THE ORIGIN OF COAL.

Poverty Bay Standard, Volume I, Issue 87, 13 September 1873, Page 3

THE ORIGIN OF COAL.

A mistaken impression is somewhat widely prevalent that, in the coalfields, we have the remains of ancient forests —in other words, it is supposed that whenever there was a forest in primeval times, there now exists a coalfield of greater or less extent. In connection with this view, also, the opinion is entertained that the forests now in existence will, in process of time, and after due geological changes, become the coal beds of future ages. But, although, as we shall presently see, the coalfields are undoubtedly due to the vegetation of former eras, it is far from being the case that the primeval forests became converted in a general way into coal. Conditions of a peculiar and to some extent exceptional character were requisite for the formation of coalfields. If we consider the evidence by the coalfields themselves, we shall see what those conditions were. The beds or seams of coal form but a small portion of the thickness of the great geological group of strata to which they for the most part appertain. This group is called the carboniferous, and not uncommonly “the coalbut even where coal is abundant, it forms only a minute portion of the whole mass. Thus it is estimated, Sir Charles Lyell tells us, that in New South Wales the thickness of the carboniferous strata amounts in all, to between 11,000 and 12,000 feet, or more than two miles ; but the various coal seams do not, according to Professor Phillips, exceed in the aggregate 120 feet, or less tlian one-hundredth part of the whole. In North Lancashire, the carboniferous strata occupy a depth of more than three and a-half miles, with the same relative disproportion in thickness of the coal seams and that of the complete series of strata. Again, in Scotia, the coal-bearing strata attain a thickness of more than three miles. Here, no fewer than eighty seams of coal have been counted (seventy-one having been exposed by the action of the sea) ; but these seams are nowhere more than five feet in thickness, and many are but a few inches thick. Thus it is evident that the formation of coal can have been in progress but a short portion of the time, during which the carboniferous series of strata was in process of deposition. Throughout by far the greater portion of that time other minerals were being deposited. It is next to be noticed that under each coal seam a stratum of ancient soil exists, in which there are commonly found the roots of ancient trees; while above the coal there is commonly a layer of shale or sandstone, in which not unfrequently the trunk of those trees are found either fallen or in their original position, and only partly converted into coal. The bark remains, but is transmuted into coal; the hollow of the trunk, decaying long before the trunk gave way, is represented by a cast in standstone. Thus, if we picture to ourselves the state of things which existed when sueh a seam of coal began to be covered up by the next higher deposit, we see that there must have been trees standing erect above a layer of vegetable matter, the roots of the trees being imbedded in the soil which forms the deposit next the coal. The vegetable layers may probably have been two or three times as thick as the resulting coal seam, and were reduced by pressure to their present thickness; but such layers cannot at any time have reached to the branches of the forest trees. Then the process of gradual deposition began. This only can happen when some substance of the soil had caused it to be submerged to a greater or less depth. Wo can infer from the depth of the strata overlaying the coal seams that this state of submergence continued in many cases for a long period of time; and it is equally clear that the formation of the vegetable layers themselves must have been a process occupying a considerable time, since tall trees grew before the next submergence took place. So soon as submergence was complete, the tall trees perished and began to decay. The stout trunks above the vegetable layer are broken off and swept away by the sea. The forest itself, properly so called, was for the most part destroyed. It was the decaying produce of the forest, intermixed with the lowlier growths which formed the coal seam as it now exists. Among these were the lower parts of the trunks of ancient forest trees. These became converted, like the rest of the vegetable matter, into coal.