576 THE POPULAR SCIENCE MONTHLY. 



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 pas- 

 sage 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 perox- 

 ide through the oxidizing action of the air and the escape of the car- 

 bonic 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 pres- 

 ent 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 



