ART. III.-On some questions concerning the Coal Formations of North America; by L. LESQUEREUX.

Ir may perhaps be said that as everybody is now acquainted with the coal, with its essential constituents and the general laws of its formation, an attempt to offer to science something new or even interesting on the subject, must prove a fruitless task. This assertion has a semblance of truth only, for it is certain that some of the various and most important phenomena connected with the formation of coal are not satisfactorily, nor even at all explained. And as they are continually brought forward for discussion, either by lecturers or systematic geologists, the subject of the formation of coal, considered as a whole, has been obscured in such a manner that it is doubtful if the most essential facts on the subject, some of which may be considered as demonstrable, are not still looked upon by many as hypothetical and individual opinions. It is with these peculiar phenomena. of the coal formations, and consequently with the exposition and the discussion of geological facts connected with them, that we have to deal in the first part of this paper.

As we cannot expect to come to a right understanding of the formation of coal without some acquaintance with the vegetation of whose remains it is made, our attention must necessarily to some extent be directed to the flora of the coal period. But it is not enough to know the peculiar nature, the anatomical and chemical constitution, of the coal plants. It is necessary to study them also in their geographical distribution, in the different coal basins of America and of other countries, and also in the successive strata of the coal at different geological horizons. And it would be desirable also to examine the vegetation of the coal in connection with other external influences, in order to become acquainted if possible with the climatic conditions that prevailed at the time of the coal formation.

The plan that we propose to follow may accidentally direct the discussion to some points which do not appear to have a close relation to the formation of the coal. But we must bear in mind that geological eras are not very distinctly limited; or at least that to have a true understanding of one of them it is necessary sometimes to examine the causes that have prepared it, or that may have brought it to a close.

The supposition that coal is a true mineral, formed in certain strata of our globe only by some chemical agency and without an accumulation of wood grown on the surface and buried afterwards, has been recently revived among us, though it had long since been put aside, and apparently forever, as contradicted by all the appearances of the coal deposits and by the nature of the

coal itself. It would be useless, again to show the groundlessness of an hypothesis to which nature does not give the slightest apparent support.

The supposition that the matter of the coal (the wood) was heapel in some hollows or basins by the agency of water, as by currents of the sea or of some river, or by some other external cause, hurricanes, partial or general floods, sinking of the ground covered with thick forests, &c., has been also generally abandoned as contradicted by general evidence. The reasons against it may be briefly enumerated. They are found: 1. In the stratification of the coal measures; and also of the coal itself, which upon close examination appears to have been formed by successive layers of matter. 2. In the presence of plants in the coal and in the shales above it, plants preserved in the integrity of their most minute and fragile parts, and in a position which shows that they have been buried at the place where they have fallen from the trees or the bushes and where they grew. 3. In the absence in the coal of any matter foreign to it, of sand, of mud, &c., the ashes of the coal being_generally in exactly the same proportion as in the wood. 4. In the thickness of some beds of coal containing a quantity of matter far greater than could be furnished by a buried forest.

The theory of the formation of the coal by the heaping of consecutive layers of plants and trees grown in place, preserved in water and buried afterwards; or the peat-bog theory as it is called by some, is then the only one admitted now as satisfactorily explaining the process of formation of the coal. The analogy of formation between the peat-bogs of our time and the beds of coal of the old measures cannot be called a theory; it is a demonstrable fact. We can now see the coal growing up by the heaping of woody matter in the bogs. After a while we see it transformed into a dark combustible compound that we name peat or lignite according to its age. We then see it hardening either by compression, or by this slow burning in water that has been so well explained by the experiments of Liebig. Most of the peat bogs of Europe, at least the oldest, have at or near their bottom some plates or thin layers of hard, black matter, that ocular examination or chemical analysis fail to distinguish from true coal. We find besides in Holland, Denmark and Sweden, thick deposits of peat separated into distinct beds by strata of mud and sand, giving the best possible elucidation of the pro

cess of stratification of the coal measures.

It is not only in their general features that both formations are so much alike. But in the minutest accidents and even local peculiarities, their agreement is clear and unquestionable to one who has studied the formations of the peat bogs of our time. We quote a few examples.

An author, speaking lately of the formation of the coal, mentions the presence in the coal of wedge-shaped masses of vascular tissues found imbedded in the midst of the more structureless bituminous maller of the coal. He accounts for this fact by supposing that these tissues are the remains of floated logs, which have finally become imbedded in the carbonaceous matter below. This supposition is rather an extraordinary one. If the coal has been formed like the peat bogs, there can not be any floated logs in the compound. If there were floated logs in the coal, this would take us back to the formation of the coal by transportation. In every peat bog, the process of burying trees is in constant operation. The preservation of the logs which cannot be covered with water when they fall on the ground, is due to the agency of a moss, the sphagnum which extends its compact tufts always saturated with water like a sponge, over every fragment of wood, from the smallest to the largest. The Sphagna work like the ants to bury their treasures; and as their growth is continuous and stopped only by the frost, the heaping of their own woody matter which forms the structureless peat added to the wood which they have to preserve and the other plants of the marshes gives an appreciable thickness for each year. In the peat bogs of Switzerland, peat grows at the rate of two inches per year, a thickness reduced to one half by compression. In the same peat bogs, the Sphagna do not require more than three years to cover the stem of a tree of moderate thickness.

The bogs then, even the largest, enter naturally and without transportation into the composition of the coal as they become part of the matter of the peat bogs. In the deep bogs of New Jersey, there is a class of woodmen whom I would call log-fishers, who sound the marshes with long poles, to find the sound logs which they dig out of the black and already combustible mould or peat, from a depth of from six to ten feet. Some old swamps of Northern Europe contain as many as four or five generations of trees of different kinds imbedded from twenty to fifty feet deep and separated by thick beds of compact, entirely decomposed woody matter or peat. Some of those bogs are so abundantly filled with sound and large logs of oaks, pines and birches, that their removal has gone on for more than half a century before there was any material diminution of the supply, and for a long time it was supposed and even maintained that the trees of those marshes were growing under ground.

The flattening of all the stems found in the coal and in its shales, and also the layers of bark observed in the same formations, without any trace of internal woody structure, have also attracted a great deal of attention and useless theoretical discussion. In the oldest peat bogs of Germany, especially in the large swamps or lignite-deposits of the Pliocene of Saxony, the

trees are found all softened and already flattened to a greater or less extent. Some of the buried forests of England show the same appearance. From some clay banks exposed by a slide in the Jura mountains, large trees of recent species, still living in the country around, have been exhumed, and though the wood still preserves its natural appearance and its tissues, it has lost its hardness of texture and has become as soft as the clay itself. Hence, as Liebig has proved by direct experiments, in the process of slow decomposition or rather slow combustion in water, the woody matter is generally softened before its hardening and entire transformation in coal.

In Denmark, there are immense meadows, extending for miles. along the shores and covering old deposits of peat or combustible matter to a depth of from six to eight feet. The entire mass consists of a half fluid paste with layers of the bark of alder and white birch, rolled, flattened or pressed like the leaves of a book. Farther back in the interior of the country, especially in the royal park of Copenhagen, the formation of this kind of peat can be followed in all its details. First a thicket of alders and birches sprout out, covering an overflowed surface of ground. The thicket is impenetrable, and soon presents a confusedness of stems and interlaced branches. Then, as the trees become older, the whole mass begins to decay, especially at the level of the water, and by and by it falls down by its own weight, becomes submerged in a few years, and from its own seeds upon the mould of its half floating, half decomposed remains, a new generation of trees appears again and the process of formation is continued in the same way. The internal woody matter of the trees, the lignine, is decomposed at first and reduced to a paste, while the bark, impregnated with resins, is preserved for an indefinite period. In the coal basin of Trevorton, Pa., there is a perpendicular wall presenting to the eye a beautiful picture of prints of Lepidodendra and Sigillariæ, crossing each other in every possible direction, all thin layers of bark_superposed without any woody or carbonized matter between. It is nothing but the surface of an old coal-swamp, formed like the peat bogs described above. The peat which it covered has formed the coal, and the woody matter floating in water above it has been mixed with mud and formed the shales.

If it is true, as we said before, that all the peculiar accidents of the coal formations can be thus exemplified and explained by phenomena now observable in the growth of the peat, is it not surprising that the peat-bog theory of the formation of the coal should be still exposed to so many contradictions, and especially be subjected to continual and hypothetical modifications, which, destroying its simplicity, render it then truly unsustainable. The following reasons have been repeated time and again. The

repeated succession of various strata in the coal measures, viz., the constant alternation of fire clay containing roots of trees, with coal and shales, both containing remains of land plants or of marine shells; with limestone containing madrepores and shells of the deeper seas; with sandstone mostly without any fossil remains: this alternation evidently shows that at the time when the formation was progressing, the sea was continually brought in contact with the coal and covered it most of the time. Hence it follows; that if the coal has been formed in marshes like our peat bogs, we ought necessarily to admit of a submergence and therefore of a subsidence of the land after each deposit of woody matter, and of an upheaval of the same land to bring it up again above the level of the sea for each successive growth of a new peat bog. This appears to some geologists an unaccountable and unnecessary use of nature's internal forces; a kind of lusus naturæ, resembling a miracle. To meet this objection, they have supposed that the peat bogs of the coal measures grew on the deltas of some large river, and therefore exposed to periodical inundations: that as fast as the peat grew, the river brought upon it mud and sand, the materials from which the shales and the strata of sandstone were formed that, nevertheless, the deltas being by some internal force constantly sinking, they were consequently sometimes invaded by the sea which covered their whole extent and in the course of time, built upon them the strata of limestone: that as soon as these strata reached the surface of the sea (a fact which probably supposes that the movement of subsidence had stopped for a while) the land plants began to appear again, the peat to grow, and the matter to be heaped up till another large periodical inundation of the river brought new deposits of mud and sand; and thus by continuous subsidence and repeated inundations, the coal, shales, sandstone and limestone strata were alternately formed.

Before giving any reasons in support of the alternation of upheaval and subsidence as supposed by the peat bog theory, we will take the liberty to examine this new theory which we regard only as a poor modification of part of the former which it assumes to put aside forever. It is generally asserted that in the coal measures, the alternation of strata is the same in the whole extent of a basin, or in other words, "that each stratum is generally horizontally extended over the whole coal-field in a continuous sheet, so that each seam is accompanied by the same strata above and below." This is only partly true. In the coal-fields of the United States, it is true only of some beds of coal and of one or two strata above the conglomerates. Every practical geologist knows well that it is impossible to identify the position of a bed of coal by means of its adjoining strata. If the same strata

SECOND SERIES, VOL. XXVIII, No. 82.—JULY, 1859.