CHAP. VII.] DYASSIC PERIOD. Ill 



knowing, but there is no reason to suppose it extended 

 far, for Wales and Ireland were doubtless connected by a 

 tract of high mountainous land. 



The slopes surrounding this lake seem in most places to 

 have been steep, and the rivers running into it were conse- 

 quently rapid, carrying down quantities of sand, and in 

 some regions large stones and boulders, as in the breccias 

 of the Midland counties. The size of the transported 

 boulders in these breccias, their angularity, and the occa- 

 sional striation of their surfaces, suggested to Professor 

 Eamsay that they had been carried by floating ice. Many 

 of the fragments can be identified with Welsh rocks, and 

 it is highly probable that the Welsh mountains were then 

 much more lofty than at present, and that snow may have 

 accumulated on them in sufficient quantity to form glaciers. 

 Some of these may have reached the level of the lake, and 

 torrential streams bursting from others may have been 

 equally active in carrying down the rock-fragments quarried 

 from the frosty regions above. 



On the northern borders of the lake volcanic forces came 

 into play, and lava-flows with beds of volcanic ash were 

 interbedded with the lacustrine sandstones (see p. 104). 



From the thickness of the mechanical deposits in this 

 north-western lake, and the rarity of magnesian lime- 

 stones, we may infer that many streams and rivers ran 

 into it, bringing a constant supply of fresh water and pre- 

 venting the formation of chemical deposits. The north- 

 eastern lake, on the other hand, seems to have suffered 

 from evaporation and concentration ; possibly, also, the 

 waters poured into it contained a larger proportion of salts 

 in solution. At any rate, there are good reasons for re- 

 garding the Dyassic dolomites as direct chemical deposits, 

 although the process of precipitation cannot be imitated in 

 our laboratories. 



The analogy between the conditions of the modern Gas- 



