Ch. XXII.] 



ORIGIN OF RED SANDSTONE. 



M7 



found, called Thecodontosaurus and Palceosaurus by Dr. Riley and Mr. 

 Stutchbury ; * the teeth of which are conical, compressed, and with 

 finely serrated edges (figs. 487 and 488). 



Teeth of Saurians. Dolomitic conglomerate ; Eedland, near Bristol. 

 Fig. 487. Fig. 488. 



S;% Teeth of Palczosaunis 

 platyodon ; nat. size. 



sLJ 



Teeth of Thecodontosaurus , 

 3 times magnified. 



Messrs. Conybeare and BucMand referred the strata containing these 

 saurians to the period of the magnesian limestone, or the lowest part 

 of their Poikilitic series, and for a long time these reptiles ranked as 

 the most ancient representatives of their class which had been found 

 in any British rocks ; but Sir H. De la Beche afterwards pointed out 

 that, in consequence of the isolated position of the breccia containing 

 the fossils in question, it was very difficult to determine to what pre- 

 cise part of the Poikilitic series they belonged.f More- lately, our 

 Government surveyors have satisfied themselves that the breccia is of 

 Triassic date, probably referable to the base of the Keuper. 



Origin of Red Sandstone and Rock Salt. 



We have seen that, in various parts of the world, red and mottled 

 clays and sandstones, of several distinct geological epochs, are found 

 associated with salt, gypsum, magnesian limestone, or with one or all 

 of these substances. There is, therefore, in all likelihood, a general 

 cause for such a coincidence. Nevertheless, we must not forget that 

 there are dense masses of red and variegated sandstones and clays, thou- 

 sands of feet in thickness, and of vast horizontal extent, wholly devoid 

 of saliferous or gypseous matter. There are also deposits of gypsum 

 and of muriate of soda, as in the blue clay formation of Sicily, without 

 any accompanying red sandstone or red clay. 



To account for deposits of red mud and red sand, we have simply 

 to suppose the disintegration of ordinary crystalline or metamorphic 

 schists. Thus, in the eastern Grampians of Scotland, in the north of 

 Forfarshire, for example, the mountains of gneiss, mica-schist, and clay- 

 slate are overspread with alluvium, derived from the disintegration of 

 those rocks ; and the mass of detritus is stained by oxide of iron, of 

 precisely the same color as the Old Red Sandstone of the adjoining 

 lowlands. ISTow this alluvium merely requires to be swept down to the 



* Geol. Trans., Second Series, vol. v. p. 349, pi. 29, figs. 2 and 5. 

 f Memoirs of Geol. Survey of Great Britain, vol. i. p. 268. 



