112 PALEOZOIC TIME. [CHAP. VII. 



plan Sea and those which appear to have prevailed in the 

 sea of the Magnesian Limestone has been pointed out by 

 Sir A. Ramsay. Just as the Caspian is believed to have 

 been originally connected with the Arctic Ocean, and as its 

 fauna is really a marine assemblage, so also the inland 

 sea which in Dyassic times stretched from England into 

 Germany seems to have been isolated from the main 

 oceans of the period, the introduction of new species and 

 genera being thus prevented, so that the fauna was only a 

 dwarfed and modified remnant of Carboniferous life. 



Mr. E. Wilson suggests the following as the probable 

 sequence of events in the north-eastern basin. 1 After 

 indicating the formation of the basement sands, he says 

 " After a time the waters would become sufficiently satu- 

 rated to cause dolomitic materials to be thrown down to 

 some extent, which, commingled with the sand and mud, as 

 also with the large supplies of ferrous carbonate likewise 

 brought down by the rivers, would give rise to the blue- 

 coloured plant-bearing dolomitic sandstones and shales of 

 the Marl Slate series. During this stage, mechanical de- 

 position predominated, on the whole, over chemical pre- 

 cipitation. . . . Somewhat suddenly (however) this state of 

 things came to an end. 2 Chemical precipitation now began 

 to predominate, and the formation of the white and yellow 

 Dolomites commenced." In the north this precipitation 

 continued uninterruptedly, but the intercalations of shale 

 and marl in the southern part of the area point to the in- 

 flowing of large rivers from the land which then existed 

 over the east of England. It was clearly these southern 



1 " The Permian Formation of N.E. England, Midland Xat.," vol. iv. 



2 The change, as Mr. Wilson suggests, was probably due to pro- 

 gressive subsidence, which, by diminishing the altitude of the surround- 

 ing land, while increasing the area of evaporation, would tend to promote 

 condensation of the lake-waters, and the consequent precipitation of 

 mineral matter in solution. 



