Page:Popular Science Monthly Volume 3.djvu/599

Rh Chalk, and it may be even comprehending the additional time occupied in the formation of the Tertiary strata. But this latter part of the subject I propose to work out before long.

One other point remains. I have elsewhere attempted to prove, and the opinion is gaining ground in England, that this long continental epoch embraces at least two glacial episodes, as witnessed first by the bowlder-beds of the Old Red Sandstone of Scotland and the north of England, and secondly by the occurrence of similar deposits containing far-borne erratic blocks and ice-scratched stones, in a portion of that part of the Permian strata that is usually considered to represent the German Rothliegende. Should this be finally admitted, it may, on astronomical grounds, some day help us in the positive measurement of geological time.

Finally, let me rapidly pass in review what I think we know of later terrestrial, as opposed to marine epochs, in the British and neighboring areas of Europe. A wide-spred partial submergence brought the old continent to an end, and during the Liassic and Oolitic epoch (Jurassic) the Highlands of Scotland and other mountain-regions in the British Islands formed, with some other European Palæozoic rocks, groups of islands, round which, in warm seas, the Jurassic strata were deposited. These relics of an older continent, by deposition of newer strata and subsequent gradual upheaval, began to grow in extent, and at length formed the great continental area through which the mighty rivers flowed that deposited the strata of the Purbeck and Wealden series of England and the continent of Europe.

A larger submergence at length closed this broad local terrestrial epoch, and in those areas now occupied by Northern Europe (and much more besides), the sea, during the deposition of great part of the Chalk, attained a width and depth so great that probably only the tops of our British Palæozoic mountains stood above its level.

By subsequent elevation of the land, the fluvio-marine Eocene strata of Western Europe were formed, including in the term fluviomarine the whole English series, embracing the London Clay, which as shown by its plant-remains was deposited at, or not far from, the mouth of a great river, which in size, and in the manner of the occurrence of some of these plants, may be compared to the Ganges. With this latter continent there came in from some land, unknown as yet, a great and new terrestrial mammalian fauna wonderfully different from that which preceded it in Mesozoic times, and from that day to this the greater part of Europe has been essentially a continent, and in a large sense all its terrestrial faunas have been of modern type.

One shadowy continent still remains unnamed, far older than the oldest of those previously spoken of. What and where was the land from which the thick and wide deposits that form the Silurian strata of Europe were derived? For all sedimentary strata, however thick and extended in area, represent the degradation of an equal amount