Page:Popular Science Monthly Volume 3.djvu/590

574 Sandstone, as distinct from the marine Devonian rocks, is only occasionally and hesitatingly allowed to have a fresh-water origin, in spite of the statement made by Mr. Godwin-Austen long ago, that it was deposited in lakes.

My present object is to prove that, in the British Islands, all the great formations of a red color, and which are partly of Palæozoic, and partly of Mesozoic or Secondary age, were deposited in large inland lakes, fresh or salt; and if this can be established, then there was a long continental epoch in this part of the world comparable to, and as important in a physical point of view as any of, the great continents of the present day.

The Upper Silurian rocks of Shropshire, Herefordshire, Monmouthshire, and South Wales, are succeeded immediately by the Old Red Sandstone series, and there is no unconformity between them.

The teeming life of the Upper Silurian seas, in what is now "Wales and the adjoining counties, continued in full force right up to the narrow belt of passage-beds which marks the change from Silurian brown muddy sands into lower Old Red Sandstone. In these transition beds on the contrary, genera, species, and often individuals, are few in number and sometimes dwarfed in size, the marine life rapidly dwindles away, and in the very uppermost Silurian beds land-plants appear, consisting of small pieces of undetermined twigs and the spore-cases of Lycopodiaceæ (Pachytheca spherica). Above this horizon the strata become red.

The poverty in number and the frequent small size of the shells in the passage-beds indicate a change of conditions in the nature of the waters in which they lived; and the plants alluded to clearly point to the close neighborhood of a land, of which we have no direct signs, in the vast development of a purely marine fauna in lower portions of the Ludlow strata. In the Ludlow bone-beds the fish-remains, Onchus and Sphagodus, and the large numbers of marine Crustacea, almost entirely trilobitic in the Ludlow rocks, indicate a set of conditions very unlike those that prevailed when the passage-beds and the lower strata of the true Old Red Sandstone were deposited, in both of which new fish appear, trilobites are altogether absent, and are more or less replaced by Crustacea of the genera Pterygotus and Eurypterus, one of which, Eurypterus Symondsii, has only been found in the lower Old Red Sandstone. Neither are there any mollusca in the Old Red Sandstone, excepting where that formation passes at the top into the Carboniferous rocks. All these circumstances indicate changes of conditions which were, I believe, of a geographical kind, and connected with the appearance in the area of fresh water.

The circumstances which marked the passage of the uppermost Silurian rocks into Old Red Sandstone seem to me to have been the following: First, a shallowing of the sea, followed by a gradual alteration in the physical geography of the district, so that the area became