ANCIENT GLACIERS IN EASTERN HEMISPHERE. 169 



" The shells in the Moel Tryf aen deposit have been fully 

 described, so far as the enumeration of species and rela- 

 tive frequency are concerned, but little has been said as to 

 their absolute abundance and their condition. The shells 

 are extremely rare, and daring a recent visit a party of 

 five persons, in an assiduous search of about two hours, suc- 

 ceeded in finding five ivliole shells and about two ounces of 

 fragments. The opportunities for collecting are as good 

 as could be desired. The sections exposed have an aggre- 

 gate length of about a quarter of a mile, with a height 

 varying from ten to twenty feet of the shelly portion ; and 

 besides this there are immense spoil-banks, upon whose 

 rain- washed slopes fossil-collecting can be carried on under 

 the most favorable conditions. 



" I would here remark, that the occurrence of small 

 seams of shelly material of exceptional richness has im- 

 pressed collectors with the idea that they were dealing with 

 a veritable shell-bed, when the facts would bear a very dif- 

 ferent interpretation. A fictitious abundance is brought 

 about by a process of what may be termed ' concentration,' 

 by the action of a gently flowing current of water upon 

 materials of different sizes and different specific gravities. 

 Shells when but recently vacated consist of materials of 

 rather high specific gravity, penetrated by pores contain- 

 ing animal matter, so that the density of the whole mass 

 is far below that of rocks in general, and hence a current 

 too feeble to move pebbles would yet carry shells. Illus- 

 trations of this process may be observed upon any shore 

 in the concentration of fragments of coal, corks, or other 

 light material. 



" Regarding the interpretation of these facts : The com- 

 monly received idea is, that the beds were laid down in the 

 sea during a period of submergence, and that the shells 

 lived, not perhaps on the spot, but somewhere near, and 

 that the terminal curvature of the slate was produced by 

 the grounding of icebergs which also brought the boul- 



