Coal: Its Occurrence and Origin; excerpted from Chapter XXX of "The New Geology" by George M. Price; 1923; pp. 454-470.
General Remarks. "Although many thousands of men," says Professor Suess, "work day and night in our Coal Measures, and although many acute observers are led by their profession to make the study of these deposits the business of their life, yet the mode of formation of the coal beds is still far from being satisfactorily explained." ("Face of the Earth," Vol. 2, p. 244.)
These words seem like a quite unpromising introduction to our subject; yet we shall do the best we can to present a rational and satisfactory explanation of the origin of the coal deposits. But first we must note the occurrence of coal and some of its characteristics.
Almost all the geological systems of rocks contain coal beds of some description; but the most important beds occur in the Carboniferous, the Cretaceous, and the Tertiary. Of these, the Carboniferous is the most important, and has been estimated to include seven tenths of the total coal beds of the world. In North America, the Carboniferous coals are east of the 100th meridian, the Tertiary are mostly west of the 120th meridian, with the Cretaceous beds in between. In Europe, the coal in the British Isles is chiefly Carboniferous, as are also some of the coal beds on the Continent; but the immensely thick brown coal beds of Germany, from 75 to 150 feet thick, are classed as Tertiary. Beds of brown coal even much thicker than this are known in Australia. China has very extensive coal beds; but they are mostly Carboniferous.
As will appear later, the distinction between these different geological systems is based on the kinds of fossils associated with the beds, and it is a common impression that only in the Carboniferous strata do we find good coal. But this is a mistake. Splendid anthracite, as good as any in Pennsylvania or Wales, occurs in the Cretaceous formations of British Columbia and Alberta, or even in the Tertiary beds there and elsewhere. But in a general way, it may be stated that the Carboniferous coals are more likely to be anthracite or bituminous, while those of the Cretaceous and Tertiary are more likely to be less carbonized, or even to be little more than pressed wood, as is the case with so many of the lignites. The latter usually occur at the surface of the ground, or close beneath the surface; while the Carboniferous beds are sometimes many thousands of feet down. This makes an immense difference in the amount of pressure which these formations have been subjected to; and this difference in pressure, together with differences in the heat and other metamorphic agencies to which they were subjected, will partly account for the differences in the coals as we find them. Yet it may be that the original differences in the plant materials entering into the beds may be largely responsible for the distinctions which we now observe.
Mineral Matter. In every coal region, there are always many very thin seams which are worthless as a source of coal, and also many seams of shale which are so impregnated with carbonaceous matter that they can be made to burn, though with a large amount of residue as ash. Good coals show only from 5 to 1 percent of ash, but the percentage often runs higher. This ash is due to earthy or rock materials which are mixed up with the coal, having been included in the deposit when it was first made. Probably all the original plant remains, when first buried, contained much mineral matter; but in many of the larger or thicker seams, most of this mineral matter has been dissolved and leached out by the acids generated in the fermenting and decomposing mass, as will be explained later. The wonder is that so much of the coal is so nearly free from mineral matter.
Color of the Associated Strata. Red or yellow rocks are usually absent in the coal regions, because of the acids generated by the organic deposits, which dissolve the iron oxides which cause these bright colors. A similar explanation is made of the so-called "underclay" which is often found at the base of a coal bed. This is a fire clay; that is, it has lost the basic alkaline materials which would act as fluxes and cause it to melt easily, so that now it is almost infusible, and can be used as the lining for blast furnaces, and in other places where intense heat must be resisted. It is often gray or almost pure white for several inches or even for several feet below the coal, and has been produced by the carbonic acid and other acids which formed in the decomposing mass of plant remains, these acids dissolving and carrying away, first the alkali material in the feldspar, and next the iron compounds, until the blanching is often complete throughout a thick bed. The great quantities of carbonic acid generated in these plant deposits will also serve to account for the very frequent occurrence of siderite, or iron carbonate, in association with the coal beds. Beds of iron carbonate several feet thick often alternate with the coal beds, being repeated many times in a vertical section. These beds of iron ore are also frequently found resting upon a pure fire clay, just as the coal does.
It is often stated that this "underclay" contains the remains of the roots of the plants (trees) which grew in the beds above, and that in many instances, the stumps of these trees are still found in situ, as they grew. It is curious how this assertion has been repeated over and over again in all the textbooks, being copied from one to another for more than half a century. However, the fact is that, while stumps are often found in the coal beds, it is only by the help of the imagination that anything like true roots can be detected as attached to them. As for the frequent assertion that the small rootlets can be still made out in the underclay, being derived from the plants which grew above, it is wholly imaginary. Of course, even if the vegetable matter were washed into the position of these beds and buried, the trees would in many instances carry their stumps and roots with them, and if the stem were broken off a few feet above the roots, the stump would in most cases sit upright, as is so frequently seen in snags in the rivers which have been floated down by floods or freshets; and many roots are always attached to the stumps in such instances. It may be confidently affirmed that there is no well-authenticated case where the presence of such old stumps in the coal beds can be proved to have been due unquestionably to growth on the spot. The wish is father to the thought in such reports.
Classes of Coals. There are four leading types of coal, -- brown coal, lignite, bituminous, and anthracite.
Brown coal is little more than a compacted mass of plant remains, and is usually more or less of a homogeneous character throughout. It may vary in color from a light yellow to a deep brown; but ordinarily not much can be determined regarding the kind of plant materials entering into its formation. Only in rare instances can anything like a positive identification of a particular type of plant be made; but if we suppose that the materials in the coal itself were similar to the plant remains preserved often with such exquisite perfection in the shales above the coal, we shall have no trouble in fixing upon the kinds of plants entering into the composition of these coals.
The brown coals burn with a sooty flame, a strong odor, and but little heat. They generally run from 55 percent to 75 percent of carbon. In density, they range from 0.5 to 1.5. Many of the Tertiary formations contain brown coal, the beds often being of an astonishing thickness; and these beds often occur near the surface.
Lignite is the name given to coal which has been made chiefly out of trees, such as conifers, and which has been only partially converted into coal. Often the texture of the wood can be distinctly made out. Lignites are common throughout the Mesozoic and the Tertiary formations. A regular gradation can be traced from the little altered lignites to the true bituminous coals and the anthracites. It even occurs that a seam will be a lignite in one locality, and if followed up only a few hundred feet, will turn into a true coal, and perhaps even into a true anthracite, the change being due to some local source such as heat, such as a dike. 1
The ordinary soft coal is termed bituminous. It is black in color, of a bright luster, and quite brittle. Usually it runs from 75 to 90 percent in carbon, and contains a greater or less amount of sulphur. In density, it ranges from 1.2 to 1.35. It burns with a clear flame; though there is a great diversity among the different varieties with respect to their behavior in the fire.
Under the microscope, the organic structure can occasionally be seen; and the kinds of plants contributing to its formation can almost always be recognized from the well-preserved leaves in the shales capping the coal bed.
Anthracite is the purest hard coal, and contains over 90 percent carbon. It has a vitreous or submetallic luster, and breaks with a conchoidal fracture. Its density is from 1.35 to 1.7. It is difficult to ignite; but when once raised to a sufficient temperature, it burns with an intense, steady heat, without smoke or odor. Many anthracites are found in regions where the rocks have been disturbed; so they are commonly spoken of as metamorphosed bituminous coals. It is usually more difficult to make out the kinds of plants in the beds above the anthracites, as all traces of leaves have generally disappeared in the metamorphism which has accompanied the transformation of the coal. However, occasionally perfect leaves or other parts of plants have been discovered, so that we know with reasonable accuracy the kinds of plants contributing to these formations.
The Peat-Bog Theory. We must now consider briefly the problem of how the coal beds were probably formed; that is, how the plant remains were accumulated which were subsequently converted into the coal.
For nearly a century, we have been pointed to the peat bogs of the cool temperate regions as the nearest approach to the present formation of a coal deposit. We have already considered the growth and character of these peat bogs. They are confined to cool climates; no such deposits exist in the tropics. Hence, as all the coal deposits found in our fossiliferous formations were certainly produced under tropical or semitropical conditions, the mangrove swamps of the tropics, or the Everglades of Florida, are often mentioned as better modern analogies. But in all cases, it is something in the nature of a swamp or a marsh or a bog which is used as a parallel. Of course, if formed by gradual accumulation on the spot where we find them, a cover of water would be necessary to keep the materials from oxidizing too rapidly. The stumps and roots said to be present in the underclay are always referred to as evidence of growth in situ. These arguments have hitherto seemed conclusive; and we are told that we have here an ancient soil, which, after having accumulated its load of peatlike material, gradually settled down beneath the waters, and had another layer of earth or sand spread out over it to form the soil for a new growth of vegetation. The land then rises -- or the sea retires, which is the same thing -- until this layer of clay or sand is just about the proper distance above the surface again. Thus the same course is repeated age after age, up and down, up and down, there being frequently 75 or 100 successive seams of coal piled one above another, each with its "underclay," and each "underclay" having what are alleged to be the roots of the plants in the beds above.
Difficulties. That the "pulsating crust" could thus rise and fall ad libitum, was regarded as being eminently reasonable during the first half of the nineteenth century, for then everyone thought our earth to be fluid inside. However, since the demonstration of the earth's remarkable rigidity, this traditional explanation of the rise and fall of the crust has taken on a much more doubtful cast; but even some who, like Dana, were compelled to accept the earth's essential rigidity, still clung to the old idea of the land's rising and falling, or s they expressed it, "oscillating" at sea level, in some way not exactly explained.
But when we consider that over wide areas, all these successive coal beds are approximately parallel to one another through the whole series of from 50 to 100 successive beds, and that thus all these countless elevations and subsidences must never have disturbed the exact horizontal position of these beds throughout hundreds or even thousands of square miles, but must always have carried them up and down with the same care with which a waiter carries a full plate of soup, another idea suggests itself: How much better it would be if we only suppose the land to be stationary and the sea to be rising and falling, transgressing over the lands and retreating from off them! Of late years, this explanation has been steadily advanced be many eminent scientists; and it would have gained decidedly more acceptance if its defenders could only have shown how these transgressions of the ocean could take place in a few localities without affecting all the coasts of all the continents -- which is far more than is required.
An objection which lies against either theory is that all these areas affected must repeatedly have gone through all the experiences of a seabeach; for "with each foot of submergence, the seabeach would be set a little farther inland, so that the whole would successively pass through the conditions of a seashore." (Dana.) Of this condition there is no evidence whatever throughout these deposits; yet it is difficult to understand how the marks of such a seashore experience could be obliterated, even if such an experience did not destroy entirely the accumulated peatlike deposits.
Here is Professor Dana's naive statement of the exactness required in adjusting the superior and the inferior limits of the land or of the water, as the case may be:
"For the making of extensive Coal Measures a nice balancing of the land surface between submergence and emergence was a requisite. With a very little too much emergence, even if only a few hundred feet there would have been no marshes in North America; for the land would have been drained. And with a little too much submergence, the limestones or barren sediments of sand or gravel would have covered the region. North America was admirably arranged and poised for the grand result." -- "Manual," pp. 710, 711.
It is needless to comment on such a statement.
The next thing that claims our attention is a problem connected with the wonderful similarity in the kinds of plants throughout the set of beds in any given locality. At the South Joggins, Nova Scotia, there are 76 successive seams of coal; in the British coal field, England, there are 87 coal beds; in South Wales, over a hundred, 70 of which are worked; in the Liege basin of the Continent, 85; and in Westphalia, 117 successive seams of coal.
No wonder Huxley estimated the length of the Carboniferous period (on the basis of uniformity) at about six million years. Some have made it many times this. But how is it that through all this vast lapse of time, a series of ages so prolonged that our historic period is but the tick of a clock compared with it -- how is it that during all this time, the particular plants growing in these localities remained constantly the same, not only unchanged in general aspect, but practically unchanged even in genera and species? Whenever in our modern world a region of spruce or pine forests is completely burned over and destroyed, the next growth is almost certain to be some entirely different kind of vegetation, such as maple of birch. In Denmark, three or four such successive forests have occupied given localities within quite modern times, while in New Brunswick and Nova Scotia, as Dawson has shown, a complete change of this character has occurred over and over again within a single generation. But strange to say, during all these uncounted millions of years (?) of the "Coal Period," while the country was being "desolated again and again, either universally or partially, by the returning waters, and over the large submerged areas kept desolate for many centuries or series of centuries again and again" (Dana, "Manual," p. 666), the vegetation continued ever the same, the very same plants being found in the upper beds as in the lower, and practically identical the whole world around, wherever the Carboniferous rocks have been discovered, whether in North America, Europe, Asia, South Africa, or South America. Surely this is a very strange inconsistency which this theory compels us to believe in.
The Underclay. We must next consider the character and conditions of a modern peat deposit. Here we have a bed perhaps 30 or 40 feet thick, made largely of moss which can grow in water alone without earth, or grow on the decaying bodies of its ancestors. Trees of any size do not act thus; for through arbor vitae, speckled alder, etc., may thrive in wet, swamplike conditions, yet they require to get their roots into real soil even though submerged. So they could not possibly thrive on deposits of purely organic materials of the thickness here spoken of, or where their roots could not reach the soil below. And so far as we are aware, this is the case with all trees, and the huge coal plants ought not to have been any exception to this rule. But suppose we admit that those ancient ferns and club mosses, even though trees in character and size, could have lived and thrived in peatlike accumulations of any thickness, and that they did not need to have their roots in real soil. But even the thickest seams of coal, say 30, 40, or 50 feet in thickness, have their "stigmaria" or rootlets in the lower part, just as do the thin beds. We must also remember that for each foot of coal, we require several feet in thickness of organic material; the usual estimate seems to be that about 10 or 12 feet of peatlike deposits would be required to produce one foot of good coal. Thus we would have 300 or 400 of 500 feet of peat as quite a common thickness for the larger sort of these ancient deposits.
Now we shall hardly be prepared to claim that the roots of even the giant plants of the "Coal period" could thus reach down through these 300 or 500 feet of deposits to the soil below. The alternative is that the roots of the first plants must have remained intact and unaffected by decay all the centuries that these 300 or 500 feet of deposits were being built up. Either way, the case seems absurd.
But it frequently happens that a fossil tree is found extending up through two or more of these successive beds of coal, together with the intervening beds of shale or sandstone. Evidently in these cases, the coal was not formed in the manner described by current theory.
Another difficulty is that if the first plants lived and thrived in the soil before there were any peatlike accumulations, the same plants could hardly be expected to grow well in the top layers when there was nothing but peat, -- no soil whatever within reach. Frequently, also these bands of shale or sand, the so-called "underclays," separating two successive seams of coal, are only one, two, or three inches in thickness, and utterly insufficient amount for a soil in which trees are supposed to grow.
Thus it seems that in every way in which we can get at this theory from the a priori point of view, the evidence breaks down completely.
The Sedimentation Theory. However, it should be noted that not all geologists teach this peat-swamp theory, with its incredibly delicate oscillations of level. The French geologists, with others on the Continent, hold strongly to another explanation, called the theory of sedimentation, which claims that the plant material of ordinary coal does not represent growth in situ, but that it drifted to its present position in much the same way as ordinary sedimentary rocks.
Without going into a detailed consideration of this theory or of the arguments by which it is supported, it is quite evident that, to those who hold it, the alleged presence of rootlets in the "underclay" must be of very little weight. And this French theory, if we may so term it, evidently explains the surprisingly regular "alternation" of conglomerate, sandstone, shale, and coal seams observed in most coal basins" (Zittel) far better than the other theory. Still it fails to explain the almost universal presence of exquisitely preserved leaves and other parts of plants in the accompanying shales; and it is perfectly helpless in the presence of crinoids and corals and radiolarians alternating regularly with the coal seams, or what is still worse, in the presence of limestones and coals, which are sometimes even mingled together.
That the coal beds themselves are usually as thoroughly stratified or bedded in their structure as are the conglomerates and the sandstones, is proved by their frequent lamination, and even their alternations in shales of color. (Dana, "Manual," p. 709.) As for the "underclay" itself, it admits of only one rational explanation in the case of either theory, and so proves nothing for either side. Even if the coal plants were deposited by wholly abnormal kind of sedimentation, with great quantities of the plants perfectly fresh and green, as may have been the case, the acids generated in the decomposing mass, carried along by the water percolating from above, would dissolve or carry to the bottom of the bed practically all the earthy matter which would necessarily at first be scattered through the mass of vegetable matter. This is why coal is so purely carboniferous, and also why we generally find perfect plant specimens only in the upper part of the seam, or in the shales above the seam.
Professor E. A. N. Arber, of Cambridge University, has given us some very enlightening remarks about the "underclays." He says that "nothing could be more unlike a soil, in the usual sense of the term, than an underclay." ("Natural History of Coal," p. 95) He further points out: "Not only are fire clays commonly found without any coal seams above them, but they may occur as the roof above the seam, or in the seam itself... Sometimes coals occur without any underclay, and rest directly on sandstones, limestones, conglomerates, or even on igneous rocks." -- P. 98. "Another difficulty in connection with the underclays is that their thickness commonly bears no relation to the extent of the seam above. Often thick coals overlie thin underclays, and vice versa."
Regarding the many instances of upright stems, this author argues that --
"These stems in some instances are certainly not in situ. Examples have been found which are upside down, and in some districts the prone stems far exceed those still upright. No doubt the majority, if not all of these trunks have been drifted." -- P. 114.
The Opinion of Suess. It is a decided relief to take up a work like that of Eduard Suess, and see how this most accomplished scientist makes quite impossible and absurd the old theory of the oscillation of the land. One of his arguments against the common theory of the formation of the coal, is that there are many instances where coal beds do not rest upon anything like an "underclay," but directly on shale or limestone. Another is that in very many cases, "thick beds of coal split up into a number of smaller seams, which become separated further and further from one another by the thickening out of intercalated wedges of sterile rock" ("Face of the Earth," Vol. 2, pp. 244-246), a thing that would be clearly impossible on the basis of the current theory. Thus in Staffordshire, England, the main coal bed, 25 feet thick, splits toward the north into 8 seams, in such a manner that the sum of the seams and the intervening beds amounts to 390 feet.
But this work of Professor Suess also has its disappointments; for while the author sees the errors of the old theory, he has little better to offer in the way of explanation, and he gives the following summary of the situation, which will bear repetition here:
"Although many thousands of men work day and night in our Coal Measures, and although many acute observers are led by their profession to make the study of these deposits the business of their life, yet the mode of formation of the coal beds is still far from being satisfactorily explained." -- P. 244
The Probable Method of the Formation of the Coal Beds. If we care to go into the question of probabilities, and can picture to ourselves a wide stretch of country clothed for long periods with a most luxuriant vegetation, and it we may suppose that the remarkable atmospheric conditions of the ancient world may have absolutely precluded any parching drought, thus rendering it quite improbable that the accumulated deposits of centuries should ever be burned up by forest fires, we shall have the probable source of materials. If now, in the great world catastrophe which seems to be indicated, these accumulations of may centuries were all washed away, dead and green together, and swept pell-mell into lakes or valleys, somewhat like the great natural "raft" on the Red River, only on a far more enormous scale, the stumps of the trees would still carry may of their roots with them, and would frequently float in the natural or upright position. In this case, too, we would ultimately have the formation of an "underclay" at the bottom of the bed from the action of the acids generated in the oxidizing mass; and this "underclay" would naturally contain many roots or something resembling them. Thus these two conditions are accounted for.
Coarse conglomerates or gritty sandstones very generally underlie the coal beds or are sometimes intercalated with them, as the Millstone Grit of England and the Pottsville conglomerate of Pennsylvania. These conglomerates and grits point to large volumes of water in sudden and violent action.
Marine Remains Mixed Up with the Coal. But let us consider another proof of how these coal beds were formed; namely, the remarkable way in which various kinds of marine life are mixed up with the coal. We have already spoken of the deepsea crinoids and the clear-water ocean corals, both of which often alternate with the coal beds; and we have drawn the conclusion that these conditions indicate an abnormal tidal action. We need not repeat the evidence here.
The proofs of vast numbers of fishes having been entombed in the deposits forming the cannel coals, are of the same import. Indeed, as Dana says:
"The great number of fossil fishes in some very carbonaceous or bituminous shales, has led to the suggestion that fish oil may have been the sole source of the oil or gas yielded by the shales. It is not improbable that it was a prominent source, since the same process which will convert vegetable tissues into coal or mineral oil, will produce a like result from animal oils." -- "Manual," pp. 655, 656.
We have already spoken of the small chance which vertebrate fishes have of being fossilized under our modern conditions. Dana himself say that they "require speedy burial after death, to escape destruction." And it should be remembered that the stagnant, poisonous waters of our modern swamps or bogs do not thus teem with fishes.
Quite evidently these enormous deposits, thus packed full of fish remains, must have been formed in some wholly abnormal manner. Nothing at all resembling such things has ever been formed within the historic period. But we must remember that these fossil conditions are absolutely world-wide in extent, and are scattered through almost all the various formations.
Pyrites. Another interesting side light upon the question of how the coal beds were formed, is supplied by the frequent presence of various compounds of sulphur. Thin layers of pyrite are very common, the percentage in the anthracites being a little over one per cent, while in the coals of Ohio and Indiana and Illinois the average is over two per cent. The coals from Nova Scotia and the anthracites of Pennsylvania seem to have very little sulphur, and they contain very scanty traces of marine fossils. But all the other coals are interbedded with strata which contain relics of marine faunas; and it is now known that the presence of sulphur is due to the action of sulphur-making bacteria which exist only in connection with sea water. Hydrogen sulphide is found in great abundance in modern peat beds to which sea water has access, and is seldom found in peat which forms under fresh water. Thus the presence of sulphur in these coal beds is "an indication that the sea water had access to these beds while the vegetable matter was still recent." ("Acadian Geology," p. 164.) Even common salt, in the form of brine, is sometimes present in the coal.
Two other features characterize the coal beds as a whole, -- the lignites and the brown coals, as well as the true coals of the Coal Measures, -- and these features should decide the question of how the coal beds were produced. These two lines of facts are (1) the well-preserved character of the plant remains; and (2) the kinds of plants composing them.
Condition of the Plant Remains. With regard to the wonderful perfection with which the plants are found preserved in the accompanying beds, Sir Archibald Geikie says:
"Not much is usually to be made out from the coal itself, for the vegetation has been so squeezed and altered that the leaves and branches of the plants can no longer be recognized... But though the larger plants have not usually been preserved well in the coal itself, they may sometimes be found in great profusion and beauty in the beds of rock above or below the coal... Now and then, the plants may be seen lying across each other, in wonderful profusion, upon the bottom of the bed of rock that overlies the coal seam and forms the roof of the mine. Though all squeezed flat like dried leaves in a book, they still retain their original graceful forms." -- "Premier," pp. 68, 69; 1893.
Most geologists who have written upon this subject have bewailed the poverty of our language to convey any adequate idea of the marvelous perfection of the forms laid out to view by the thousands through the opening up of such beds of shale or fine sandstone.
Evidently such things could not have lain for centuries rotting in a swamp or peat bog. But this splendid preservation of the plant remains is a universal characteristic of all the coal-bearing rocks, not alone those of the Carboniferous system, but also those of the Jurassic, Cretaceous, or Tertiary, or even the lignites of the Pleistocene. They are all much alike in these respects; they all contain wood or leaves, flowers and fruits, in marvelous state of preservation, "with all the perfection they have in an herbarium." (Dana.) On this point, certainly, we must conclude that these ancient accumulations of plant remains were not formed as our modern peat bogs or swamps are now being made.
Kinds of Plants. But the kinds of plants contributing to the formation of the coal beds, have also a very important bearing on the problem of how these beds were formed. The plants of the Coal Measures proper are always spoken of as ferns, cycads, equiseta, lycopods, etc.; and our ignorance of how they actually grew has made it seem reasonable to suppose that they may have grown on wide, damp plains, with plenty of moisture above and below. But it must be remembered that "the ablest botanists," as Dr. Page observes, are "yet unable to assign them a place among existing genera." ("Textbook," p. 134.)
However, these Carboniferous coals constitute only a portion of the total coals of the world, all the subsequent systems containing vast coal deposits. And if we ask ourselves what kinds of plants produced these coals of the Cretaceous, Tertiary, and other formations we have to reply that they are chiefly plants and trees which do not grow in swamps or bogs, and can not, by any stretch of the imagination, be supposed to have contributed to the formation of any peatlike accumulations in the long ago. For example, we have in the Upper Cretaceous such kinds as sassafras, laurel, tulip tree, magnolia, Aralia, cinnamon, sequoia (like the "big trees" of California), poplar, willow, maple birch, chestnut, alder, beech, elm, with the leaves of some palms, and hundreds of others. In the Tertiary of England and the Continent, we have such trees and shrubs as fig, cinnamon, various palms, varieties of Proteaceae (like those of India and Australia), cypress, sequoia, magnolia, oak, rose, plum, almond, myrtle, acacia, with many other genera now found only in America. The Miocene strata of Greenland have yielded great numbers of the same genera. It would thus seem that in whatever way we examine this problem of how these ancient plant deposits were formed, we are confronted with evidences of some very abnormal action of the elements, essentially different from any conditions now prevailing.
Climate. David White has given us a very careful study of the climatic conditions indicated by the coal deposits. He first tells us that "well-developed coal has been found in the strata of every period since the Silurian" ("Origin of Coal," p. 1); and then proceeds to say that --
"During the times of deposition of most of the principal coal groups the
climate has been characterized by --
"(1) General mildness of temperature, approaching in most cases tropical or subtropical;
"(2) Conspicuous equability or approximation to uniformity of climatic conditions; ...
"(3) A generally high humidity; ...
"(4) An amazingly wide geographical distribution of these genial and equable climates, which occurred seemingly in almost uniform development simultaneously in the high and in the low latitudes of both the Northern and the Southern Hemisphere." -- P.68.
Next he gives his reasons for these conclusions, though we can give here only
a brief outline of the facts which he presents. But these conclusions are
based on the following criteria:
"1. The relative abundance or luxuriance and large size of terrestrial vegetation -- that is, rankness of growth -- indicating favorable conditions of temperature, humidity, etc.
"2. Character, condition, and amount of land-plant material ... indicates humility.
"3. Great radial distribution, seemingly over the greater part of the earth, and especially over wide ranges of latitude, of identical species and genera in characteristic association, indicating the extension of approximately uniform climatic conditions in these regions.
"4. Presence of types known to be adapted to or confined to the warm temperatures or moist climatic conditions of the present day.
"5. Structures of the plants themselves. Features showing rapidity of growth; that is, abundant rainfall, mild or warm temperatures, etc. -- conditions favorable to rapid growth:
"(a) Very large size of the cells, many with thin walls, and large intercellular spaces, indicating rapid growth and abundant moisture, noticeable in the woods found in and with most coal.
"(b) Large size of fronds and leaves.
"(c) Frequency of laciniate or much-dissected, drooping fronds and pendant branches or twigs.
"(d) Smoothness or bark, which is often thick, pointing toward warm humid swamps.
"(e) Absence of growth rings in the woods of the older coal formations, showing climatic conditions favorable to practically uninterupted growth, and the absence of long dry seasons of winter frost."
F. H. Knowlton, in commenting on these facts, agrees fully with White in the above conclusions, and declares that there was "a non-zonal arrangement" of climate prior to the Pleistocene. (Bulletin of Geological Society of America, Vol. 30, p. 541.) He further declares that the temperature of the oceans was everywhere the same and without "widespread effect on the distribution of life." (P. 548.) And he summarizes this matter of climate in the following words:
"Relative uniformity, mildness, and comparative equitability of climate, accompanied by high humidity, have prevailed over the greater part of the earth, extending to, or into, polar circles, during the greater part of the geologic time, since, as least, the Middle Paleozoic. This is the regular, the ordinary, the normal condition." -- P. 501.
Knowlton further confirms the conclusions of White, and says the argument of the latter "applies with equal force to all horizons," and even goes so far as to affirm that even the red beds, so often pointed out as proofs of arid or desert conditions, "may have been formed under conditions of warm, moist climates." (P. 506.)