Old Red Sandstone. 105 



south of Builth, where true passage-beds occur, the 

 ordinary shells of the Upper Ludlow rocks become far 

 less numerous, and are almost all of small size, including 

 species of Modiolopsis and ModMa, Lingula cornea, 

 Platychisma helicites, a small Discina, a small Theca, 

 a few small Crustacea, of the genera Leperditia, Gythe- 

 rellina, &c. The water was freshening and getting 

 unfitted for marine life. 



The remains of Cephalaspis Lyellii (fig. 26) are 

 occasionally found all through the Old Eed Sandstone 

 of this large area. The absence of marine shells and 

 the nature of the fossil fishes of the Old Ked Sandstone 

 long ago led Mr. Godwin- Austen to infer that the 

 formation was deposited, not in the sea, as had always 

 been asserted, but in a great fresh-water lake, or in a 

 series of lakes. In this opinion I thoroughly agree, 

 for the nearest living analogues of many of the fish are 

 the Polypterus of the African rivers, the Ceratodus of 

 Australia, and in less degree the Lepidosteus of North 

 America. The red colour of the rocks also helps to 

 the same conclusion. Each grain of sand and marl is 

 red, because it is encrusted with a thin pellicle of 

 peroxide of iron, which could not have been deposited 

 from mere solution, as a crust enveloping each grain of 

 sand at the bottom of a great open ocean ; but if car- 

 bonate of iron were carried in solution into lakes, it 

 might have been precipitated as a peroxide through 

 the oxidising action of the air and the escape of the 

 carbonic acid. 1 



1 There is no analogy between the coarse red sandstones and 

 finer marls of the Old Ked Sandstone, and the very fine red ooze 

 dredged from the deeps of the South Atlantic. The latter is a 

 residue produced by the decomposition of Foraminifera, and in no 

 way resembles the coarse mechanical strata of Old Red Sandstone. 



