks composed of carbonate 
 of lime and of magnesia are known as magnesian 
 limestones, or dolomites. The amount of magnesia 
 in a dolomite varies considerably, from eight to thirty 
 per cent. Almost all limestone contains a small in- 
 gredient of clay and siliceous material. The purest 
 limestones have less than one per cent of these in- 
 gredients. In some limestones they may amount to 
 as much as twenty per cent, or even more. A cal- 
 careous rock that contains more clay than carbonate 
 of lime or magnesia is usually called a marl, especi- 
 ally when not indurated. 
 
 The usual color of limestone is light gray but a 
 limestone may have any color, from the many hn- 
 
30 Handbook of Aids to 
 
 purities which may enter into its composition. Some 
 limestones are colored red from the presence of iron 
 oxide. This red may vary from yellow and orange, 
 through red to purple. Limestones may be green 
 or greenish from the presence of other compounds of 
 iron, as in the case of glauconitic and chloritic in- 
 gredients. The cuttings of limestones that come 
 from below the level of moisture in the ground 
 very generally have a bluish gray tinge, mostly 
 owing to the presence of ferruginous material. 
 Under the influence of weathering such limestones 
 acquire the familiar yellowish, straw color. Some 
 limestones are dark gray or even black, due in 
 some cases to the presence of manganese, pyrite 
 or in other cases to the presence of bituminous 
 material. As regards their texture, limestones 
 vary greatly. Most limestones which are not dolo- 
 mitic have a fine texture and under the impact 
 of the drill they break into fragments that are more 
 or less equal in diameter in all directions, though 
 there are many exceptions to this rule. Some lime- 
 stones are thinly laminated and break into flaky 
 fragments. Some limestones are highly porous and 
 others again are quite compact, and break with 
 smooth and curving fracture almost like flint. The 
 hardness of limestone is such that it can easily be 
 scratched with soft iron. Soft limestones can be 
 scratched with the finger nail. The usual progress 
 of drilling in a limestone is at the rate of ten feet 
 in twelve hours, with a standard rig, and a little 
 less with a rotary rig, using a Hughes bit. It is 
 said that a pure limestone has a tendency to wear 
 the angles of a drop bit so as to round its cutting 
 edges. 
 
 Drillers usually identify limestones by mere optic 
 examination of the cuttings. In many cases this is 
 unsatisfactory. Beds of gypsum and anhydrite have 
 often been in this way identified as limestones. The 
 only reliable and quick test by which limestone may 
 be identified, is by the application of dilute hydro- 
 
Identification of Geological Formations 31 
 
 chloric acid. This is best done by placing a few 
 drops of the acid on a flat glass and letting a clean 
 washed small fragment of rock fall in this acid. 
 Fragments of limestone will invariably effervesce in 
 the acid, and the effervescence is always prompt and 
 active. Dolomite effervesces slowly. The finer the 
 fragments the mere violent is always the efferves- 
 cence. 
 
 Varieties of Limestone 
 
 When examined closely, hardly any two limestones 
 will be found alike. With practice, some of the va- 
 rieties of limestone can be distinguished with the 
 naked eye, or at least with a hand lens. 
 
 Oolitic Limestone 
 
 Oolitic limestone consists of small, almost perfect- 
 ly spherical grains of the size of fish roe down to 
 fine sand. When the oolitic grains are of sufficient 
 size, the rock is easily identified with the naked eye. 
 Some oolites consist of grains so small that they 
 can be identified best in a thin section examined un- 
 der the microscope. The ooliths are believed to have 
 acquired their round form by rolling and by the ac- 
 cretion of calcareous material present in abundance 
 in the water where the rock was formed. It is also 
 believed that in many instances the formation and 
 growth of ooliths have been influenced by micro- 
 scopic organisms. In many ooliths it can be made 
 out that the crust of the grains has been deposited 
 around a small center of mechanical origin. Oolitic 
 limestones occur in formations of all ages. The pro- 
 portion of ooliths to the mass of the rock varies from 
 small to large fractions. Under the impact of the 
 drill, oolitic limestone is to a large extent separated 
 into the original oolitic grains and it is not infre- 
 quently mistaken for sand. In this state horizons of 
 oolitic material occur at several geological horizons. 
 
S2 Handbook of Aids to 
 
 In the lower part of the Ellenburger limestone such 
 rock has in places been silicified, so that we find it 
 containing layers of oolitic chert. The ooliths in 
 this horizon are relatively large and almost perfect- 
 ly spherical. They are also trenchantly marked off 
 from the matrix in which they are imbedded. In 
 the Pennsylvanian, oolitic limestones are very rare, 
 but again in the Permian, especially of- the western 
 part of the state, they are frequent. Some occur in 
 the Permian redbeds. In many of these Permian 
 limestones the ooliths are of small size, and not al- 
 ways clearly denned in outline. In the Comanchean 
 and Upper Cretaceous, oolitic limestones are prac- 
 tically absent. They are rarely if ever found in 
 limestones of Tertiary age in the Southwest. . 
 
 Shell Breccia 
 
 Some limestones consist of fragments of shells 
 which have been broken up and more or less worn 
 by water currents. These may be called shell brec- 
 cias, organic conglomerates, or organic sandstones, 
 according to the degree of wear of the organic frag- 
 ments from which they are made. Many limestones 
 are a mixture of fine calcareous slime material and 
 fragments of organic bodies. Such are most of the 
 limestones in the Pennsylvanian. The Anacacho 
 limestone of the Upper Cretaceous formation is for 
 the most part an organic sandstone with coarser ]ay- 
 ers that can be classed as shell breccias. Rocks of 
 this kind frequently take their name after the bulk 
 cf the organic materials of which they are composed. 
 Coral limestone consists mainly of coral fragments. 
 Crinoidal limestones consist of joints of crinoid 
 stems. Limestone of this latter kind is known in 
 Texas only in the Pennsylvanian formations. 
 
 Chalk 
 
 In some parts of the sea, in the past as well as 
 at present, foraminifera have existed in such abun- 
 
Identification of Geological Formations 33 
 
 dance in the sea that their minute shells have formed 
 an ooze on the bottom of large areas in the ocean. 
 When such deposits are consolidated, they form the 
 porous, soft rock known as chalk. When exposed 
 and weathered, chalk is usually of a white color. 
 When encountered in wells it is sometimes of a 
 bluish-gray color, and so soft as to be only slightly 
 different from some marls. Chalk is confined al- 
 most exclusively to the Austin formation in the 
 upper Cretaceous. 
 
 Lithographic Limestone 
 
 This is characterized by an exceedingly fine and 
 uniform texture and the name may be applied to 
 any limestones of this texture. So far as known, 
 no lithographic limestone suitable for use in the 
 arts has been found in Texas, but limestones of a 
 very fine texture characterize at least two geologic 
 horizons. One of these horizons is the upper part 
 of the Ellenburger limestone; another is the Buda 
 limestone of the Cretaceous. Thin limestones of 
 like texture occur also at certain horizons of the 
 Permian above the Clear Fork formation. These 
 are thin and usually somewhat impure. Some of 
 them are characterized by a peculiar delicate light 
 blue color. 
 
 Dolomite 
 
 Dolomite and dolomitic limestones are character- 
 ized by definite and clear crystallization of the mass. 
 On the application of acid, effervescence is slow, un- 
 less the rock is finely pulverized. The crystals vary 
 considerably in size but are usually uniform in this 
 respect in individual layers and beds. Usually dolo- 
 mitic limestones are a little more resistant to the 
 drill than other limestones. Dolomitic limestones 
 make up the lower two-thirds of the Ellenburger 
 formation. Locally there are some, layers of dolo- 
 
34 Handbook of Aids to 
 
 mitic limestone in the lower part of the Comanchean. 
 Dolomitic limestone is rarely found in the Pennsyl- 
 vanian and in the Upper Cretaceous. It is abun- 
 dant in the Permian. 
 
 Caliche 
 
 Caliche is a white, porous rock, indurated to any 
 extent, or perfectly floury. Such deposits are 
 formed by the deposition of lime from water evap- 
 orating from the surface of the ground or from the 
 water table below the surface. Usually it occurs as 
 a part of the surface soil. In the region of the High 
 Plains of northwest Texas it underlies the yellow 
 adobe soil at depths varying from zero to thirty 
 or forty feet. In this region it is soft white rock, 
 somewhat resembling chalk, and it is locally known 
 as the "rim rock." It is made up of exceedingly 
 fine calcareous crystals of carbonate of lime, and 
 effervesces with great briskness on the application 
 of acid. 
 
 * 
 
 Travertine 
 
 Travertine is a crystalline or occasionally struc- 
 tureless calcareous rock of somewhat more than 
 common hardness. It is sometimes encountered as 
 fillings in underground caverns and is in many 
 cases colored red by the presence of ferruginous ma- 
 terial. On account of its crystalline structure and 
 its color it has in some cases been reported by drill- 
 ers as granite. 
 
 Bituminous Limestone 
 
 This is a limestone impregnated with asphalt or 
 other bituminous material. It can usually be recog- 
 nized by its odor upon heating, or upon crushing 
 after it is thoroughly dried. Limestones so impreg- 
 nated are usually quite resistant to the drill. For 
 
Identification of Geological Formations 35 
 
 the most part they are thin. The thickest known 
 are m the Permian of West Texas, near the middle 
 of the Permian section. 
 
 GYPSUM AND ANHYDRITE ROCK 
 
 Any large accumulation or bed of these minerals 
 is to be regarded as rock and should be identified 
 by drillers in describing the ground explored. 
 
 When sulphate of lime is precipitated in the sea, 
 it may result in the formation of either of these two 
 rocks. Both are characterized by their softness. 
 They can be scratched with the finger nail. Neither 
 of the two is attacked by acid, and this furnishes 
 a ready method for identification. A soft sedimen- 
 tary rock which resembles limestone in texture and 
 which will not effervesce in acid, is in nine cases 
 out of ten either gypsum or anhydrite. Anhydrite 
 may sometimes be known from its behavior in drill- 
 ing. When it is ground up into very fine powder 
 by the drill, it is apt to take up water and set like 
 plaster of Paris, adhering strongly to the bit, even 
 to such an extent as to interfere with the work. 
 Anhydrite as well as gypsum occurs in association 
 with the salt beds of the salt domes in the coast coun- 
 try and there they are, like the salt deposits, limited 
 in horizontal extent but often quite deep. Usually 
 the anhydrite occurs under the cap rock and always 
 above the salt. It has frequently been reported as 
 limestone by drillers in the coastal fields. While 
 small crystals and small bodies of gypsum and anhy- 
 drite may be found in many clays, it is only in the 
 Permian and perhaps some of the Comanchean beds 
 in the western part of the state that these minerals 
 exist as extensive formations. Most of the sulphate 
 of lime in the Permian sea seems to have been de- 
 posited originally in the form of anhydrite, for it is 
 seldom that any gypsum exists in these beds below 
 300 or 400 feet, from the surface. Below this depth 
 nearly all sulphate of lime is anhydrite. Extensive 
 
36 Handbook of Aids to 
 
 beds of anhydrite underlie the Panhandle and the 
 Llano Estacado and extend nearly to the central part 
 of the state in the lower part of the Permian. An- 
 hydrite as well as gypsum has frequently been re- 
 ported as limestone. The most fine grained kinds of 
 gypsum and anhydrite are also called alabaster rock. 
 
 ROCK SALT 
 
 Rock salt is what the name implies, salt in large 
 masses. In the salt domes of the coast these masses 
 are known to be several thousand feet in thickness. 
 They do not here reach any great distance horizon- 
 tally, but are limited to the salt domes, which seldom 
 extend more than a few miles in any direction. The 
 only formation which contains rock salt in exten- 
 sive horizontal beds, is the upper part of our Per- 
 mian in the Panhandle and in the Llano Estacado 
 regions. Here we find salt beds from a few up to 
 300 feet in thickness, evidently extending scores of 
 miles underground. They represent evaporated brine 
 from embayments in the sea in which the Upper 
 Permian was laid down. At times in this sea, salt 
 was evidently accumulating at the same time with 
 fine sand and silt, and the result is that we find 
 beds which consist of half salt and half silt. In 
 the Spur well, such beds were some scores of feet 
 in thickness. 
 
 In drilling through rock salt, especially with a 
 rotary, it sometimes happens that the ground-up 
 rock is entirely dissolved by the water used. In 
 such cases it has sometimes puzzled inexperienced 
 men that there are no "returns." The rock is 
 all dissolved and there are no cuttings from the 
 ground penetrated. 
 
 FLINT AND CHERT 
 
 All calcareous muds that have been consolidated 
 into limestone contained originally some siliceous 
 
Identification of Geological Formations 37 
 
 material* Under the influence of moisture present 
 in the ground, different minerals have a tendency 
 in all rocks to segregate. Each mineral tends to 
 form separate bodies of its own kind. By a process 
 somewhat resembling the growth of crystals, there 
 are thus formed frequently in sedimentary rocks 
 bodies of materials of different composition from 
 the mass of the rock itself. These bodies are called 
 concretions. Their shape usually is more or less 
 determined by the structure of the rocks. In bedded 
 rocks there is a tendency for concretions to be flat 
 or loaf-like. When the siliceous materials present 
 in limestones have gathered in concretions, we call 
 these bodies flint or chert. Concretions of this kind 
 have formed in many limestones and are frequently 
 characteristic of certain layers in these limestones. 
 Flint has a hardness a little in excess of the steel in 
 a file. It will cut glass and indent some steel. It 
 may have any color, but is usually gray or grayish- 
 white when found below the surface. It can usually 
 be known from its fine texture, and from its tend- 
 ency to split into very sharply angular fragments. 
 When viewed with a magnifying glass the chert is 
 translucent along thin edges. As quartz is not af- 
 fected by acids, flint and chert can readily be told 
 from limestone when present in cuttings. With 
 practice it is readily recognized without any test. 
 Flint is frequent in the Pennsylvanian limestones 
 and in parts of the Ellenburger limestone of the 
 Ordovician. It is infrequent in our Permian lime- 
 stones, but quite abundant in the Edwards limestone 
 of the Comanchean rocks. In the Upper Cretaceous, 
 flint is rare, and it is almost unknown in our Ter- 
 tiary sediments. 
 
 CLAY-IRON-STONE 
 
 Just as silica is present in limestone, so we have 
 carbonate of iron present in shales. In some shales 
 this carbonate of iron is distributed throughout the 
 
38 Handbook of Aids to 
 
 deposit, while in other cases it has formed concre- 
 tions and gathered together into nodules and loaf- 
 like bodies ranging in size from a few inches to many 
 feet in diameter. Occasionally an accumulation of 
 carbonate of iron forms separate layers, which, no 
 doubt, were laid down originally in their present 
 condition. When heated in a flame, carbonate of 
 iron is reduced in part to metallic iron and becomes 
 magnetic, so that if a magnet is passed through 
 the cuttings the roasted fragments of clay-iron-stone 
 will be picked up. It is also attracted by acid but 
 the effervescence is much more slow than it is in the 
 case of limestone and dolomite. Clay-iron-stone is 
 of a fine texture and gray in color. It is quite hard 
 and very tough and resistant to the drill. Most 
 clay-iron-stone concretions are traversed by thin 
 veins of calcite which is frequently colored brown 
 from the presence of iron. 
 
 Concretions of this kind are frequent in all dark 
 clays and shales. Drillers often record them as 
 boulders. They are, of course, not true boulders, 
 since they have grown into shape in the place where 
 they are found. Concretions of carbonate of iron 
 occur in a few horizons of the dark shales of the 
 Pennsylvanian. They are frequent and often of 
 large size in the Taylor Marl and in the Navarro 
 formation of the Upper Cretaceous, and they are 
 quite common in the clays and marls of the Tertiary. 
 The true nature of these formations can easily be 
 ascertained by the fire test, as already mentioned. 
 
Identification of Geological Formations 39 
 
 THE PRINCIPAL FORMATIONS IN TEXAS 
 INTRODUCTORY 
 
 What is known among geologists as "formations" 
 are extensive sheets of more or less similar rock 
 material that have been deposited under somewhat 
 similar conditions, over large areas. Just how 
 much is to be called one formation, is to a great 
 extent dependent on the judgment of those who de- 
 scribe the formations. In cases where the forma- 
 tions are clearly marked by difference in the nature 
 of the materials of which they consist, or by differ- 
 ences in the fossils they contain, there is seldom any 
 disagreement as to their limits. The horizontal ex- 
 tent of formations is very variable, ranging from 
 a few miles to several hundred miles. As the ex- 
 istence of water, oil, and other minerals, which are 
 sought by drilling, can to some extent be foretold 
 from the nature and position of the formation in 
 which they occur, it is of much importance to the 
 practical driller to be able to distinguish the dif- 
 ferent formations he penetrates. This is usually 
 done by examining the cuttings as the drill descends. 
 Such identification of formations requires a most ex- 
 tensive as well as intimate knowledge of the rock of 
 which they consist. In making them, thfe driller has 
 an advantage over the geologist in that he has op- 
 portunities to make frequent examination of the ma- 
 terials through which he drills. On the other hand, 
 the geologist usually has the advantage in possess- 
 ing extensive knowledge of the formations from ex- 
 amining them in exposures at the surface, and in 
 widely separated places. This enables him to better 
 interpret his observations on the material brought 
 up by the drill. It is higTily desirable that the ef- 
 forts of drillers and of geologists should be com- 
 bined in securing exact knowledge of our formations 
 
40 Handbook of Aids to 
 
 as they exist underground. These notes are writ- 
 ten in the hope that they may be of some aid in this 
 direction. They are brief descriptions of the prin- 
 cipal formations which are encountered by drillers 
 in the southwest. 
 
 In these notes, the formations have been described 
 in order from above downward. This is the order in 
 which the driller encounters them. It is hardly nec- 
 essary to call the attention of the reader to the fact 
 that all of these formations, from the top to bottom, 
 are not found at any one place. Some are always 
 absent. 
 
 The latest formation overlies the older rocks al- 
 most everywhere, especially in stream valleys and 
 on the coast. There is a great accumulation of late 
 deposits along the Gulf Coast. Formations of ear- 
 lier age than these are encountered, superficially, in 
 succession further inland away from the Gulf, until 
 we come to the Central Mineral Region in Llano 
 and surrounding counties. Here we encounter some 
 of the very oldest rocks. From this point north- 
 westward we find again successively younger beds 
 out to the northwestern part of the state, where 
 recent deposits overlie the older ones on the High 
 Plains. In the region west of the Pecos River, the 
 country has been broken up into parts of relatively 
 small size, and we find all the formations outcrop- 
 ping at the surface within quite short distances of 
 each other* 
 
 Everywhere it will be found that some of the 
 formations described are missing, in local sections. 
 Gaps of this kind are in many cases due to earlier 
 periods of erosion, by which some formations were 
 removed before later ones were deposited. In such 
 case the beds below and above the missing member 
 or members, are said to be separated by unconformi- 
 ties. 
 
 A peculiar condition exists on the coast. By far 
 the greatest part of all borings on the coast have 
 
Identification of Geological Formations 41 
 
 been made in the so called "salt dome oil fields'* 
 in that region. More than a score and a half of 
 salt dome structures have been found on the coastal 
 plain, mostly within a hundred miles of the Gulf 
 Coast. These structures are dome-like and cover 
 relatively small areas from one to three or four 
 miles in diameter. Their origin is not xet fully un- 
 derstood. They may be said to resemble plug-like up- 
 lifts of ground, where beds lying elsewhere at a depth 
 of 3000 to 4000 feet have been pushed up to with- 
 in 1000 feet of the surface or even up to the surface- 
 Where most of the drilling in the Gulf Coast oil fields 
 has ben done, the original position of the formations 
 is therefore quite destroyed and the descriptions and 
 measurements which are given in the following note 5 . 
 on the Pleistocene and Tertiary do not apply to the 
 positions which these formations have in the salt 
 dome oil fields, where most drillers have had their 
 experience. They apply only to the positions that 
 these rocks occupy in the wide tracts which separate 
 these domes from one another. 
 
 PLEISTOCENE AND RECENT FORMATIONS 
 
 Our latest deposits are made up of the soil, and 
 of alluvial sediments of sijt, sand and gravel along 
 streams everywhere. On the Gulf border, littoral 
 sediments, such as clays, sands, and gravels, and 
 mixtures of such materials of varied color, extend 
 to depths of from a few to 1600 feet. These thin 
 rapidly inland, and disappear entirely in from fifty 
 to ninety miles from the Gulf. Logs of wood, shells 
 like those from the present Gulf waters and bones 
 of larger animals, such as elephants, horses, the 
 bison, and the deer, are occasionally to be found. 
 In a recent boring in Kleberg County, the minute 
 oval and spirally marked spore-fruits of Chara were 
 found in cuttings coming from 800 feet down, in 
 these beds. The High Plains of the west are also 
 covered with loam sand and gravel of this age. 
 
42 Handbook of Aids to 
 
 Almost everywhere the Pleistocene and recent form- 
 ations are entirely unconsolidated. 
 
 THE TERTIARY FORMATIONS 
 
 The tertiary in Texas measures about 5000 feet 
 in greatest thickness. It underlies the Pleistocene 
 of the Gulf Coast and extends in a belt diagonally 
 from Louisiana to Mexico. This belt is about 120 
 miles wide on the Rio Grande, 70 miles wide in the 
 middle of its course, and 180 miles wide in the 
 east part of the state. The oldest formations of the 
 Tertiary outcrop farthest inland and the youngest 
 formations appear nearest the Gulf. The entire 
 Tertiary dips toward the Gulf at an average rate of 
 from twenty-five to seventy feet per mile. 
 
 The Tertiary can be divided into three parts: the 
 upper, the middle, and the lower. 
 
 THE UPPER TERTIARY 
 
 On the coastal plain, the Upper Tertiary consists 
 of deposits much like those of the Pleistocene, such 
 as clays of various shades of color, sand, and gravel. 
 The clay is often reported as gumbo by drillers. In 
 cuttings from this part of the Tertiary, fragments 
 of bone and of silicified and lignitized wood are 
 sometimes found. Small shells of fresh water mol- 
 luscs (such as clams and snails) and fragments of 
 larger molluscs are somewhat common, in cuttings 
 from these beds on the coast. Some of the sands 
 are coarse, some fine in texture. In the clays, small 
 calcareous concretions are frequently found. Inland, 
 the upper Tertiary deposits are mostly old alluvial 
 accumulations yet but little known. Their thick- 
 ness is very variable, at different points, but prob- 
 ably nowhere exceeds four hundred feet in the in- 
 terior of the state. 
 
Identification of Geological Formations 43 
 
 THE MIDDLE TERTIARY 
 
 On the Gulf Coastal Plain, the Middle Tertiary 
 consists of two divisions. The Dewitt formation, 
 from to 800 feet thick, lies uppermost. It con- 
 sists mostly of slightly indurated sand with some 
 streaks of clay. These sands and clays often con- 
 tain bones and ancient oyster banks. The lower 
 part of the Middle Tertiary, known as the Fleming 
 Clay, is from 200 to 400 feet thick. It is very gen- 
 erally reported by drillers as "gumbo," as it is fine- 
 textured and sticky. It is noted for containing 
 many small nodules or concretions of calcareous ma- 
 terial. 
 
 In the High Plains of the Panhandle, and the 
 Llano Estacado, the Middle Tertiary is represented 
 by fairly loose sands and clays, mostly less than 100 
 feet thick, and from light gray and greenish-gray 
 to brown in color. Fragments of bones of mammals 
 are quite frequent. 
 
 The Catahoula sandstone may be said to be the 
 lowermost formation of the Middle Tertiary, meas- 
 uring from 500 to 800 feet. It is coarsest below and 
 contains some clay above. The coarser sand of this 
 formation, especially near its base, is sometimes 
 characterized as "rice sand." This is because many 
 of the larger sand grains are oval. They consist of 
 a slightly translucent quartz resembling agate or 
 chalcedony. The sands of this formation are usu- 
 ally soft but locally they may be hard. Fossil wood 
 is quite common in this formation. 
 
 THE LOWER TERTIARY 
 
 The Lower Tertiary occurs only on the inner Gulf 
 Coast Plains, where it is represented by no less than 
 seven different formations. 
 
 Under the Catahoula is the Jackson formation 
 in iSabine, San Augustine, and Angelina counties. 
 It consists of some 250 feet, or less, of marly clay 
 
44 Handbook of Aids to 
 
 with sea shells and large calcareous concretions, 
 usually known as "boulders." It can often be iden- 
 tified under the microscope by the presence of many 
 angular grains of volcanic dust. 
 
 Next in downward order is the Yegua clay, rang- 
 ing from 375 to 750 feet in thickness. The clay is 
 greenish-gray and dark gray. It contains lignite 
 in places and is characterized by concretions and 
 crystals of gypsum, and by frequent minute siliceous 
 tests of diatoms, some of which are of relatively 
 large size and spherical in form. 
 
 The Yegua is underlain by the Cook Mountain 
 formation, which consists of sand and clay. Both 
 the sand and the clay often contain green grains of 
 glauconite, frequent fossils, many concretions, and 
 often continuous layers of clay-iron-stone. Thick- 
 ness, 400 feet. 
 
 The underlying Mount Selman formation is some 
 350 feet thick and consists of materials somewhat 
 like those of the Cook Mountain, but in part red, 
 and with more lignite. 
 
 The Wilcox formation measures some 1000 feet 
 in thickness. This formation consists of marly 
 clays and beds of sand some of which yield much 
 water. There is also much lignite and some glau- 
 conite. Fossil shells and plants are frequent . in 
 some of the marl. 
 
 The Midway is the lowest of the Tertiary forma- 
 tions. From Hopkins to Falls County this forma- 
 tion contains considerable white limestone, which is 
 an indurated shell breccia or a sandstone made up 
 of mostly worn organic grains mingled with some 
 foraminifera, and cemented by a calcareous matrix. 
 The greater part of the formation which is some 400 
 feet in thickness consists of thin-bedded marly clays, 
 with imbedded small fossils and black shreds of 
 vegetation. Minute tests of foraminifera are quite 
 common. 
 
Identification of Geological Formations 45 
 THE UPPER CRETACEOUS FORMATIONS 
 
 The Upper Cretaceous formations are a succession 
 of clay, shale, or marl, chalky flags, and sandstone. 
 They outcrop in a belt extending from the northeast 
 corner of the state westward to beyond Dallas, from 
 there south to San Antonio, and thence west to Del 
 Rio. The thickness of the entire Cretaceous is near 
 5000 feet. In east and central Texas, the formations 
 of this age dip toward the Gulf at a varying rate 
 from a few to some eighty feet to the mile, and 
 south and east from the indicated belt they pass 
 under the deposits of Tertiary age, going deeper and 
 deeper as we approach the Gulf. Here they may lie 
 more than 5000 feet below the surface. West of the 
 Front Range, the Cretaceous occurs north of and 
 around the Chisos Mountains, between the Chinati, 
 the Eagle, and the Quitman mountains, east of Davis 
 Mountains, and in some other less extensive areas. 
 
 The latest of the Upper Cretaceous is known as 
 the Tornillo beds and these consist of vari-colored, 
 red, yellow, and greenish-gray and dark clays. These 
 occur in a belt surrounding the Chisos Mountains, 
 and are of little interest to drillers. 
 
 The Tornillo clays are underlain by some coal- 
 bearing beds, called the San Carlos beds, in Presi- 
 dio County and the Rattlesnake beds in Brewster 
 County. These may be the equivalents of the coal- 
 bearing beds at Eagle Pass. So far these forma- 
 tions have been but little drilled. With the over- 
 lying Escondido beds in Maverick County, they must 
 have a thickness of at least a thousand feet in the 
 southeast part of Maverick County. The clays are 
 mostly dark, tough, and gumbo-like. These alter- 
 nate with soft sandstones from 10 to 100 feet in 
 thickness. These are frequently strongly cemented 
 with calcareous material and again quite soft, al- 
 most loose sands. There are also layers of oyster 
 shell beds. 
 
46 Handbook of Aids to 
 
 In the Anacacho Mountains and east into Medina 
 County the Eagle Pass coal-bearing formation seems 
 to be replaced by the Anacacho limestone. This 
 formation is. some 400 feet thick in the Anacacho 
 Mountains, but thins gradually to the east. It con- 
 sists of a white limestone composed of some oolitic 
 material, much worn calcareous beach sand of or- 
 ganic origin, and some fine shell gravel. For the 
 greater part, the rock is strongly cemented by in- 
 filtrated and other fine-textured calcareous material. 
 It is the thickest oolitic limestone among our Cre- 
 taceous formations. The somewhat open textured 
 beach gravel in the lower part of the formation in 
 Uvalde County* contains the Uvalcle asphalt. 
 
 Below the coal-bearing beds of the Upper Creta- 
 ceous we have in north Texas the Navarro beds, 
 which are regarded as measuring some 300 or 400 
 feet in thickness. These are mostly gray marls and 
 sands which nearly everywhere contain some glau- 
 conite, by which they are usually recognized. 
 
 In the north, the Navarro beds are not easily to be 
 separated from the underlying Taylor marl by the 
 examination of cuttings. This formation is one of 
 the well known "gumbos" of the upper coastal 
 plains. It is an easy drilling formation, through 
 the 700 to 800 feet of which rotaries have been 
 plunged in some cases in less than three days in 
 the Corsicana, the Thrall, and the Caddo fields. In 
 some places small fine-textured sandy layers occur 
 in the Taylor marl and such sands are usually glau- 
 conitic, as is also frequently the marl itself. Occa- 
 sionally small teeth of fish come up with the returns 
 from the upper third of this formation and through-- 
 out nearly the whole occur prisms of calcareous ma- 
 terial from shells of Inocerami. The marl is almost 
 invariably more or less bituminous. 
 
 The Austin Chalk, from 200 to 300 feet in thick- 
 ness, underlies the Taylor marl. This is usually 
 easily recognized by its texture, its light color and 
 
Identification of Geological Formations 47 
 
 its softness. In some places it is bluish gray. This 
 can also be recognized by the abundance of small 
 formaminifera which it yields on trituration and 
 washing. The prisms of shells of Inocerami are al- 
 ways present in the cuttings, and frequently also 
 larger pieces of 'these shells. The Austin Chalk is 
 a quite uniformly developed formation wherever 
 it occurs in the southeast two-thirds of the state. 
 West of the Pecos it is less easily distinguished 
 from the overlying Taylor and the underlying Eagle 
 Ford. 
 
 The Eagle Ford formation is quite unequally de- 
 veloped in different parts of the state. To the north 
 and northeast it merges downward into the Wood- 
 bine sands and consists, like this, of marls, clays and 
 sands. Here it is several hundred feet thick. 
 Southward it gradually becomes thinner until at 
 Austin it is less than 50 feet. Westward from San 
 Antonio, it again increases in thickness. Following 
 it beyond the Pecos River, we find that it measures 
 some 600 feet. From Austin, south and west, it 
 becomes more and more calcareous and west of Dev- 
 il's River it is mostly thin-bedded limestone. This 
 formation is everywhere more bituminous than other 
 parts of the Cretaceous and this feature is most 
 marked where it has the least thickness, as from 
 Waco to San Antonio and from San Antonio to Del 
 Rio. It nearly everywhere gives showings of oil. 
 Within the same limits it consists in part of black 
 marly shale containing in places as much as ten or 
 even twenty per cent of oil or other volatile bitu- 
 mens. This black shale and also the more calcareous 
 layers which approach limestone or sandstone, can 
 usually be known in cuttings by the occurrence of 
 fish scales or other fish remains on flat cleavage 
 fractures. Such scales are less frequent in the for- 
 mation in the north and in the west, but they can 
 generally be found in cuttings from these localities 
 also, if persistently looked for. In Bexar County 
 
48 Handbook of Aids to 
 
 many drillers record this formation as "lignite" in 
 their logs. 
 
 The Eagle Ford is underlain from Waco north- 
 ward by the Woodbine sands and clays. These are 
 not always readily to be separated in cuttings from 
 the Eagle Ford. In the Woodbine, sands are more 
 prevalent than in the overlying formation, and this 
 formation generally shows some traces of plants, 
 such as shreads of leaves and pieces of wood, while 
 it rarely has any fish remains. Pyrite is common 
 in places near the base of this formation. 
 
 THE LOWER OR COMANCHEAN CRETACEOUS FORMA- 
 TIONS 
 
 The Comanchean Cretaceous (Lower Cretaceous) 
 is essentially limestone with some marly clay am 
 sandstone. It is one of the most extensive series of 
 rocks in Texas, covering a belt of country from 
 Gainesville to north of San Antonio and from there 
 westward over the entire Edwards Plateau to the 
 Front Range and into New Mexico. Smaller, but 
 quite extensive areas occur west of the Front Range. 
 Its thickness ranges from zero to at least 1500 feet 
 in the Big Bend Country. It has been drilled quite 
 extensively, mostly for water. 
 
 Different ones of the Comanchean formations can 
 be separately identified. 
 
 South of Waco and westward past San Antonio 
 and out to Brewster County, the Buda limestone un- 
 derlies the Upper Cretaceous. This is a pure white 
 and almost compact limestone, seldom less than 
 thirty or more than seventy feet in thickness. It 
 is sharply marked off from the dark Eagle Ford 
 clay above and from the soft Del Rio clay below. 
 No other formation in Texas compares with the 
 Buda in the combination of its white color and fine- 
 ness of texture. 
 . The Del Rio clay, which underlies the Buda, has 
 
Identification of Geological Formations 49 
 
 a great capacity for caving. In places it is absent 
 and, where present, it varies in thickness from a 
 few feet to two hundred feet. This formation can 
 almost everywhere be identified by a little "ram's 
 horn" shell, Exogyra arietina, and another equally 
 common shell, which looks like a very short string 
 of small beads, Nodosaria texana. Both can gen- 
 erally be washed from the returns, from this clay. 
 
 In the central part of the state the Georgetown, 
 which underlies the Del Rio, consists of frequent 
 alternations of limestone and marls usually full of 
 shells. To the southwest this is replaced by some 
 very compact limestone. 
 
 Under the Georgetown follows the thickest and 
 most extensively developed member of the Coman- 
 chean, the Edwards limestone. This measures frori 
 fifty to four hundred feet in thickness. It is quite 
 uniform in character, partly of coarse crystalline 
 texture, in some places cavernous, and almost every- 
 where it contains a few persistent layers of gray 
 flint. 
 
 Below the Edwards there are again some changes 
 to marly material and often some strata containing 
 fine sand, the Paluxy sands, which are generally 
 water-bearing, and are oil-bearing near South 
 Bosque. At this depth the Glen Rose formation be- 
 gins. This can usually be identified from its cut- 
 tings by the appearance in them of some small cir- 
 cular, concavo-convex, dark, disk-shaped fossils, 
 Orbitulina texana. The limestones below the Ed- 
 wards are quite generally noticeably finely dark- 
 speckled, and are almost invariably darker than the 
 overlying rock. They measure, with the interbed- 
 ded marls and sands, from three hundred to nearly 
 a thousand feet in thickness. 
 
 The basal deposits of the Comanchean consist of 
 mostly blue and partly red marls and white, gray, 
 or yellowish sand, varying from packsand to gravel. 
 They are mostly quite soft, but are in a few places 
 
50 Handbook of Aids to 
 
 very hard, practically quartzite. This sand, which 
 is the Trinity sand, makes the great water-bearing 
 formation in northern, and parts of central Texas. 
 
 THE JURASSIC FORMATIONS 
 
 Deposits of this age, consisting chiefly of lime- 
 stone with, some conglomerates and dark shale, are 
 known only in the Malone Mountains and are of no 
 general importance to drillers in Texas. 
 
 THE TRIASSIC FORMATIONS 
 
 This consists largely of red clays and sand, with 
 which are also some gray sands and some conglom- 
 erates. 
 
 The Triassic underlies the greater part of the 
 Panhandle and the Llano Estacado, where it over- 
 lies the Permian and extends in detached outliers 
 as far east as to Glasscock County at the south and 
 beyond Dickens and Amarillo to the north. The 
 sands and conglomerates of these beds usually 
 have good water. These sands are mostly highly 
 micaceous. The conglomerates in the lower part 
 are characterized by well rounded yellow or white 
 quartz pebbles. All parts of the formation are apt 
 to contain logs of trees changed to lignite or in a 
 silicified condition. Some gypsum is also found, 
 and black manganese oxide is not infrequent. The 
 Triassic is not known to have any limestones. 
 
 THE PERMIAN FORMATIONS 
 
 \ 
 
 The greatest known thickness of the Permian is in 
 the Glass Mountains, where it -measures some 9000 
 feet and consists largely of limestone, in part dolo- 
 mitic. Permian of the same general character oc- 
 curs also in the Guadalupe Mountains, north and 
 east of the Chinati Mountains, and probably in some 
 other localities west of the Pecos. 
 
Identification of Geological Formations 51 
 
 The Permian beds which are apt to be of most 
 interest to drillers occur on the Plains. This form- 
 ation extends west from the principal Pennsylvanian 
 area to the Pecos River and to the east escarpment 
 of the Panhandle High Plains and the Llano Esta- 
 cado. West from this escarpment it is overlain by 
 the Triassic and other later formations, rising again 
 in New Mexico. How far south it underlies the Co- 
 manchean north of the Rio Grande and east of the 
 Pecos River is not known. It has been found in a 
 boring at Sheffild. The upper 1500 feet of this part 
 of the Permian consists largely of red marls and clays 
 commonly known as the "red beds" which also con- 
 tain beds of anhydrite and gypsum and quite fre- 
 quent beds of salt, some of the latter measuring as 
 much as a hundred feet in thickness. These beds 
 are mostly eroded away east of the line joining 
 Ballinger, Abilene, and Petrolia. The lower part 
 of the Permian, which underlies the "red beds" in 
 this region and which appears on the surface east 
 of the line indicated as far as to the Pennsylvanian 
 area, consists of mostly blue, in part reddish, and 
 purplish shales, clays, and marls; and there are 
 gray, red and white sandstones, which are nearly 
 always of fine texture and only slightly micaceous. 
 Even this part of the Permian in places contains 
 some gypsum and salt. The limestones are mostly 
 thin and in many cases bituminous and fine-grained, 
 and some are oolitic. There are also dolomites, which 
 are mostly also thin, and may show bituminous ma- 
 terial. Nearly all water found in the Permian of 
 the plains is salt or even briny. Some of the shales 
 and marls in the Permian of the Plains have a pe- 
 culiar delicate dove-blue color. 
 
 THE PENNSYLVANIAN FORMATIONS 
 
 The Pennsylvanian underlies extensive parts of 
 Texas. It is exposed in the north central part of the 
 state which may be roughly encompassed by a peri- 
 
52 Handbook of Aids to 
 
 phery extended from Montague to San Saba, from 
 there to near Paint Rock, and then to Henrietta. 
 West from this belt it dips under the Permian and 
 has been entered in deep borings at San Angelo and 
 at Spur. West of the state it again comes up to 
 the surface east of the Front Range in New Mexico 
 as well as in Texas, so that it no doubt underlies, 
 at depths from 3000 to 5000 feet, the entire Pan- 
 handle and the Llano Estacado. The Pennsylvanian 
 no doubt underlies the Comanchean Plateau from 
 the latitude of San Angelo to the Mexican Border. 
 Eastward from the line joining San Saba and Mon- 
 tague, the Pennsylvanian underlies the Comanchean 
 at increasing depths to quite near the Balcones Es- 
 carpment. Whether it also underlies the Coastal 
 Plain east of the Balcones Escarpment can not yet 
 be regarded as proved, though it seems likely. In 
 this direction it is covered first by the Cretaceous 
 and farther out east and south by the Tertiary and 
 may be looked for at depths from 5000 to 6000 feet 
 or still deeper near the coast. West of the Pecos, 
 the Pennsylvanian no doubt underlies most of the 
 region, though at greatly varying depths. It out- 
 crops in the Franklin, the Hueco, the Chinati and 
 in other mountains and in the Marathon and the 
 Solitario uplifts and is elsewhere mostly covered by 
 later deposits, in places at depths of evidently many 
 thousand feet. 
 
 The formations making the Pennsylvanian series 
 measure in different parts of the state from noth- 
 ing, where eroded away, to five or six thousand feet, 
 as in the Marathon region. They consist of shales, 
 sandstones, and limestones, in part in frequent al- 
 ternations. The shales and clays are mostly en- 
 tirely free from calcareous material, by which fact 
 they can usually be recognized from Cretaceous 
 clays. In the lower part of the series they are most- 
 ly dark or bluish gray. Higher up they are partly 
 reddish, brownish, or purplish, which colors alternate 
 
Identification of Geological Formations 53 
 
 with gray, bluish and dark colors. In the central 
 part of the state they contain some small seams of 
 coal. Many of the clays are fine-textured, and show 
 slickensides or jointing. The darker shales general- 
 ly contain miscroscopic foraminifera, and they sel- 
 dom fail to contain minute black shreds of carbona- 
 ceous material. They also frequently contain con- 
 cretions ("boulders") of clay-iron-stone or even con- 
 tinuous strata of this nature. The sandstones range 
 into occasional fine conglomerates. They may be 
 open-textured or compactly cemented with lime. 
 Some are petroliferous, as at Electra, Moran, and 
 Strawn. As a rule the sands are moderately fine 
 and the grains angular. Most water found in Penn- 
 sylvanian sands is salty. The limestones present 
 great variations. In thickness they range from a 
 few inches to hundreds of feet. They are in rare 
 cases dolomitic. Generally they consist to a con- 
 siderable extent of organic fragments. In most of 
 them may be found Fusulinas and especially seg- 
 ments of crinoid stems, which can quite readily be 
 found and noted. These are common also in many 
 shales and clays of the Pennsylvanian. The color is 
 varied but mostly gray. Some are sandy, some 
 shaly. In the thickest limestones, chert and flint 
 are common at some horizons. 
 
 THE BEND FORMATION 
 
 This is the deep formation which is oil-bearing 
 in the Ranger field. It is possibly of Pottsville age, 
 and clearly lies near the middle of what is gener- 
 ally called the Carboniferous strata. It is known 
 to outcrop in McCulloch, San Saba, Mason, Gillespie, 
 Blanco, and Burnet counties, and underlies the later 
 Carboniferous to the north of this region and no 
 doubt in many places underlies the Comanchean and 
 possibly also later Carboniferous strata west and 
 south of the Central Mineral Region. It is likely 
 
54 Handbook of Aids to 
 
 enough that the Bend is to be correlated with the 
 Dimple formation in Brewster County, which is 
 Pennsylvanian in age. 
 
 The Bend formation consists mostly of limestone 
 but with this is also some shale. One of the shaly 
 members occurs in the limestone. The other forms 
 the basal member of the formation. Part of the 
 limestone and nearly all of the shale is dark, almost 
 black. Much of this limestone can be told from later 
 limestones in the Pennsylvanian by its somewhat 
 more developed crystallization. When observed in 
 thin sections this crystallization is seen to give the 
 rock a blotched or streaked appearance, with irreg- 
 ularly shaped and distributed areas of crystalline 
 and granular tracts. The largest microscopic cal- 
 cite crystals have a decided tendency to radiate from 
 what appears to be microscopic organic centers. 
 Thin microscopic sections of this rock are rarely 
 entirely free from spicules of sponges and in some 
 layers these are present in profusion. The forma- 
 tion also contains some black chert. Some calca- 
 reous layers in the upper shale are filled with sponge 
 spicules and are quite siliceous. To some extent 
 the shales in this formation may be recognized by not 
 making as much fine mud under the drill as is the 
 case with most of the shales of later Pennsylvanian 
 deposits. 
 
 THE MISSISSIPPIAN, THE DEVONIAN, AND THE SILU- 
 RIAN FORMATIONS 
 
 Rocks of these ages are of very little importance 
 to drillers in Texas, unless it should be found that 
 the Bend is of late Mississippian age, which can not 
 yet be regarded as demonstrated. The Fussulman 
 limestone in the Franklin Mountains, and the upper 
 part of the Maravillas chert in the Marathon uplift 
 probably are of Silurian age, and the Caballos no- 
 vaculite in Brewster County, a conspicuous white 
 flint, is probably Devonian. 
 
Identification of Geological Formations 55 
 THE ORDOVICIAN FORMATIONS 
 
 In the central part of the state the Ordovician is 
 represented by the upper part of the Ellenburger 
 limestone which is a hard, fine-grained, and com- 
 pact white rock that changes downward into a some- 
 what more coarse-grained but still hard and yellow- 
 ish dolomite, containing some oolitic layers and some 
 white chert. The lower part of the Ellenburger be- 
 longs to the Cambrian. The two make one continu- 
 ous formation, a thousand feet thick, without any 
 interbedded shales or clays. It has been entered by 
 a deep boring at Myra, in Cooke County, where it 
 comes up to within some 2200 feet of the surface, 
 and has also been encountered in some borings in a 
 triangular area roughly outlined by a line connect- 
 ing Parker, Lampasas, and McCulloch counties, 
 where this formation underlies the Bend, which is 
 oil-bearing. It outcrops in McCulloch, San Saba, 
 Mason, Llano, Burnet and Gillespie counties, and 
 no doubt will be found underlying later rocks in the 
 region adjacent to these counties. In Brewster 
 County rocks of this age have been found in the 
 Marathon uplift and contain dark flint and chert 
 and consist of thin-bedded, dark, bituminous lime- 
 stones and shale. The El Paso limestone, also of 
 this age, is a dolomite 1000 feet thick on the south- 
 west of the Hueco Mountains, underlying the Huecp 
 limestone in that region. The Montoya limestone 
 near El Paso is also Ordovician. 
 
 THE CAMBRIAN FORMATIONS 
 
 Formations of Cambrian age occur in San Saba, 
 Mason, Llano, Burnet, Gillespie and adjacent coun- 
 ties, and have been drilled into for water or for 
 oil in these and in some adjacent counties. Much 
 of the Cambrian consists of gray or red sandstone. 
 Some of this is mixed with lime and.glauconite. The 
 upper part of the Cambrian is a hard, dolomitic lime- 
 
56 Handbook of Aids to 
 
 stone, the Ellenburger limestone described above. 
 This measures several hundred feet in thickness. 
 West of the Pecos the Cambrian is all sandstone. 
 It is known as the Van Horn, the Bliss, and the 
 Brewster sandstones. 
 
 ARCHEAN AND IGNEOUS ROCKS 
 
 Rocks of Archean ages are mostly granite, gneiss 
 and schist, and they occur in Llano, Burnet, Gilles- 
 pie, and Blanco counties. They have been entered 
 under overlying later rock in some borings in these 
 and adjacent counties. The schists can usually be 
 known by their abundant mica and the granite by 
 its pink feldspar. Schistose rocks have been reached 
 in some deep borings at Georgetown and near San 
 Antonio. Chloritic rocks, and dark basaltic rocks 
 occur in Kinney, Uvalde, Medina, Travis and Wil- 
 liamson counties, and may exist also in Hays, Bexar, 
 and Guadalupe counties. These are of igneous or- 
 igin. The former may be known by its dark green 
 color and by its softness, the latter by its dark color, 
 crystalline texture, and extreme toughness and hard- 
 ness. West of the Pecos, lavas and basalts cover 
 much of the land. They are underlain in many 
 places by volcanic tuffs, and sands in which drilling 
 operations are often successfully made for water, as 
 in Jeff Davis, Presidio and Brewster counties. 
 
Identification of Geological Formations 57 
 
 TABLES FOR IDENTIFICATION OF 
 SOME MINERALS AND ROCKS 
 
 Owing to the different chemical and physical prop- 
 erties of minerals and of rocks they may usually be 
 recognized by characteristics that become evident on 
 close inspection, or which appear from their reac- 
 tions to some simple tests to which they may be 
 submitted. Some of these tests were known to the 
 ancients and are practiced in various trades today. 
 Some of the processes involved are fundamental to 
 great industries. Non-magnetic iron ores change 
 to iron when smelted, and then become magnetic. 
 Carbon dioxide gas is evolved from carbonates, such 
 as calcite or carbonate of soda, when acted on by 
 hydrochloric acid. This is the way "soda water" 
 is made. It is the same gas we see forming when 
 a fragment of limestone effervesces in the acid. Lime 
 is burned from limestone and it slacks in water. So 
 does a small fragment of the same rock, when heated 
 to redness and then placed in a drop of water. 
 
 By taking advantage of these and other well 
 known processes, trying first one and then another, 
 it is possible to correctly identify all of the more 
 common sedimentary rocks. They involve the fund- 
 amental rudiments of determinative mineralogy. To 
 aid the beginner some tables have here been pre- 
 pared, showing characteristics of different materials 
 as they appear to the unaided eye or under a common 
 magnifying glass, and also some characteristic reac- 
 tions which minerals and rocks show when subjected 
 to a few tests. These examinations and tests are 
 as follows: 
 
58 Handbook of Aids to 
 
 1 . . Observation of general appearance. 
 
 2. .Examination of texture. 
 
 3 . . Observation of color. 
 
 4 . . Observation of streak. 
 
 5 . . Test for hardness. 
 
 6.. Acid test. 
 
 7. .Test for fumes. 
 
 8.. Fire test. 
 
 To make the required observations it is not nec- 
 essary to have much apparatus. What little is need- 
 ed can be obtained at very small expense and effort. 
 The following will suffice: 
 
 1 . . A small magnifying glass, such as a linen tester, 
 which can be bought in any jewelry store, or 
 from any dealer in optical goods. 
 
 2 . . An acid bottle filled with a twenty per cent solu- 
 tion of hydrochloric acid. The bottle should 
 have a glass stopper. 
 
 3 . . A pocket knife. 
 
 4 . . A "tin" spoon, or a thin sheet of iron, as from 
 a "tin" can. 
 
 5 . . A piece of broken window glass. 
 
 6 . . A small horseshoe magnet. 
 
 7 . . Pieces of broken porcelain or chinaware. 
 
 In case of most materials it will not be necessary 
 to make all of the observations and tests described. 
 One or another may be made first, according to the 
 judgment of the observer. In every case, when a 
 conclusion has been reached, the brief description 
 of the identified rock or mineral found in this text 
 should be consulted before the identification is 
 recorded. 
 
 A practice has recently been resorted to by in- 
 experienced drillers who record some materials in 
 the log merely as "rock.* This gives no ^clue at all 
 to the nature of the material so described, except 
 perhaps that it is harder than the materials above 
 and below it. The fact is that all formations con-^ 
 sist of rock: sand and clay are rock, as well as 
 sandstone, limestone or granite. Evidently the term 
 is used when the log-maker is unable to tell, or too 
 
Identification of Geological Formations 59 
 
 hurried to note whether the "rock" is limestone or 
 sandstone. Sometimes inability to make the neces- 
 sary distinction is quite excusable as in case of a 
 cemented sandstone of fine texture. "Hard rock, 
 undetermined/' would be a more proper record in 
 such a case. A sandstone cemented with calcareous 
 material will effervesce in acid, but if a few frag- 
 ments be crushed and treated with a sufficient quan- 
 tity of acid until effervescence ceases, the quartz 
 grains of the sand will remain and can be seen 
 through a magnifying glass. 
 
60 
 
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Identification of Geological Formations 61 
 
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Identification of Geological Formations 63 
 
 
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Identification of Geological Formations 65 
 
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Identification of Geological Formations 67 
 
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Identification of Geological Formations 69 
 
 
 
 
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14 DAY USE 
 
 RETURN TO DESK FROM WHICH BORROWED 
 
 EARTH SCIENCES Li 
 
 This book is due on the last date stamped below, or 
 
 on the date to which renewed. 
 Renewed books are subject to immediate recall. 
 
 *H97r 
 
 General Library 
 
tiH 
 
NOTICE 
 
 Headers of this text, interested in the 
 geology of Texas, will find it advantageous 
 to have at hand for reference Bulletin 44 
 of the University of Texas* This has 
 been prepared by the Bureau of Economic 
 Geology, and Is a review of the geology of 
 the state as far as it has been made out to 
 the present time. It contains- maps and 
 sections for ready reference and clear pre- 
 sentation of the known surflcial distribution 
 and the thickness and nature of the forma- 
 tions described on]; briefly in this .text. 
 Bulletin 44 will be mailed promptly on the 
 receipt of the price ($1.00) r 
 
 Address J e A. Udden, Bureau of Eco- 
 nomic Geology, Austin, Texas*