236 HISTORY OF THE EUROPEAN FAUNA. 



land-ice. I have stated on several occasions that the 

 view of the marine origin of the boulder-clay agrees 

 best with the known facts of distribution, and with 

 the history of the European fauna (pp. 80-86, and 

 p. 129). It may be urged that if the lower boulder- 

 clay were contemporaneous with the British Crags 

 which succeeded the Red Crag, how can we explain 

 the fact that these crags contain plenty of shells, 

 while in the lower continental boulder-clay there 

 are scarcely any ? 



But as yet our knowledge of the conditions of life 

 of the marine mollusca and of their distribution is 

 extremely scanty. We are apt to imagine that the 

 bottom of the sea is covered by a more or less 

 uniform thick layer of shells; but whenever a careful 

 survey of the nature of the deposits now forming 

 there has been made, such is by no means found to 

 be the case. Some of the best results obtained by 

 that useful body, the Liverpool Marine Biological 

 Committee, have been precisely in this direction. 

 A most interesting account has been published by 

 Professor Herd man and Mr. Lomas on the floor 

 deposits of the Irish Sea, in which the authors state 

 (p. 217), that "a place may be swarming with life 

 and yet leave no trace of anything capable of being 

 preserved in the fossil state, whereas in other places, 

 barren of living things, banks of drifted and dead 

 shells may be found, and remain as a permanent 

 deposit on the ocean floor." 



Owing to the fact of the peculiar geographical 



