Page:Popular Science Monthly Volume 3.djvu/593

Rh color that was produced by the encasing of each individual grain of sediment with a thin pellicle of peroxide of iron.

The proof that the Old Red Sandstone was deposited in inland lakes is strengthened by a similar case in well-known ancient inland sheets of water, as shown by the red marls of the Miocene lakes of Central France.

It is known that in Ireland and in Scotland the Old Red Sandstone consists of two divisions, upper and lower, the upper division lying quite unconformably on the lower. In South Wales there are symptoms of the same kind of unconformity, for the upper beds of the Old Red Sandstone gradually overlap the lower strata. But, on consideration, this last circumstance does not appear to present any real difficulty with regard to the physical conditions of the period. If the great hollow in which the Dead Sea lies were gradually to get filled with fresh water and silted up, 1,300 feet of strata might be added above the level of the present surface of the water, without taking into account the depth of the sea and the deposits that have already been formed; and the upper strata all round would overlap the lower, apparently much as the Old Red Sandstone strata do in Wales and the adjoining counties. If the Caspian and other parts of the Asiatic area of inland drainage got filled with water, the same general results would follow.

Neither does the decided unconformity between the Upper and Lower Old Red Sandstones both in Ireland and in Scotland present any insuperable difficulty as to the fresh-water origin of the strata. It indicates only great disturbance and denudation, and a long lapse of geological time unrepresented by strata between the disturbance and denudation of the older beds and the deposition of the newer. Here also we have a parallel case in times comparatively recent, for the fresh-water Miocene strata of Switzerland and the adjacent countries have been exceedingly disturbed, heaved up into mountains, and subjected to great denudation, while at a much later geological date—that of to-day—we have all the large fresh-water lakes that diversify the country north of the Alps in the same general area.

It is unnecessary to dilate on the well-known continental aspect of a large part of the Carboniferous strata which succeed the Old Red Sandstone, especially of the Coal-measures, which in the north of England and in Scotland are not confined to the upper parts of the series, but reach down among strata which elsewhere are only represented by the marine beds of the Carboniferous Limestone. The soils (underclays), forests, and peat-mosses of the period, now beds of coal; the sun-cracks, rain-pittings, bones, and footprints of Labyrinthodont Amphibia on mud now hardened into shale; the estuarine and fresh-water shells all point to vast marshes and great deltoid deposits, formed in a country which underwent many changes in its physical geography, and yet retained its identity throughout.