DEPARTMENT OF THE INTERIOR 

FRANKLIN K. LANE, Secretary 

UNITED STATES GEOLOGICAL SURVEY 

GEORGE OTIS SMITH, Director 

PROFESSIONAL PAPER 108-A 

BAKED SHALE AND SLAG FORMED BY THE 

BURNING OF COAL BEDS 

BY 

G. SHERBURNE ROGERS 

Published March 3,1917 

Snorter contributions to general geology, 1917 

. (Pages 1-10) 

"\V ASHINGTON 

GOVERNMENT PRINTING OFFICJ<J 

. 1917 
DEPARTMENT OF THE INTERIOR 

FRANKLIN K. LANE, Secretary 

UNITED STATES GEOLOGICAL SURVEY 

GEORGE OTis SMITH, Director 

Professional Paper 108-A 

BAKED SHALE AND SLAG FORMED BY THE 

BURNING OF COAL BEDS 

BY 

G. SHERBURNE ROGERS 

Published March 3, 1917 

Shorter contributions to general geology, 1917 

(Pages 1-10) 

WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1917 
CONTENTS. 

Introduction .... . ... . . . ................. . ..... . ... ... .. . . . ..... . . . . .. . . . ....... . . ...... . ... .. .. ... ..... . 

Causes of burning .... .. ........................ ...... .. .. ..... ... .. ....... . . .. .. . . ... . .. ... .. . . . ..... . . . 

General effects of burning on the overlying strata .. ... .. . . ... .. . . .. , . ... .... .. . .. ... . ... .... ... . . . ... ..... . 

Petrologic character of the rocks formed .. . ......... .. . . . . . . ..... .. .. ... ....... .. ... . ...... . ... ... .. .. . . .. . 

Character of the original sedimentary rocks ...... ... .. . ... . ...... . ... . ..... ...... .. ... . ...... . .. ...... . 

Baked rock .......... ... .......... .. .. . .. .. . . . . : . .... . ..... .. .... . ....... . ........ . ............. . . . . 

Vitrified shale . ............. .. ........ . . . .. . . . ......... . ....... . ... . . . ............. . .... ___ . _ .. ____ . 

Glassy slag ..... . ... . .. . . -. . ......... .. . . ... .. ....... .. ..... . .. . . . .. . . · .. , . . . . ...... . .. .. _ . . _ . . . _ . __ . .. . 

Recrystallized slag .......... .. .. .. ... . ... ... . . ... . ..... . ........... __ .. . _ . . _ . . ... __ _____ __ . ___ . __ . _ .. 

Chemical changes ... . .. . . . . . ........... .. . .. . . . . ... .. ....... . . .. .. . ...... . ... . ... . ... .... .. ...... ....... . 

ILLUSTRATIONS. 

PLATE I. Effects of the burning of coal beds in eastern Montana: A, A chimney or core of hard fused rock projecting through partly baked strata near Miles City; B , Coal bed 11 feet thick on Custer Creek near 

Page. 

1 

1 

4 

5. 

5 

6 , 

6 

6 

6 

I 8 

Page. 

Yellowstone River, between Miles City and Terry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

II. Photomicrographs of vesicular slag formed by the burning of coal beds: A, Glassy slag with spherulitic structure; B, Holocrystalline slag composed chiefly of diopside and plagioclase... .. . . ... .. .... 6 

III. Pliotomicrograph of holocrystalline slag.............................................. .. . . . . ...... 8 

NOTE.-The papers included in the annual volume "Shorter contributions to general geology" are issued . 

separately, with the final pagination, as soon as they are ready. The last paper will include a volume title-page, 

table of contents, and index for the use of those who may wish to bind the separate parts. A small edition of 

the bound volume will also be issued, but copies can not be supplied to those who have received all the parts. 

II 
SHORTER CON'TRIBUTIONS TO GENERAL GEOLOGY, 1917. 

BAKED SHALE AND SLAG FORMED BY THE BURNING OF COAL BEDS. 

By G. SHERBURNE RoGERs. 

INTRODUCTION. 

The baking and reddening of large masses of 

strata caused by the burning of coal beds is a 

str~king feature of the landscape in most of the 

great western coal-bearing areas. The general 

character and br6ader effects of the burning 

have been described by many writers/ but 

the fact that in places enough heat is generated 

to fuse and thoroughly recrystallize the overlying . shale and sandstone has received less 

a,.ttention.. Some of the natural slags thus 

formed simulate somewhat abnormal igneous 

rocks, but others consist largely of rare and 

little known minerals. A wide range in the 

mineral composition of such slags is to be expected, depending on the composition of the 

original sediment and the conditions of fusion 

and cooling. These products of purely thermal 

metam0rphism offer a fertile fiel.d for petrologic investigation. 

The writer has observed the effects produced 

by the burning of coal beds in several localities 

in Montana, particularly along upper Tongue 

River in the southern part of the State, in the 

district lying southeast of the mouth of Bighorn 

River, and in the Little Sheep Mountain coal 

field north of Miles City. A number of specimens of the rock formed have been examined 

under the microscope, though time has not 

been available for a systematic examination. 

The writer is greatly indebted to Mr. E. S. 

Larsen for assistance in the study of some of 

the minerals. 

1 Allen, J. A., Metamorphism produced by the burning of lignite beds 

in D.akota and Montana territories: Boston Soc. Nat. Hist. Proc., vol. 

16, p. 246, 1874. See also Lewis, Meriwether, and Clarke, William, History of the expedition under the command of Lewis ahd Clarke, vol. 1, 

pp. ~80, 189, Philadelphia, 1814; Hayden, F . V., Geological report of the 

exploration on the Yellowstone and :Missouri rivers, pp. 56-103, 1869; 

and Dawson, G. M., Report on the geology and resources of the region 

in the vicinity of the forty-ninth paralle:, pp. 1G4--1G9, Montreal, 1875. 

70460°-17 

CAUSES OF BURNING. 

Most writers on the burning of coal beds have 

attempted to explain the mode of ·their ignition, .which they usually ·ascribe to lightning, to 

prairie or forest fires, to the agency of man, 

or to spontaneous combustion. AU these 

agencies have probably been effective in one 

place or another. 

Lightning has doubtless ignited some · beds, 

especially those exposed on th~ higher hills, 

but it is apparently inadequate to account for 

the burning of beds over great areas. There 

are also well-authenticated reports of the ignition of beds by prairie fires, 2 but it may well 

be doubted whether this agency has often 

been effective in the sparsely grassed badlands,. 

where burning is so common. Similarly, the 

writer knows of one locality in which a bed of 

coal "11 feet thick was ignited by the camp 

fires .of a round-up outfit, but much of the 

burning evidently took place many thousands 

of years ago and some of it probably dates back 

to Pleistocene time. 3 Ignition from . an ex-;-

traneous source is perfectly possible and may · 

be the most plausible explanation in any given 

instance, but the fact that burning on the outcrop is so common as to be actually characteristic of the coal beds over an area greater 

than 200,000 square miles indicates a more 

widespread mode of origin and one less depen ... 

dent on special local conditions. 

Ever since coal first became widely used as 

fuel the possibility of its spontaneous combustion has been recognized. Disastrous fires 

have started through spontaneous combustion 

in mines, many others at the surface in piles of 

2 Allen, J. A., op. cit., p. 261. 

a Idem, p. 258. Also Collier, A. J., and Smith, C. D., The Miles 

City coal field, Mont.: U.S. Geol. Survey Bull. 341, p. 45, 1909. 

1 
2 SHORTER ·coNTRIBUTIONS TO GENERAL GEOLOGY, 1917. 

culm or slack coal, and others in stocks of coal 

held in storage or undergoing shipment, 

especially at sea. Because of its economic importance the phenomenon has been carefully 

studied by many observers, and although there is considerable disagreement as to detail the 

main determining conditions have been well 

outlined. Thus, it is . generally recognized 

that only coals that contain a moderate to high 

proportion of volatile matter are subject to 

spontaneous combustion; that the more finely 

divided the coal the more liable it is to ignite; 

and that a small increment of heat from an 

outside source is an important contributing 

cause. 

Most of the coal in the Western States that 

has burned at the outcrop is of lignitic or-subbituminous grade; this coal when exposed to 

the air loses moisture and tends to slack or 

crumble to fragments. All freshly mihed coal 

absorbs oxygen and according to Lewes 1 may, 

if in fairly small fragments, absorb two to 

three times its own volume. This absorption 

of oxygen takes place at ordinary temperatures 

but proceeds more rapidly at higher; temperatures, and as the process itself generates some 

heat it is self-accelerating. The heatfirst generated may be due merely to the compression 

of the gas, but a slow oxidation of. resins or 

unsaturated hydrocarbons in the coal, with the 

formation of humic or other acids, probably 

soon begins. The elaborate experiments of 

Parr and Kressmann 2 indicate that this reaction assumes positive · activity at about 80° C. 

With rise in temperature the occluded oxygen 

. becomes ·more active, combining with certain 

hydrocarbons and converting them to carbon 

dioxide and water. According to Parr and 

Francis 3 this reaction begins only when the 

coal becomes heated to 120° to 140° C., but it 

represents a critical stage in the process of 

oxidation, for so much heat is generated that 

the temperature thereafter rises rapidly. At a 

little over 200° C. autogenous oxidation begins 

and at 350 to 450° c~ actual ignition takes place. 

1 Lewes, V. B., The .carbonization of coal, p. 23, London, 1912. See 

also Parr, S. vV., and Barker, Perry, The occluded gases in coal: Illinois 

Univ. Eng. Exp. Sta. Bull. 32, 1909; White, David, The effect of oxygen 

in coal: U.S. Geol. Survey Bull. 382, pp. 63-71, 1909; and Chamberlin, 

R. T., Notes on explosive mine gases and dusts: U. S. Geol. Survey 

Bull. 383, pp. 15, 60, 1909. 

2 Parr, S. W., and Kressmann, ~· vV., The spontaneous combustion 

·of coal: Illinois Univ. Eng. Exp. Sta. Bull. 46, pp. 24, 52, 1911. 

a Parr, S. "\V., and Francis, C. K., The modification of Illinois coal by 

low temperature distillation: Illinois Univ. Eng. Exp. Sta. Bull. 24, 

1908. 

According to experiments by Fayol,4 however, 

coal in a state of fine dust ignites at a much 

lower temperature, gas coal igniting at 200° C. 

and lignite at 150° C. 

The oxidation of pyrite or marcasite in the 

coal was once considered the chief cause of 

spontaneous combustion, but it is now known 

that coals practically free from iron sulphides 

may spontaneously ignite. Some a:uthorities 

therefore hold that the influence of pyrite is 

negligible and that at the most it merely 

increases the general rise in temperature and by 

swelling splits up the coal, thus exposing fresh 

surfaces to the action of atmospheric oxygen.5 

Parr and Kressmanri....:find7 however, that the 

oxidation of pyrite ii+ :the presence of moisture 

results in a distinct increment of heat, and as 

this reaction takes place at ordinary temperature they believe that it is one means by which 

the mass is heat~d to the point at which active 

oxidation of the coal itself commences. · They 

state that the oxidation of one-fifth of the 

pyrite in a coal containing 6 per cent generates 

sufficient heat to raise the temperature of the 

mass 70° C., assuming no loss by radiation.~ 

As the conductivity of coal is very low compared with that .of the surrounding rocks, according to figures given by Prestwich/ it seems 

probable that loss of heat by radiation is slow 

and that small amounts of heat generated 

within the coal may well be conserved to produce far-reaching results. 

It is generally recognized that the presence 

of moisture favors spontaneous combustion. 8 

In fact, Dennstedt and Biinz,9 as a result of experiments, conclude that "self-ignition increases in a ratio corresponding to the amount 

of moisture [water of constitution] in air-dry 

coal." Opinions differ1 however, as to whether 

its action is chiefly mechanical, catalytic, or 

chemical. It is also known that inflammable 

gases are occluded in coal, and though it may 

be doubted that these gases are the direct cause 

of spontaneous combustion it is evident that if 

the coal is heated by oxidation to the temperature at which these gases can unite with atmosf Fayol, Henri, Etudes sur l'alteratio.O: et la combustion spontanee 

de la houille exposee a l'air: Soc. ind. min. Bull., ~d ser., vol. 8, pt. 3, 

1879. . 

• Lewes, V. B., op. cit., pp. 21-22. 

e Parr, S. W., and Kressmann, F. W., op. cit., p. 34. 

7 Boulton, W. S., Practical coal mining, vol. 2, p. 331, London, 1907. 

sParr, S. W., and Kressmann, F. "\V., op. cit., p. 52. 

9 Dennstedt, M., and Biinz, R., Die Gefahren der Steinkohle: Zeitschr. 

angew. Chemie, vol. 21, P:::>· 1825-1835,1908. 
BAKED _SHALE AND SLAG FORMED BY BURNING OF COAL BEDS. 3 

pheric oxygen they may become an important 

contributing cause. Penhallow 1 believes that 

the burning of ligni-te beds is caused by the spontaneous ignition of escaping gases, particularly 

sulphureted hydrogen, carbureted hydrogen 

(methane~), and phosphureted hydrogen. 

chemical character of four typical coals that 

have burned extensively on the outcrop is 

shown by the first four analyses, and the character of a coal that has nowhere burned by the 

last analysis. A considerable range ~ com:-

position is shown by these analyses, though all 

show high moisture and volatile matter but 

Analyses of lignite and coal from Montana and Wyoming. fairly low sulphur. It will be noted that the 

Made at the Pittsburgh laboratory of the Bureau of Mines; F. M. Stan- coal that has nowhere burned is . much higher ton and A. C. l!1eldner, chemists in charge.] 

in sulphur than any of the others; whether its 

10898 64G9 11111 8465 14755 lack of tendency to burn-is due to the offsetting 

--------!·---------- of this factor by the low percentage of mois- SAMPLE AS RECEIVED. 

Proximate: 

Moisture ......... . Volatile ma.tter .. _ . Fixed carbon ..... . Ash ................... . Sulphur ............... . 

Ultimate: 

Hydrogen .. ~ ..... . Carbon ............ . Nitrogen .......... _ Oxygen ....... _ . __ 

42.8 26.8 22.6 . 29.4 19.8 

25. 7 32. 8 31. 9 25. 4 30. 7 

26. 9 27. 9 39. 5 38. 8 35. 2 

4.6 12.5 6.0 6.4 14.3 . 24 . 64 . 51 . 37 1. 50 

7. 21 6. 04 . ----- -.... - ------ 36. 21 42. 72 - . - - - - -.. -- - - - - - -- . 62 . 60--.-----.--.------

51. 08 37. 53 -.. - - - - - -- -- . -. - -- 6, llO 7, 340 8, 811 7, 170 8, 579 

ture and the high ash content is a matter ?f 

conjecture. 

British thermal units .. Loss of moisture on air 

drying ... "·------ .... 

The physical factors promoting spontaneous.· 

combustion are a finely divided condition of 

the coal, a slight increment of heat from an 

outside source, and a sufficient volume of coal 

to retard ·toss of heat by radiation. A pile· of 

coal exposed to the direct rays of the sun 

would thus be liable · to become heated arid 

finally ignite. In general, the outcrop of a 

35.3 20. 5 3. -7 19.3 8· 5 coal bed, even in the arid western coal fields, 

AIR-DRIED SAMPLE . 

Proximate: 

Moisture .......... . Volatile matter .... . Fixed carbon ..... . 

Ash ................... . Sulphur ............... . 

11. 6 7. 9 19. 6 12. 5 12. 3 

39. 7 41. 3 33. 1 31. 4 33. 5 

41.5 35.1 41.1 -48.1 38.5 

7. 2 15. 7 6. 2 8. 0 15. 7 . 37 . 81 . 53 . 46 1.64 

weathers down slowly and is more or less 

covered by talus, so that the oxidation of the 

coal is too slow to generate much heat. Where 

a young, rapidly cutting gulch intersect~ ·a 

coal bed, however, a perpendicular face of coal 

may be formed, and as this is undercut by the Ultimate: , Hydrogen ........ . Carbon ........... __ 

Nitrogen .......... . 

5. 09 4. 73 . . . . . . . . . . . . . . . . . . . stream a considerable mass of finely broken but 

55· 97 53· 74 ------ ------ ------ · fairly fresh coal may accumulate at the base· . 96 . 75------------------ . Oxygen ..... ___ .. . 30. 44 24. 28 . _ .... _ ... ___ ... _. of the bed. Under such conditions oxidation 

British thermal units .. . 9, 440 9, 238 9, 153 8• 880 9• 376 would be fairly rapid, especially as the normal 

LIGNITE OR COAL KNOWN T? BURN ON O}JTCROP. 

10898. Lignite from Culbertson lignite fie'ld, Mont. Red Bank open-cut 

mine, sec.IO, T. 28 N., R. 59 E. Fort Union formation. 

6469. Subbituminouscoalfrom Buiialocoalfield, Wyo. Mitchell mine, 

sec. 26, T. 51 N ., R. 82 W. Fort Union formation. 

17711. Subbituminous coal from Tullock Creek coal field, Mont. Prospectin sec. 30, T. 1 N ., R. 38 E. · Fort Union formation. 

8465. Subbituminous coal from Bull Mountain coal field, Mont. McCleary prospect, sec. 26, T. 9 N., R. 30 E •. Fort Union formation. 

, COAL NOWHERE BURNED ON OUTCROP. 

temperature of the coal, if exposed to the direct 

heat of the sun, would reach 130° F. (55° C.), 

or more on summer afternoons. 

In the writer's opinion most of the burning 

of western coal beds has been spontaneous and 

has originated under conditions similar to those 

just described. C. H. 1Wegemarin,2 who has 

studied particularly the relation of topography 

to burning, finds that burning is most prevalent 

14755. Subbituminous coal from Tqllock Creek coal field, Mont. Pros- along rapidly cutting- streains and believes 

pect 12 miles southeast of Bighorn. Lance formation. that it is partly a function of the character of 

It is evident from the foregoing summary 

that both chemical and physical factors enter 

into spontaneous combustion. As regards 

chemical· compositi~n, coals highest in volatile 

matter, moisture, and sulphur are most liable 

thus to ignite. In the accompanying table the 

1 Penhallow, D.P., Tertiary plants of British Columbia: Canada Geol. 

Survey Pub. 1013, pp. 148-149, 1908. 

the topography. The writer has 'observed 

beds actually burning at six -localities, and in 

all of them combustion evidently started in a 

gulch on . a small, rapidly cutting stream. 

Field studies indicate that two other generalizations can be made: First, thin beds are less 

commonly burned than thick ones, probably 

2 Personal communication. 
4 ·SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1917. 

because piles of coal large enough to retain 

.self-generated· heat can not accumulate along 

their outcrop; and second, beds of impure coal 

burn less commonly than those of clean coal. 

feet back from the outcrop, and as the coal 

burns out and these beds cave they tend to 

break into irregular fragments. Incipient fusion may occur on the edges of these fragments, 

which therefore have a tendency to cohere 

GENERAL EFFECTS OF BURNING ON THE OVERLYING STRATA. and thus form a stable rock 30 or 40 per cent 

of whose volume is air space. It is chiefly 

The rocks overlying a burned-out coal bed through the crevices in this rock that oxygen 

present widely different appearances, ranging is supplied to the coal burning farther back. 

from Teddened and only slightly hardened How far the burning extends back from the 

shale or sandstone through vesicular glassy outcrop is a n1atter on which there is little 

slag to gray medium-grained rock. These positive info'rmation. Field studies indicate 

broader·differences are due chiefly to the degree ~hat a bed under 20 feet or less of cover may 

of heat to which the material has been subjected~ burn out completely under large areas, and 

and their relati.ons to the coal bed may be similarly that all the coal underlying a small 

:plore clearly understood if the process of burn- butte or a narrow neck of land may burn out 

ing is briefly considered. even though the overburden is 100 feet or more 

Whatever the cause of ignition, the · com- thick. Bowie 1 cites an instance -in which 

hustion apparently always starts at the surface the coal burned out to a point almost 500 feet 

and spreads first along the outcrop. The from the outcrop, as ascertained in the course 

presence of the burning bed is disclosed by the of mining the unburned portion of the , bed_. 

(3moke and fumes which rise from it, and by According to F. R. Clark/ development at 

the heat at the surface of the earth near the Sunnyside, Utah, has shown that burning may 

outcrop or. above the bed; which becomes so extend even farther back under high spurs 

intense that all vegetation is killed. As the jutting out from the main ridge. In general, 

coal burns out the overburden generally caves, however, where there is a cover .of more than 

and large fissures may be opened in the surface 50 or 100 feet, and where the outcrop of the 

of the ground above the bed. During this bed is fairly straight, it seems probable that 

stage of the burning the heat is for the most burning does not extend more than 200 or 300 

part dissipated and the overlying strata are feet b~ck from the outcrop. 

only slightly aff.ected. However, thin red · Although the rocks within a few feet of the 

bands may be formed by the baking of clay top of the burning bed may be partly fused, 

partings in the bed, and as these slump down those beyond are in general merely baked and 

they become curiously contorted and form, still further up are only_ slightly reddened. 

with white· bands of ash or cinders, a structure The great bulk-of the rock retains itsoriginal 

resembling somewhat that of a crumpled schist. texture, ·and though much of the shale is vitriPlate I,· B, shows a burning coal bed 11 .feet tied or hardened the sandy strata may be only 

thick exposed on Custer Creek, near Yellow- slightly affected. The most striking effect is 

stone River, between Miles City and Terry, the reddening of the rock, which is due to the 

Mont. To the left the bed is slowly burning action of moderate heat under conditions favbut has not perceptibly affected the overlying orable to simple oxidation and which theresandstone; to the right it is entirely consumed fore implies no very extensive chemical 

at the outcrop and the overburden has slumped changes. 

down irregularly. Though_ fusion is for the most part confined 

As the burning progresses back from the to the rocks directly above the burning bed, 

outcrop the heat is conserved and tends to act it may locally extend many feet above. The 

.more strongly -on the overlying rocks, until gases formed during combustion of the coal es.:. 

finally a point is reached where combustion is cape chiefly through ·fissures in the overlying 

smothered by the lack of oxygen. The under- strata and apparently are hot enough to fuse 

lying strata are scarcely affected, if at all, and the rock thoroughly along their paths of escape. 

in many places the coal does not burn entirely 

to the base of the bed. Baking and hardening 1 Bowie, Alexander, The burning of coal beds in place: Am.Inst. Min. Eng. Trans., vol. 48, p. 181, 1915. · 

of the overlying strata doubtless begin a few 2 Personal communication. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 108 PLATE I 

A. A CHIMNEY OR CORE OF HARD FUSED ROCK PROJECTING 

THROUGH PARTLY BAKED STRATA NEAR MILES CITY. 

Photograph by C. D. Smith. 

B. COAL BED 11 FEET THICK ON CUSTER CREEK NEAR YELLOWSTONE RIVER 

BETWEEN MILES CITY AND TERRY. 

To the left the bed is slowly burning but has not yet perceptibly affected the overlying strata; to the 

right it is entirely consumed, leaving only contorted white bands of ash and darker bands formed 

by the baking of clay partings. 

EFFECTS OF THE BURNING OF COAL BEDS IN EASTERN MONTANA. 

BAKED SHALE A~D SLAG FORMED BY BURNING OF COAL BEDS. 5 

As the melting point of silic.a is above 1,600° C., I oxygen, a considerable proportion . of carbon 

however, it is highly improbable that this ac- monoxide is probably for~ed, an~ If the rock 

tion is accomplished by the spent gases, the is ferruginous the reduCing, actwn may . be 

products of combustion, alone. It is reasonable very striking. Directly above the. burnmg 

to suppose that the coal itself becomes highly bed and along the paths. of the escaping gases 

heated, as well as the overlying rocks, and that the iron is partly or wholly :educed and gray, 

considerable distillation goes on a short dis- green, yellow, or black slag IS formed. When 

tance back of the burning face. According to the reduction is partial magnetite may be proBowie/ in the King mine, on North Fork of duced, in some places in sufficient quantity 

Gunnison River Colo. to affect the compass needle. When the rock 

' ' is thoroughly fused under reducing conditions, the coal adjacent to the burned-out area seemed to have 

its cohesive quality entirely destroyed. It was soft and however, a great deal of the ferrous iron formed 

very easily mined but could not be utilized even for steam is taken up in the formation of new minerals 

fuel as the volatile constituents of the coal seemed to and a light-colored slag results. A reducing 

hav~ been expelled by the heat. We had a zone of sev- atmosphere may also be developed in beds of 

eral hundred feet of this kind of coal. carbonaceous shale that are highly heated by 

It is probable that the gases arising through the burning coal bed, but most of the rock outfissures in the overlying rocks contain a consid- side of the chimneys is ·merely baked .under 

erable proportion of combustible matter, which oxidizing conditions to the characteristic redexplodes or burns on reaching a supply of dish color. 

oxygen, and that the intense heat necessary to 

fuse rock 7 5 feet or more above the bed is thus 

transmitted. 

PETROLOGIC CHARACTER OF THE ROCK~ 

FORMED. 

Apparently small fissures may control the CHARACTER OF THE ORIGINAL SEDIMENTARY 

paths of the gases and thus start the lormation ROCKS. 

of well-defined chimneys. As the material 

forming the immediate walls of the fissures 

becomes molten it tends to sink and clog the 

passage, so that the gases may be slightly 

diverted and thus be caused to act on_ a greater 

volume of material. A similar effect is produced when a fissured zone rather than a single 

fissure controls the escape of the gases. In this 

case, however, the heat is less concent.rated and 

may not be sufficient to melt more than the 

edges of the interfissural blocks. A single fissure may lead to the formation of a core of 

molten material extending many feet up 

through partly baked rock, whereas a fissured 

zone apparently produces a larger and less 

sharply defined core of partly fused fragments. 

On cooling, the fused or partly fused rock coheres and forms a roughly cylindrical mass of 

hard "clinker." Chimney-shaped masses of 

this kind resist erosion to a greater degree than 

the partly baked strata around them and 

weather to the curious pinnacles that commonly 

surmount clinker bluffs or buttes (Pl. I, A). 

Aside from the strictly thermal effects of 

the heated gases, they also play an important 

part as reducing agents. As the combustion 

of the coal takes place in a scanty supply of 

1 Bowie, Alexander, op. cit., p. 182 .. 

Although the degree of heat and the rapidity 

of cooling govern to a large extent · the general 

character of the materials formed, the fundamental factor is of course the composition of the 

original rock. The coal-bearing formations of 

the Western States are made up largely of shale 

and sandstone. The shale may be yellow, 

bluish or greenish gray, or brown and carbonaceous; less commonly it is reddish. The sandstone, which is usually gray, yellow, or brown, 

is generally more or less arkosic and may carry 

25 per cent or more of feldspar, hornblende or 

pyroxene, and mica. Under the microscope 

the rock from some localities or formations is 

seen to be made up of predominantly angular 

grains, and the feldspars and ferromagnesian 

minerals are surprisingly fresh. In other 

specimens, however, the grains are more 

rounded, the feldspars kaolinized, and the 

ferromagnesian minerals altered to chlorite or 

serpentine. Accessory mineralH, such as garnet, 

zircon, and magnetite, are present in most of 

the sandstone in minor quantity, and there is 

usually much fine clayey interstitial material, 

commonly iron stained. Some of the rocks 

examined, notably those of the Lebo shale 

member of the Fort Union formation (Ter-

·tiary), are evidently derived from fairly basic 
6 H RTER TRIB TI .. T E TER \ L 

inter 

y 1 17. 

mblino· lu 

f d 

allw 

n id rabl 

n ·tituting a 
j, 

BAKED SHALE AND SLAG FORMED BY BURNlNG OF COAL BEDS. 7 

minor facies of the metamorphic products. positive; - axial angle nearly 90°; dispersion 

The recrystallized material is generally, if not strong, and red greater than violet; extincinvariably, confined to the chimneys. As a tion angle 23°-25°; and indices of refraction 

rule it is gray to dark greenish gray, and exceP,t about 1.655 and 1.675. The mineral is pleofor its vesicularity is very similar in appearance chroic, deep yellow to apple-green. These 

to a fairly basic igneous rock. The grain is gen- properties indicate that it is not epidote and 

, erally fine, though in one specimen a crystal a suggest that it may be clinoenstatite. The 

tenth of an inch in diameter resembling pyrox- colorless mineral is biaxial negative in charene was observed. All gradations between acter, has a large axial angle, and has strong 

holocrystalline material and glassy slag may be dispersion, red being less than violet. Its 

found, and in most of 'the specimens examined birefringence is about that of quartz, and its 

a small amount of interstitial glass occurs. In refractive· indices are about 1.615 and 1.625. 

one slide remnants of the original quartz grains These properties suggest andalusite or some 

appear together with amorphous glass and form of wollastonite, though the mineral 

small, slender newly formed crystals. does not resemble either of these in habit. 

A specimen from Tongue River near the Two other minerals are present that could 

Wyoming-Montana line (T. 9. S., R. 40 E.) was not be identified. One is pleochroic, yellow 

studied under the microscope and found to con- to colorless; it is uniaxial negative in charsist chiefly of diopside and basic plagioclase. acter and its refractive indices are about 1.62 

The mineral called diopside has the following and 1.64. The other, which is not abundant, 

optical characters: Biaxial positive; axial angle is isotropic, colorless, and lacks cleavage; its 

. about 60°; dispersion weak with red greater index of refraction is about 1.61. In addition 

than violet; extinction angle, 36°; and in- the rock contains a basic labradorite, occurring 

dices of refraction about 1.675 and 1.702. The in small but well-twinned laths, magnetite, 

plagioclase has a maximum extinction angle of and a few grains of garnet, probably almandite. 

38° in microlites and refractive indices of 1.57 Several rounded xenoliths of shale · surrounded 

and 1.58, which indicate a basic labradorite or by contact rims of the yellow and white 

bytownite of the approximate composition minerals already described were observed. 

Ab1An3• As shown in the accompanying photo- (See Pl. III.)-

micrograph (Pl. II, B), the diopside crystals A specimen collected in T. 1 N., R. 36 E., 

have a tendency to form radiating groups, and not far from the one just described, appears 

the plagioclase occurs in well-formed laths to have been derived from a more highly 

and microlites. Some garnet, apparently al- aluminous shale. It is hnly partly recrystallized 

mandite, was observed, and magnetite in small and consists largely of glass, but acicular 

grains is fairly abundant. As near as can be crystals of sillim~nite (fibrolite) are abundant, 

ascertained the rock from which this material and several grains of what seem to be cordierite 

was derived is a sandy shale; consisting largely were observed. Spinel may be present but 

of kaolin, feldspar, and altered ferromagnesian could not be positively identified. Close to 

minerals, together with subordinate quartz and the chimney in which this mineral occurs a 

some calcite. mass of specular hematite about a foot in 

A sp_ecimen collected near the head of diameter was found. The hematite appears 

Sarpy Creek (T. 1 N., R. 37 E.) resembles' to be closely associated with the chimney, 

the specimen from Tongue River megascopi- although it lay rather in ijhe partly baked 

cally, but under the microscope appears to be shale through which the chimney projects. 

a very different rock. A larger number of This remarkable occurrence is discussed on 

minerals are present and the characters of the pages 9-10. 

most important of these do not correspond All the rocks described appear to be irreguwith those of any of the ordinary rock-form- lar in composition, and different portions of 

ing minerals. The rock consists largely of the same specimen may contain different 

greenish phenocrysts-at first sight suggesting minerals. It is probable that a detailed 

epidote-s.et in a mosaic of colorless grains study would reveal many more species, and 

resembling melilite. The optical characters some have indeed been reported by other 

of the green mineral are as follows: Biaxial observers. In slag~ from eastern Wyoming 
8 SHORTER CONTRIBUTIONS TO G,ENERAL GEOLOGY, 1917. 

Bastin 1 finds oligoclase, pyroxene, and cordi- Within the chimneys and d~rectly above the 

erite, in addition to magnetite and hematite. burning bed, where the rocks are fused under 

He describes a vein of slag about one-tenth of conditions favorable to reduction, more exan inch in diameter-penetrating red argillite tensive changes take place. The conditions 

as follows: . · · - are, of course, very different from those under 

In passing from th~ reddish argillite toward the center which igneous rocks are formed, and are analoi the vein the contact zone shows a gradual decrease ogous rather to those in the slag furnace. 

~n the .normal shale constituents and a development, in The pressure is low, mineralizing. agents are 

mcreasmg amount and coarseness, of purplish-blue b d 

pleochroic cordierite. Next the coars~r part of the a sent, an much of the water that the rocks 

vein this mineral is present to the exclusion of all others. contained is doubtless quickly driven off. 

In this contact zone the red iron oxide of the argillite The · material involved, however, is different 

has been wholly reduced to magnetite. The central from that which enters into the composition 

portion of t~e :e~n is a some~hatvesicular, holocrystalline of most furnace slags, being generally argilmass, conslAtmg of _ abundant magnetite in irregular 1 h 

masses, some hematite, usually lining the vesicles and aceous rat er than calcareous, and many of 

following fractures, and abundant cordierite, feldspar, the minerals formed are therefore probably 

and pyroxene. unlike those common in artificial slags. In 

Hibsch 2 has described similar material this connection the classic researches of V ogt 

from the Mittelgebirges (northern Bohemia) and Morozewicz, on prepared slags of known 

under the name Kohlenbrandgesteine. In the composition, are of interest. 

recrystallized specimens he finds magnetite, It is impracticable to correlate many pieces 

cordierite, epidote in well-formed crystals, of slag with the unaltered rock from which 

plagioclase, tridymite, , and abundant dark they _were derived, _ owing to the 'irregularity 

grains that may belong to the spinel group. in composition of the strata. Furthermore, 

unless fusion is very complete and the molten 

CHEMICAL CHANGES. mass is well mixed its mineral composition 

The· most widespread chemical change pro- may also be irregular. However, the diopsideduced in the overlying strata by burning coal plagioclase rock described above is e_vidently 

beds is dehydration under conditions favor- derived from a sandy shale, and after microable, to oxidation. This change affects chiefly scopic study a sample of this shale was analyzed 

the limonite and the siderite or other ferrous by the writer in the hope that the analysis as 

iron co-mpounds, which are converted into recast might explain the mineral character of 

hematite, with consequent reddening of the the slag. The composition of the shale is 

rock. Some of the hydrous silicates may be shown by analysis 1 in the accompanying table. 

partly or wholly dehydrated without ·fusion The sample analyzed had been partly baked,. 

under some conditions, but if this change which explains the ferric condition of nearly all 

takes place it is difficult to detect. In one the iron. The analysis accounts in a general 

specimen, however, the writer observed a large way for the mineral composition of the slag, 

fresh grain of green augite in a sandy shaie indicating rather high iron, lhne, arid magnesia. 

that had been baked but not fused. The According to Morozewicz, 4 diopside or heden_, 

augite could not be a product of s~condary bergite is formed in a slag in which the molec:-

crystallization, and all the other constituents ular ratio of Fe+ Mg : Ca is less than 3 : 1. 

of the rock were considerably altered. This This analysis shows a ratio of only 94 to 89,. 

suggests that it may have been re-formed by or if all the iron is calculated as ferrous, _ of 

the dehydration of alteration products, such as 140 : 89, and pyroxene of the diopside-heden-· 

chlorite or serpentine, though the writer is bergitc type is therefore to be expected. 

aware that such a process is not well supported According to the same investigator,5 augite is 

by"laboratory results.s generally forined when the melt contains 

more than 50 per cent of silica, and .though no 1 Bastin, E. S., Note on baked clays and natural slags in eastern 

Wyoming: Jour. Geology, vol. 13, pp. 408-412, 1')(}5. 

~ Hibsch, _J· E., Geologische Karte des bohmischen Mittelgebirges: 

Mm. pet. Mitt., vol. 27, pp. 35-40, 1908. 

3 Clarke, F. W., and Schneider, E. A., Experiments upon the constitution of certain micas and chlorites: U. S. Geol. Survey Bull. 113, 

pp. 27-33, 1893. 

augite was observed in this slag it might, if 

irregularly distributed, easily escape observa-· . I 

4 Morozewicz, Josef, Experimentelle Untersuchungen der M:inerale. 

im Magma: Min. pet. Mitt., vol. 18, pp. 113-128, 1898. 

5 Idem, P"?· 113, 120, 123-124. 
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 108 PI,..ATE Ill 

PHOTOMICROGRAPH OF HOLOCRYSTALLINE SLAG. 

BAKED SHALE AND SLAG FORMED BY BURNING OF COAL, BEDS. 9 

tion. The analysis, however, indicates a more 

acidic plagioclase than that found in the slag 

and suggests that an excess of silica would 

probably be left to crystallize as quartz or 

tridymite, which are apparently absent. These 

discrepancies are presumably due to irregularities in the composition of both the shale 

analyzed and the slag. 

Analyses of baked shale from Fort Union formation, Montana. 

[G. S. Rogers, analyst.] 

1 

Si02.----.-.-..................... 65.41 

Ti02 ••••••••••••••••••• ___ • _...... 1.17 

Al20 3 -- ••••••••••••••••••••••• _.. 11.94 

Fe20 3 •••••••••.•••••• _ •••• _. _____ •• 7. 38 

FeO ......... "·-------- _ ......... _ __ . 76 

MnO ....................................... . MgO ................ -----......... 1. 50 

CaO ............................ _ . 5. 02 

K20----------------...... .. . . . . . . . I. 96 

~:00~----~~--~:~~---------~:::: :::::::::: 2: ~~ H20+---------------------------·- . 50 

C02 •••••••••••••••••••••••••• _. _.. 1. 57 

803 •••••••••••••••••••••• · •••• _ • _. . · • 22 

P20s---------------.------- .. ----- Trace. 

100.34 

1. Sandy shale from upper Tongue River, T. 9 S., R. 40 E. 2. Clay shale from T. 1 N., R. 36 E. 

2 

58.30 

. 28 

20.50 

4.23 

.46 

. 18 

1. 31 

4.40 

2.28 

1.46 

1. 03 

1. 90 

3.37 

. 19 

Trace. 

99.89 

The third specimen described, containing 

sillimanite, cordierite, and probably spinel, is of 

interest as indicating the minerals that form in 

a m.elt more highly aluminous than any normal 

igneous magma. The composition of the shale 

from which this slag is derived is shown by 

analysis 2. The specimen analyzed had been 

moderately baked. It is uniformly fine grained 

and under the microscope appears to consist 

largely of clayey material together with a few 

grains of quartz and some calcite. 

The analysis indicates a slag similar in composition to some of those studied by Morozewicz/ who was able to formulate the conditions under which the several aluminous minerals form. If the molecular ratio of alumina 

is greater than the sum of the ratios of lime, 

soda, and potash, the magma is supersaturated 

with respect to alumina. If the general formula of the magma is written 1RO.mA120 3 .nSi02 

the following rules aP,ply: When magnesia and 

iron are very low, if n is less than 6 corundum 

will form, but if n is greater than 6 sillimanite 

will crystallize out. When magnesia and iron 

I Op. cit., pp. 22-83. 

are present in appreciable amount, if n is less 

than 6 corundum andspinel are produced, but 

if n is greater than 6 spinel and cordierite will 

form. Analysis 2 indicates the formula 

125R0.201Al20 3 .971Si02, or 1R0.1.6Al20 3 .-

7.7Si02; as m is greater than 1 and n is greater 

than 6 the melt is supersaturated with respect 

to alumina, and the alumina should form spinel, 

cordierite, and sillimanite. The sum of the 

molecular ratios of iron and magnesia is 64, or 

less than the difference between those of RO 

and Al20 3 • It may be supposed that cordierite 

and spinel would be formed until the magnesia· 

and iron were nearly exhausted, and that the 

balance of the alumina would then be consumed 

in the formation of sillimanite. Unfortunately 

the slag examined was incompletely crystallized, · and the order of formation of the minerals could not be determined. Cordierite is 

reported by both Bastin and Hibsch, and a 

mineral resembling spinel was found by Hibsch, 

which indicates that the aluminous silicates are 

probably common in the slags formed by the 

fusion of shales, and as magnesia and iron are 

generally present in considerable amount cordierite and spinel are doubtless more common 

than sillimanite or corundum. 

As already mentioned, a mass of hematite 

weighing several pounds was found close to the 

chimney from which the slag described above 

was taken. A partial analysis of the hematite 

is as follows: 

Analysis of hematite from T. 1 N., R. 36 E., Montana. 

[Chase Palmer, analyst.] 

Ferric iron (Fe20 3)............ • • • • • • • • • • • • • • • • • • 83. 57 

Sulphur (S) ...... , ............................ _ Traces. 

Chloride (insoluble) (Cl)---------------- .... _ .. __ Traces. 

Metallic iron (Fe) ......................... ____ . None. 

Ferrous iron (FeO) ....................... _,..... None. 

The origin of this mass of hematite is not 

easy to tmderstand. So far as the writer knows 

hematite has never been observed elsewhere 

in these flat-lying and only partly consolidated 

strata, and the specimen described must 

therefore have been formed in some way 

through the agency of the burnivg coal. It 

might be ascribed to the oxidation of a large 

pyrite or ironstone nodule, but a careful search 

failed to reveal any such nodules in the unaltered beds near by. On the other hand, if 

it is considered to have resulted from segregation during heating or fusion, the details of 
10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1917. 

the process are very difficult to explain. As 

shown by analysis 2 (p. 9), the surrounding 

rock contains less than 5 per cent of iron, and 

rather extensive concentration would therefore 

have to be postulated. According to Vogt/ 

he~atite rarely occurs in ordinary furnace 

slags, as it can crystallize only when ferrous 

compounds are present in very subordinate 

amounts. In the surrounding rock ferrous 

iron is practically absent, but in order to 

. concentrate the ferric iron complete fusion 

would appear to be necessary,. and as this seems 

·always to take place under conditions favorable 

to reduction, much of the ferric iron would 

b~come ferrous. If the iron had occurred 

directly in the chimney as native iron, or even 

as magnetite, it might therefore be supposed 

that a smelting process akin to that in the 

ordinary blast furnace had taken place, but 

as it was found at the edge of the chimney and 

is completely oxidized to hematite this view 

does not appear to be tenable. 

A third hypothesis that might be suggested 

is that the hematite formed as it forms around 

volcanoes, through the decomposition of ferric 

chloride or sulphate by steam or heated air. 

The ground water of the region in which the 

hematite was found carries a considerable 

proportion of sodium chloride and of sodium, 

calcium, and magnesium sulphate, and as these 

salts are not uncommon as incrustations on 

1 Vogt, J. H. L., Beitrage zur Kenntniss der Gesetze der Mineralbildungin Schmelzmassen, pp. 215-217, 1892 

the rocks 3tn adequate supply of chloride or sulphate is undoubtedly present. On the assumption that hydrated ferric oxide (limonite) is 

capable of uniting with sodium chloride under 

the influence of intense heat, it seems probable 

that the ferric chloride so formed would ascend 

as vapor with the other gases and would be 

deposited as hematite in the upper part of the 

chimney. It is by no means certain that the 

primary reaction between disseminated limonite or siderite and soluble chlorides or sulphates is chemically possible under the conditions prevailing, but the presence of traces 

of basic (insoluble) chloride in the hematite is 

difficult to explain on other grounds and 

therefore tends to support this hypothesis. 

Native iron of secondary origin has been 

observed near coal beds in other localities. 

On North Saskatchewan River, 70 miles 

above Edmonton, Alberta, lignite beds have . 

burned -and have reduced the large clay ironstone nodules in the overlying strata to metallic 

iron, masses of which weighing from i5 to 20 

pounds have been found. 2 At three localities 

in Missouri drill holes have penetrated small 

masses of soft malleable iron in the sedin\entary 

rocks above coal beds,3 but the genetic relations of the iron and coal in this case are not 

so clear. 

2 Tyi-ren, J; B., Naturally reduced iron: Am. Jour. Sci., 3d ser., vol. 33, 

p. 73, 1887. . 

a Allen, E. T., Native iron in the coal measures of Missouri: Am. 

Jour. Sci., 4th ser., vol. 4, pp. 99-104, 1897. 

0