THE ANNALS

OF

ELECTRICITY, MAGNETISM,

AND

CHEMISTRY;

AND

GUARDIAN OF EXPERIMENTAL SCIENCE.

JANUARY, 1843.

On the Formation of Coal.-A Paper read at the Newcastle Literary and Philosophical Society, August, 1841. By MR. JAMES

MILNE.

THERE is no subject about which geologists appear more uncertain than the origin and formation of coal. The most prevalent opinion hitherto has been, that it must have originated from floated timber and other vegetable matter, sunk to the bottom of the ocean, and there by slow degrees been covered with silt, debris, and other substances wafted over it by the water. This seems to be the opinion of Dr. Buckland in his "Bridgewater Treatise," and more lately, since this was read, by Mr. Williamson at the meeting of the British Association in 1842. The microscopic discoveries of Mr. Hatton, of Newcastle, and the fields of floated timber which have been discovered in some estuaries in America, have given a plausibility to this theory. Still I shall endeavour to show that there are insuperable facts against that hypothesis, and that in general the large coal fields have not been formed by that process.

The microscopic discoveries which has been referred to, though they may prove that some coal may be of vegetable origin, will not prove that all is of vegetable origin or of floated timber. Even Mr. Hatton says, "that in the finest of this coal (meaning the Newcastle caking coal) the cells are completely obliterated;" that in the varieties which go under the name of parrot, cannel, and splint coal, "the first kind of cells are rarely seen, and the whole surface almost displays an uniform series of the second class of cells filled with bituminous matter, and separated from each other by their fibrous divisions." This shows that in the great body of the coal strata which is found of the caking coal, there is nothing found in it analagous to the fibrous woody cells and that even in the other kinds of coals only one kind of cell is perceptible. It is possible also that the instances alluded to, were not general throughout the coal strata; Ann. of Elec. Vol. X, No. 55, Jan., 1843.

:

to account for which there is no difficulty, without being obliged to adopt the theory, that all coal was originally formed of vegetable

matter.

The first objection to the theory of sunk forests which I have to suggest is, the inconceivableness of the enormous extent of the coal strata without an interruption, without a single spot being wanting or uncovered, and all floating at one period on the surface of the ocean like a green mantle; then all sunk to the bottom like a sheet of lead. This equable sinking is quite inconsistent with any cause which we can assign, or have any knowledge of.

The second objection which presents itself is, how could such thin strata, if of vegetables, have been so equally spread over the surface of the ocean, and continued of one unbroken strata when sunk to the bottom? How many of the coal strata are only a few inches thick. How is it possible for trees and shrubs to have been so equally spread, unless they had been wove together like a web before the act of sinking?

Another objection arises from considering the nature of the roofs, or coverings, which in many instances consists of sandstone, and the impossibility of this taking place under any circumstances over a forest of wood, six, eight, or twenty feet thick, without the sandstone filling up the interstices which must have existed between the trunks and betwen the branches; for surely trees could not have formed one solid mass, till after they were compressed by the superincumbent strata, which then did not rest upon them. If the sand stone or roof was formed of a deposition from water or other liquid which crystallized and subsided, how is it possible that it should not have sunk and filled all the open interstices between the trees and shrubs, and thus mixed with the coal. Yet this is not the case. The coal seems to have been in a semi-fluid state when it was first formed into strata, which will afterwards be accounted for.

It will probably be admitted that by the greater heat and moisture of the earth in the first ages, that vegetation would be great and rapid, and that the whole crust of the earth, not covered with water, would be covered with forest trees and shrubs closely aggregated. But some great catastrophy must be sought for, capable of producing the sequel. These trees must be all torn up or broken over and laid on the bosom of the water, by some great commotion of the earth or ocean. It is not impossible to conceive of such a debacle by some immense breaking up of the crust of the earth : because such breakings up have evidently taken place when the great mountain chains were formed. One part rising up and and another subsiding, forming Alps and Apennines, hills and valleys. All the great mountain chains have been the effect of the upheaving of the crust at one place in the direction of the chain, and its depression at some other part to the same extent. This would cause such an agitation in the water by the immense waves that would be raised, as would be quite adequate for tearing up and breaking over

the strongest of the forest tribe, and laying them prostrate on the waters, on which they would float for some time and then sink; but it is not possible that they could be so close or thick as to form the coal strata, nor that they would all sink at the same time. The probability of these debacles taking place frequently, is strongly attested by the many great mountain chains in various parts of the globe, for it is not likely that they all took place at the same time. Indeed the cross veins and dykes are proofs of their having been upheaved at various periods. The undulating motion given to the water would soon break over the strongest trees above the root, tear up others, and wash up the roots of those broken over, and thus supply a vast floating forest to the theorist. To this extent I am willing to go with him; but how to spread them equally over the surface of the water, how to sink them equally and at once to the bottom of the ocean, and how to convert them into coal and give them their present chemical and mechanical characters, I am unable to form any rational conception. How came the pure sandstone to be deposited both below and above the coal strata, which is found in many instances to an immense depth, finely crystallized? Was it a deposition from a substance floating in the water, or was it a solution precipitated and crystallized by the cooling of the atmosphere acting on the solution? Is it not inconsistent with all our knowledge of nature, to suppose that a pure white sandstone could be forming from a solution in the water, just before the floating timber was sunk, which formed the sole or base on which the stratum of sunk forests rested; and also that the same sandstone should again form the roof or covering above the sunk forests, immediately after it had sunk (by deposition and crystallization), without admitting that all the interstices would have been filled with sandstone?

In searching for a rational theory to account for the origin of coal, which has hitherto been unsatisfactory, it may be asked, if it is not very probable that at the period when the coal formation was originating, that the atmosphere at the time any of these debacles and upbreakings of the crust of the earth took place, would have suspended in it gases, ashes, and other floating effluvia to a great extent and that the earth at that early period must have possessed an atmosphere full of carbon and carbonic acid? These are circumstances which no author has sufficiently taken into consideration in accounting for the formation of coal.

In the present age, single volcanic eruptions have spread clouds of smoke, dust and ashes, over hundreds of miles distant to a considerable thickness in depth; but a single eruption in the present age, could bear no proportion to the convulsions caused by the upheaving of the Alps or the Himalayan chain, or of the grand display of volcanos which must have been in active operation during the period that marks their immense number in various places in the world. The atmosphere during the time these were in activity must have been one cloud of smoke for many hundreds of miles in

extent, and might in some way supply materials for the different kinds of coal.

The great depth of stratified rock above the primary being, according to Dr. Buckland, not less than ten miles, would require immense breakings up of crust to furnish the debris and silt wanted; so much so, that it has led to the question whether some other comet or planet may not have been dashed upon this earth and furnished some of these materials. This opinion is not however much countenanced by the numerous formations that have successively taken place, at intervals from the time of the first of the stratified series to the latest of the coal formation, the materials of which must have been furnished successively at various periods; and which could only be furnished by the various upbreakings of the crust of the earth. It is impossible to conceive that all the variety of the different strata, some course, some fine, with sandstone, limestone, &c., could ever originate from any thing like the varia tions and wasting that at present are taking place in the world.

In prosecuting this enquiry, all the indispensable conditions and self-evident facts which cannot be disputed, are first taken into view, in order to observe how far they will conduct us to the formation of a theory and to discover what is wanting to complete it. The following are what are considered as ascertained facts::

I. That the great coal formation has been formed under water generally.

II. That each stratum has been either a deposition of fine mud or silt, suspended in the water or atmosphere, or a precipitate of powder from matter held in solution in the water, or a cystallization from a solution in water, or partly each.

III. That the number and variety of strata in the coal formation demonstrates, that a long space of time must have elapsed between the formation of the lowest and uppermost of the series.

IV. That from the number of beds of coal in the same field there is sufficient evidence that something like the same cause must have continued to operate at intervals of time, of equal or unequal length. V. That matter of very great extent must have been supplied to form the series of stratified rock, &c., in the coal formation.

VI. That from the number of fossil plants in the coal formation it is evident that the surface of the earth had been capable of producing vegetables, but that from the scanty fossil remains of land animals that have yet been discovered, there is reason to conclude that the earth at that time was not favourable for the existence of land animals.

VII. That the temperature of the globe was much higher at that period than it is at present.

VIII. That the roofs and soles of the coal strata are in some places of sand stone and in others of shale.

IX. That the roof and soles sometimes contain fossil trees and other fossil vegetables.

X. That in perfectly crystallized coal no vegetable fossils can be perceived.

XI. That in some varieties the vegetable structure is discernable. XII. That the bark of trees has often been converted into perfect coal in situations distant from the coal stratum, as in sand stone quarries, &c.

XIII. That the shale in many coal fields gradually changes its character till it forms the true coal.

XIV. That in the middle of a coal seam there is frequently a band of inferior coal, which partakes more of shale than the pure coal, and which in working is generally separated and sold as inferior coal.

XV. That in some instances the coal stratum, when newly formed, has been of a soft semi-fluid consistence, and allowed some portion of the roof to sink through it to the floor, and the space the fallen roof occupied filled by the substance of the coal.

Coal.

Roof.

Floor.

XVI. That the coal formation has undergone great changes with regard to the position in which it was first formed. This is evinced from the troubles and dislocations in the strata; and also from considering that from the effect of gravity the semi-fluid coal would have formed a mass at the lowest point, where the inclination is very great.

XVII. That the coal formation has in some places been depressed and in others upheaved, but being found generally now above the level of the sea, it must have been generally raised, though not universally so.

XVIII. That the earth's crust, must at a great many places have been broken up, and at various periods during the coal formation and prior to it that these breakings up have given rise to the vast mountain chains in all the different countries in the world, and also to the great depressions both of sea and land: that these have taken place successively, is seen from the veins, slips, troubles, and dykes, crossing each other in the mining districts, and from the conformation of the rocks.

The above are all facts that cannot well be disputed. What follows are probable suppositions required to complete the theory, and are not inconsistent with any of the known laws of matter.

May we not suppose, first, that the probable state of the earth, just before the first stratum of the coal formation, would be that of great heat and moisture, highly favourable to a strong vegetation? Would it not be liable in a greater degree to earthquakes and volcanoes, than at the present time? Of the latter, there is sufficient evidence, from the numerous craters which are still to be seen in