|SECOND GEOLOGICAL SURVEY OF PENNSYLVANIA - REPORT OF PROGRESS
P. DESCRIPTION OF THE COAL FLORA OF PENNSYLVANIA
AND OF THE
CARBONIFEROUS FORMATION THROUGHOUT THE UNITED STATES
BY LEO LESQUEREUX; ©1879
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©George Langford III, 2011
|On the nature of the vegetation of the Carboniferous era,
and its agency in the economy of the world.
Having described the plants of the Carboniferous age, from a study of
their remains in the coal measures, it seems advisable to consider the
vegetation of that age as a whole; first, in its nature compared to the
characters of the flora of our epoch, and to its function in geology as
the prime agent producing mineral coal; and then in regard to the
distribution of the species, geographical and strati-graphical.
The first will be essayed in this chapter. In the next chapter we will see how far the ancient distribution of species can be inferred from the present distribution of their remains; and how far we can go in assigning separate fixed horizons in the vertical columns of the rocks to individual species or groups of species.
§ 2. The coal flora was made up 1, of Cryptogamous plants: Filices or Ferns, Lycopodiaceae, Equisetaceae—and 2, of Sigillariae and Cordaiteae: two groups or families of uncertain affinity to recent plants, but generally accounted Gymnosperms of peculiar conformation, related to the Cycadeae, and partly, perhaps, to the Conifers.
§ 3. The Lycopodiaciae and Equisetacaceae of the present epoch are represented by plants of small size, sparingly distributed over the whole world, with stems no larger than a man's finger.*
But in Carboniferous times all of them were of greater size, most of them large trees.
The Lepidodendron trunks measured from six to thirty centimeters in diameter, and were proportionally high.
The Calamites were smaller trees; with stems ten to thirty centimeters thick; their growth was extremely rapid; and they stood crowded close together as now in the southern swamps the Canes, which cover the surface of the ground with a dense mass of vegetation rising high into the air.
Ferns have been called the most conservative beings of the world. The Ferns of the coal flora resembled in some of their characteristic features the Ferns of the present day, as they are seen in the humid tropics. But their growth was far more luxuriant. Many of them were virtually trees, in size and aspect, far exceeding the noblest of existing Fern-trees; and none of the bushy Ferns we have now can compare in dimension with those of the Carboniferous.
The Cordaiteae and Sigillariae also were, in the main, trees of great size.
The Sigillariae were interspersed among the Lepidodendron and Fern-trees.
The Cordaites formed dense forests, quite comparable for their distribution to the pine woods which shade with gloom the tide-water region of the Atlantic and Gulf States.
§ 4. The character of such a vegetation expresses the conditions of atmospheric influence under which it grew.
Judging by the habits of their living relatives, these ancient plants grew partly immersed in the shallow water of extensive swamps.
Some, like the Sphenophyllum and Annularia, stretched their stems and expanded their foliage upon the surface of the water, while the Calamites leafed out above them.
Others,like the Sigillaria and Lepidodendron, while rooting in the swampy vegetable mould, or borne upon a solidly compacted raft of creeping Stigmariae afloat upon the lagoons, lifted their times aloft high above the underwood, seeking sufficient light and air for the due unfolding of their leaves. We know that their leaves were soon deciduous; for we generally find them preserved at the extremities of the branches only.
But all, without exception, Cordaites, Sigillaria and Ferns, were swamp plants, and that in more than the mere sense of living in or on a marsh; they were themselves so impregnated with moisture from the super-abundant humidity of the surrounding atmosphere, that they must have produced boggy ground even on elevated land, were there any such at that time.
§ 5. In the leaves of the coal plants we read their history.
The Lepidodendron had lanceolate-acuminate, small leaves, analogous in shape and function to those of Conifers.
The Fern-trees had huge fronds, curved downwards like umbrellas, displaying to the influence of the air a wide surface, sub-divided into branches, leaves with minute lobes, teeth, and even hairs. In this manner they multiplied their points of contact with the air for the condensation and absorption of its nourishing elements.
The bushy Ferns had large undivided leaves, and constituted an undergrowth like that of the swampy valley bottoms of the Amazon, or like that of the lowlands of tropical islands like Cuba. Exotic Ferns now thrive in our conservatories only in an atmosphere of great humidity and of moderately warm but uniform temperature. I say moderately warm; for there are regions in our temperate zones, the exceptional humidity of which effects without the aid of a high temperature a luxuriant growth of Ferns, suggestive of the vegetation of the tropics. Thus Schimper [ Paleont. Veget. I, p. 358.] says:
The Fern-trees of the present age show plainly enough by their geographical distribution what sort of atmosphere they require for their prosperity; for they are seen especially inhabiting such mountain slopes and. summits as are bathed in fog and cloud; and affecting such altitudes as suit the periodical condensation of prevailing winds. The upper and lower limits of the Fern-zone on the Sandwich islands are distinctly defined, and mark the upper and lower limits of the zone of condensation. So it is in the islands of the Gulf of Mexico, on the mountains of tropical Asia, and in fine wherever the Fern-trees are known to grow.
The Lepidodendron leaves, small, linear-lanceolate, acute or acuminate in shape, presented an unusually large surface to the air, compared with other leaves. They could condense and absorb more vapor. The abundance of Lepidodendron in the coal age, therefore, indicates a prevailing humidity of climate. Our Conifers, with leaves constructed on the same principles, thrive especially well along our southern seaboard, covering it in fact with an immense forest. We find them also on the tops of our mountains; on the plains of the north, which are so often covered by fogs and clouds; along the rocky sides of small streams, and especially in boggy places, where Tamarack and Bald-cypress root themselves, like the ancient Sigillariae, in the swamp mud.*
§ 6. The sameness of the types of coal plants in all countries where coal beds have been found and examined, proves the prevalence of an invariable or uniform moderately high temperature during the coal age, just as plainly as the facts stated in the preceding paragraphs prove the prevalence of great atmospheric humidity. The exuberant coal flora of Spitzbergen in 80° N. latitude, that at the equator, that also in Australia, have a common facies.
From the coal measures of Barren island, in 74° 30' N. latitude, Heer has identified two species of Cardiopteris, Pataeopteris Rameriana, Sphenopteris Schimperi, all found in the coal beds of Germany; Lepidodendron Weltheimianum, Stigmaria ficoides, two Cyclostigma, Knorria imbricata and Bornia radiata, species abundant in, the lower coal measures of America, as well as of middle Europe. And the same types have been observed in the coal beds of Spitzbergen.
§ 7. The cause of this great uniformity of temperature, which seems to have prevailed over the whole surface of the globe in early geological ages, or at least during the Carboniferous times, I am not called upon to discuss. Several hypotheses have been offered. But it seems clear enough that extensive low lands, alternately and very slowly rising and sinking near to sea level*
§ 8. An excess of carbonic acid in the coal age atmosphere must be added to an excess of moisture, and a uniformity of temperature, if we would complete the explanation of the vegetable growth of coal.
The preponderance of this acid in the air is as favorable to vegetation as it is hostile to animal life. Such a preponderance would at least help to explain the more magnificent proportions of the coal plants, such as they were, and the remarkable scarcity of animal land forms in the coal measures.
Except some batrachians and insects, no air-breathers seem to have inhabited the land.*
It is rightly called the Age of Plants, for earth's atmosphere was then not only fitted in the true ways just specified, to nourish vegetation, but unfitted to nourish any other kind of life.**
§ 9. Of the exuberance of the Carboniferous vegetation, the great thickness of some coal beds is indeed sufficient evidence.
The almost incredible quantity of vegetable matter needful,. on the hypothesis that a large coal bed is merely a compacted mass of the fallen trunks, roots, branches, and leaves of a growth in situ, has especially excited opposition against it.
But such opposition must succumb to positive evidence presented for examination in our own day in the case of a kind of vegetation quite comparable to that of the Coal period. One has only to penetrate the cedar swamps of the north, or the Dismal swamps of the Carolina seaboard, tocomprehend the degree ofactivity to which vegetable life may rise under certain circumstances.
The surface of a cedar swamp is covered by decaying trees and shrubs, heaped together in every stage of decomposition, and at every angle of inclination. This surface-heap is from four to ten feet thick. To make one's way across such a swamp is sometimes impossible and always a most difficult feat. Even the aborigines preferred to make a detour of thirty miles around it, to crossing such a swamp only three miles wide.
In the south, large trees, especially the Magnolia and the Bald Cypress ( Taxodium), grow at distances upon the bogs; but the underwood is mostly a compound of Canes from twenty to thirty feet in height, crowded so closely together that a path must be forced with the hatchet.
What then must have been the mass of vegetable remains heaped upon the surface during a coal period when growth took place under circumstances twice as favorable as in our great swamps.
Could we then show proof that the remains of Carboniferous vegetation were thus heaped upon the ground, that additions to the pile were constantly made for a long time, and the whole transformed into coal by slow degrees afterwards, the problem of the formation of a coal bed would be satisfactorily explained. This I will now attempt:
§ 10. The transformation of the woody substance of dead plants into ulmine by oxidation takes place everywhere in comparatively dry air, ulmine being a constituent of the humus or common earth.
When the dead woody fiber however is shielded from the more energetic action of the atmospheric oxygen by immersion in saturated wet air, or under water, it escapes decomposition for an indefinite length of time, and is gradually transformed, by a sort of eraumacausis or slow burning, into a soft black material, composed of the same elements as wood. The substance of peat consists of this soft material. In time and under pressure it grows more compact, like lignite, and then becomes dense hard coal. Finally, by the loss of its more volatile hydrocarbons it passes into anthracite.*
§ 11. In the peat bogs of our time, the transformation of the woody matter takes place either under water, or beneath a water-soaked spongy covering, precluding the access of air. On the slopes of mountains, protection is found in the air itself, surcharged as it is with water in the condition of fog.
In the great humidity of the carboniferous atmosphere we find then the first cause of the preservation of carboniferous vegetable remains and their conversion into coal. For, the heaps of fallen vegetation became, by absorption, reservoirs of water, protecting themselves against rapid oxydation. Afterwards when actually submerged and covered with sheets of mud, sand, gravel, etc., their protection from destructive oxydation became perfect for all subsequent ages, even after the elevation of the continent high above sea level.
The process was a simple one; and being repeated in our sight at the present time, can be studied at leisure, and admits of no dispute.
§ 12. First, as to objections based upon the great thickness of some coal beds.
It has been thought that this necessitates the hypothesis of the transportation from a distance, by water, in various ways of at least part of the materials and the heaping of them together upon the sea bottom or in lakes.
It has been thought necessary also to imagine large con- tributory accumulations of macerated marine plants, the growth of which was as prodigiously active then as it is now.
§ 13. The second part of the hypothesis can be disposed of in a few words.
It is certain that nature takes good care of all that it produces, so that no particle of matter is really lost. It is certain that marine vegetation has its purpose and end as much as that of the land. But nature, even in its multiplicity of recompositions, cannot produce a new compound from elements which have no existence in the matter under decomposition.
All the hydrophytes (water plants) whether of fresh or salt water are merely cellular in structure ; have no fiber, no woody element. Land plants * on the contrary are composed of vascular
In the decomposition of Algae the entire mass undergoes one and the same transformation: that to which only the cellular tissue of land plants is subjected, without any residuum of woody fiber whatever. Algae can therefore produce nothing in the shape of coal—nothing but liquid fuel, oil.
Their decomposition moreover is so rapid, that when sea-wrack is gathered for manure it has to be spread upon the fields at once; otherwise, as farmers say, it melts and loses its fertilizing properties.
The Algae cannot be preserved against decomposition any more than can the fleshy parts of animals. They cannot burn nor emit any amount of caloric.
Add the geological fact that all remains of plants found either in the shales which cover coal beds or in the body of the coal itself are land plants, with woody fiber, and the demonstration is complete that however vast the accumulation of decayed vegetation in a coal bed, none of it is that of marine vegetation, Algae or cellular plants ; but all of it is that of air breathing land plants, out of the vascular tissue of which was formed the coal.
§ 14. As to the ability of a standing vegetation to supply all the material of a coal bed, on the spot, without need of other transported matter from a distance, it is only necessary to refer again to what I have written above respecting the heaped droppings of modern cedar swamps, the luxuriance of which must certainly be far inferior to that of the Carboniferous swamp-forests, in ability to produce wood. Those who have not explored the mode of growth of peat, or perhaps even visited a peat bog, cannot conceive the magnitude of the results of such a vegetation. To be well understood, it must be studied in marshy places of difficult access, even dangerous to approach, and therefore, under circumstances which few naturalists care to encounter. A volume could scarcely suffice to contain the observations to be made on the subject. I will merely quote some figures, to give an idea of the quantity of vegetable wood produced under different circumstances.
§ 15. Researches and experiments, made by the Department of Forests of France, show that in one hundred and twenty years, one acre of forests =forty-three thousand five hundred and sixty (43,560) square feet [The measures are reduced to American standard.] produces twenty-two thousand and eighty-five (22,085) cubic feet of pine wood, weighing (at thirty pounds per cubic foot) six hundred and sixty-two thousand five hundred and fifty (662,550) pounds.
One acre of a peat bog, with an estimate average growth of one foot in one hundred and twenty years, produces 43,520 cubic feet, which, when dried and compressed to a density of twenty pounds per cubic, (at which peat has the same heat power as pine wood at 30 pounds) weighs 871,400 pounds, or 208,750 pounds more than pine wood.
It may be easily admitted that the production of vegetable matter in the coal age was twice as active as now, that therefore twice as much woody fiber was grown to make coal; and therefore again, that one foot of coal could have been produced in one hundred and twenty years.
This amount, one foot, is adopted as a basis of calculation in view of the compression and prolonged decomposition of the vegetable mass; being one half the thickness produced in a given time assumed in the case of peat; with a heat power double that of peat.
The annual layers of peat, measuring one inch at the top of the bank, are compressed so as to measure about one tenth of an inch at a depth of only ten to twelve feet beneath the surface; while the layers of a coal bed are often seen to measure only the twentieth of an inch (one millimeter). This shows that the swamp growth of the coal age was twice as rapid as that in our peat bogs; and that two and two thirds as much combustible matter was formed in a given time than is formed in the same length of time in one of our modern forests.
§ 16. Viewing the subject in the light of these facts, all the objections urged against the analogy of the production of the coal to the growth of peat can be set aside; since Nature uses at the present day the same methods for arriving at the same results.
Everybody living on the borders of a peat bog, knows of its growth. Records of pre-historic human races, and of men of later historical times—relics of the stone, the copper, and the iron ages,—Celts, Gauls, and Romans—are found in layers of peat at various depths beneath the surface of the bogs; and the rate of growth of the matter has thus been approximately ascertained.
In Germany, Holland, England, etc., the surface of extensive peat bogs has been triangulated and leveled repeatedly to ascertain its gradual elevation; and not only have such measurements verified the fact, but the estimated rate of growth thus obtained has corresponded to the computed amount of surface matter produced in a given time by the vegetation of the swamps.
§ 17. For a comparison of the great thickness of some coal beds with that of deposits of peat at the present time, the data are quite explicit.
There are deposits of wood, mined in Denmark to the depth of seventy-five feet, formed by successive and alternate growths of peat and forests on the same spot.
Other true peat bogs in Sweden and Russia have been ascertained by borings to be more than one hundred feet thick; the whole mass composed of ripe black combustible material.
Such a mass, subjected for geological periods of time to the process of slow burning and condensation under superimposed deposits of sand and clay, would constitute a bed of coal from thirty to fifty feet thick.
As such enormous accumulations of woody matter are seldom observed on the present surface of the globe, so also coal beds of corresponding size are rare in the Coal Measures. The average thickness of peat growing under favorable circumstances (as for example, along the shores of the Baltic and North sea) is not more than ten or twelve feet; and this corresponds in solidity to a thickness of five or six feet of a coal bed, which is a common size, although the average thickness of all the coal beds in Pennsylvania falls much below five feet.
§ 18. Coal not a Delta deposit. —The vegetation of the Coal epoch, as remarked above, was every where governed by the same atmospheric circumstances, and had the same character. There is nothing in the Coal Measures themselves to show the existence of high mountains. There would follow an absence of considerable rivers, capable of transporting heavy materials. The land was either stationary or continued its slow and gradual movement of upheaval, bringing up the surface to near or above the level of the water, where, in shallow basins, shielded against the invasion of the sea by sand walls, the woody material were heaped during varied periods of time. Or contrarywise, the land was gradually sinking beneath the ocean level submerging one coal bed after another, and furnishing temporary sheets of open water to receive the intermediate deposits of sand and mud. But during the stage when the ocean level remained stationary, and the coal bed was growing, lagoons or irregular water channels traversed great extents of the area.
It is therefore impossible to understand wherefrom the remains of the vegetation should have been derived for transportation ; and still more so to account for their transportation itself.
It is only necessary to contemplate the action of our present great rivers, the Mississippi or the Amazon, to see that the woody matter which they bring from afar and deposit in their deltas is not concentrated in masses or layers by itself, but on the contrary is scattered and distributed throughout thick layers of sandy mud deposited from the turbid water in which the wood floated. The drifted materials of deltas and delta islands composed of sand, mud and trees intermixed cannot constitute a bed of coal.
§ 19. To meet this difficulty Bischoff supposed a sifting process; the heavier material constituents coming to rest in dead water first, the clay next and the vegetable float (trees, leaves, etc.) last, in the form of a layer covering the others.
But if this view were tenable, then, 1. Each coal bed would represent a single case of transportation, one season of freshet, one year; and the number of layers of coal in the coal measures would be practically infinite. 2. Each coal bed would be more impure at the bottom and increase in purity towards the top. How contrary to facts both these conclusions are, everybody knows.
Moreover, such a deposit as the Pittsburgh coal bed would be an impossibility. For, whence could come sufficient float vegetation to form a layer fourteen thousand (14,000) square miles in extent, and averaging eight feet in thickness ? We cannot imagine fourteen thousand square miles of forest carried down some mighty river, or system of rivers, to make it. But even if we could, that amount of forest would suffice (in bulk) for only one foot of coal; the remaining seven feet would remain unaccounted for.. The necessary supposition, that the whole event took place at once, in order to obtain but one coal bed is rendered, incredible by the regularity of the bed over its immense area.
§ 20. By the slow decomposition of the plants in passing into coal, they lose their form, and are generally converted into a homogeneous compound in which none of their characters can be recognized. But even in this condition they can be studied and determined generically, by submitting the coal to the action of acids, by which the woody fibers are loosened and separated.
In the case of some coals, thin lamellae of fibrous dry charcoal interposed between the layers of compact bright coal, form clearly drawn pictures of the outlines and nervation of Ferns. Often also the forms of vegetable stems and trunks converted into sulphide of iron (pyrite) remain unimpaired even into compact coal.
§ 21. But the most of the well preserved remains of plants are found in the laminated shales overlying the coal beds. These roof shales were always deposited at the end of the life of a coal bed, when the swamp had been invaded by water so rapidly or to such an extent as to first lessen the activity then stop the growth of its whole vegetation.*
The invading water being more or less turbid but moving in all directions with extreme slowness, while its surface was exposed to daily evaporation, deposited successive thin layers of mud between which were locked up and in time pressed flat the dead leaves, twigs and the stems, which floated for a while at the surface and gradually rested at the bottom. Most of these were detached front plants still growing around the swamps, or upon hammocks and knolls still unsubmerged, and constituting so many islets in the water. As soon as they were inclosed separately between layers of mud and protected from any further rapid decomposition they became subjected to the process of petrifaction by infiltration of earthy elements in their tissue.
It is chiefly then upon the surfaces of these laminae of shale that the history of the composition and formation of coal is written in beautiful hieroglyphics, the letters of which are leaves, branches and trunks of trees; and the deciphering of the language thus preserved constitutes the science of Palaeobotany.
In other kinds of deposit such as sand, the woody material, especially that of tree trunks, was first slowly softened by a decomposition hastened by the porosity of the embedding matter, and then gradually replaced by elements held in solution by the water; in the end nothing being left of the vegetable but the print of its bark; and this is what is most commonly seen in the coal-measure sandstones.
In other cases silica or carbonate of lime has taken the place and the form of the tissue of vegetable remains, the structure of which can thus be studied if the mass be thinly sliced and placed under the microscope.
Transformations of this kind were not rare in the coal age, and they would suggest a greater amount of silica held in solution in the waters then than now, did we not know that the silica of the sandstone itself is taken up by the percolating rain water and redeposited at certain points where chemical changes are going on.
Whole forests of silicified Fern-tree trunks standing in the place where they grew, are found in sandstone beds of the Coal Measures; for example, on Shade river, south of Ohio, or along the Great Kanawha river, Kentucky, from Charlestown to its mouth. In these trunks the whole matter is silicified by a chemical process such as has converted into stone the famous stone forests of the desert between Cairo and Suez in Egypt, and of Colorado, the Yellow Stone, and other countries of the west.
The texture of the wood is distinctly preserved and can be studied by anatomical process as distinctly as in the wood of living plants.
In other cases, as on the shore of the bay of Funda, and at the west of Cape Breton, the standing trunks have been converted into stone by infiltration of sand and mud; as is generally the case for the trunks of Lepidodendron and Sigillaria which have left only the impression of their bark into sandstone and clay. When the trunk of standing trees is decaying in the inside, the bark may remain firm for a length of time, and the sand and mud either percolate through it, or when the stems are broken short the earthy matter is poured into them, and fill the hollow pipe with a mould or cast of sandstone or of clay.
This is proved not only by the character of the cast and the coaly envelope of bark which is sometimes preserved, but also by the discovery of insects and invertebrate animals inside the stem at its base.*
Other plants (as well as animals) have been preserved in nodules of carbonate of iron, the origin of which is to be ascribed to Diatoms or infusoria, congregating and building up around the vegetable fragments or dead animals, enclosing them in the end completely, and preserving their surface characters with a remarkable minuteness of detail. These concretions are widely scattered through all Coal Measures, but are especially abundant near Morris, in Illinois, on Mazon Creek, the name of which occurs, therefore, frequently in the descriptive part of this volume.