Or, if we take the Caspian alone as an example, there we have a brackish inland sea which was once joined to the ocean, as proved by its molluscan fauna. Changes in physical geography have taken place of such a kind that the Caspian is now separated from the ocean, while its waters, gradually growing Salter by evaporation, are still inhabited by a poor and dwarfed marine molluscan fauna. If by increase of rainfall the Caspian became freshened, evaporation not being equal to the supply of water poured in by rivers, it would by-and-by, after reaching the point of overflow, be converted into a great fresh-water lake larger in extent than the whole area of Great Britain. Under these circumstances, in the Caspian area we should have a passage more or less gradual from marine to fresh-water conditions, such as I conceive to have marked the advent of the Old Red Sand-stone.
The total absence of marine shells, and the nature of the fossil fishes of the Old Red Sandstone, also help to prove its fresh-water origin, for we find the nearest living analogues of the fishes in the Polypterus of the rivers of Africa, the Ceratodus of Australia, and in less degree in the Lepidosteus of North America. In the upper beds of the formation there is distinct proof of fresh water, in shells of the genus Anodonta mingled with ferns and other land-plants.
One other sign of the inland character of these waters remains to be mentioned—I mean the red color of their strata. As a general rule, all the great ocean formations, such as the Silurian, Carboniferous Limestone and Jurassic series, are gray, blue, brown, yellow, or of some such color. The marls and sandstones of the Old Red series are red because each grain of sand or mud is incrusted with a thin pellicle of peroxide of iron. When this coloring-matter is discharged the rock becomes white, and the iron that induces the strong red color in the New Red Marl, which much resembles that of the Old Red series, is found to be under two per cent, of the whole. I cannot conceive how peroxide of iron could have been deposited from solution in a wide and deep sea by any possible process, but, if carbonate of iron were carried in solution into lakes, it might have been deposited as a peroxide through the oxidizing action of the air and the escape of the carbonic acid that held it in solution. It is well known that ferruginous mud and ores of iron are deposited in the lakes of Sweden at the present day. These are periodically dredged for economic purposes by the proprietors till the layer is exhausted, and after a sufficient interval they renew their dredging operations and new deposits are found. With a difference the case is somewhat analogous to the deposition of peroxide of iron that took place in the Old Red Sandstone waters. It is obvious that common pink mud might have been formed from the mechanical waste of red granite, gneiss, or other red rocks in which pink felspars are found, but such felspars are tinted all through with the coloring-matter, and such a tint is very different from the deep-red
