THE ORIGIN OF COAL.

Waikato Times, Volume IV, Issue 188, 24 July 1873, Page 2

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 supjroscd 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 no«r in existence will, in process of time, and after due geological changes, become the coal beds of future ages. 13ut, although, as we shall presently tee, the coalfields are undoubtedly duo to the vegetation of former ems, it is far from being the cose 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 smnll 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 coal ;" but even where coal ii abundant, it forms only a minute of the whole mass. Tlius it is estimated, Sir Charles Lyell tells us, that in New South Wales the thickness of the carboniferous strata amounts in all, to between 11,0000 and 12,000 feet, or more than two miles ; but the various coal seams do not, according to Professor Phillip?, exceed in the aggregate 120 feet, or less than one-hundredth part of the whole. In North Lancashire, tho carboniferous strata occupy a deptli 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 Nova Scotia, the coalbearing strata attain a thickness of more than three miles. Here, no fewer than eighty seams of coal have been counted (se^enty-ono 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 scam a stratum of ancient soil exists, in which t here arc commonly found the roots of ancient trees ; while above the coal there is commonly a layer of slmle or snndstone, in which not uufrequently the trunk tf those tives are found either fallen or in their original position, and only partly converted into coal The bark lvnuims, but i- transmuted into coul ; tho hollow of the trunk, decaying lon>» before Iho trunk gave way, is represented by a eat in sandstone. Thus, if we picture to oursolve-* the stale of things winch existed when such a seam of coal bi',:an to b«» covered up by the next higher deposit, wo sco that tlicie iiiust have been trees M muling erect above aimer of vegciablo matter, the roots of the trees being imbedded in the soil which forms tho deposit next the coal. The vegetable layers may probably ha\o boon two or t'arre times as thick as the resulting coal seam, ni.d were reduced by pressure to their present thickness ; but such layers citnnot at any time have reached to the branches of the forest trees. Then the process of gradual deposition began. Tim only can happen when some tubstanoe of the (oil had caused it to be submerged to a greater or less depth. We can infer from the depth of the strata overlaying the coal seams that this state of submergence continue i in many cases for a long period of time ; and it is equally clear that the formation of tho vegetable layers themselves must have been a process occupying a considerable time, since tall trees grew before the next submergence took place. So scon as submergence wai complete, the tall trees perished and began to flecny. Tho stout trunks above the vcßi'tnbl. 1 layer are broken off ami swept away by the se.i. The forest itself, properly so tailed, was for the moit part destroyed. It wai tho decaying produce- ot tho furest, intonuixed with the lowlier growths which formod the coal »cam as it now exists. Auioug these were tho lower parts of tho trunks of i ancient forest trees These became converted .like tba rest of the vegetable matter, into coil. — Examiner, i