Vol. 54.] AND THE CORALLINE CRAG. 351 



channel must have been much greater than it is at present through 

 the Straits of Dover. 



The hypothesis that the Coralline Crag represents, not the bottom 

 of a sea in which the mollusca found in it lived (as do the Isocardia- 

 beds of Belgium), still less that it originated at any considerable 

 depth, ^ but that it was accumulated by currents coming from the 

 south-west, which heaped up, in comparatively shallow water, the 

 remains of marine organisms, in banks more or less parallel with 

 the then existing coast of the Grerman Ocean, and at some distance 

 from the mouth of any river discharging into it,^ seems to be in 

 accordance with all the facts of the case. It explains why we find, 

 at the same level, seams, in one place of large, in another of smaller 

 shells, and in a third of fine comminuted material. We can 

 understand that when, by the temporary and local diversion of the 

 currents, no sediment was for a time deposited on any portion of 

 the banks, they would there become occupied by sheets of reef- 

 building polyzoa, which would afterwards be smothered and unable 

 to exist, when another alteration brought over them quantities of 

 the fine mud.^ Crustaceans and echinoderms would live under such 

 conditions, as they now do on the Turbot Bank, but their numbers 

 would be few in proportion to the drifted shells of mollusca. 



Por the reasons given above, it seems probable that the conditions 

 under which the Coralline Crag originated were similar to those 

 now obtaining in the northern part of the Irish Sea, where strong 

 tidal currents, sweeping through the narrow channel that separates 

 Ireland from Scotland, are causing the accumulation of banks 

 containing dead shells, at no great distance from the shore and 

 parallel to it. 



In a future paper I hope to deal with the questions of the 

 classification and mode of origin of the various deposits of the Upper 

 Crag formation of Suffolk and Norfolk. 



lY. Recapitulation. 



In the foregoing pages I have set forth the reasons which lead 

 me to think : — 



1. That the Lenham Beds, containing a considerable proportion 

 of characteristic Miocene or Italian Lower Pliocene mollusca 

 (13 out of 67) unknown or very rare in the Coralline Crag, 

 are older than that formation. 



^ Our estimate of the depths of the western (and iLtoral) portion of the Crag 

 basin during the deposition of the Coralline Crag should depend, not on the 

 character of its mollusca, which are not, as a rule, in situ, but on the view that 

 we may take as to the strength and volume of the currents then prevailing. 



^ The Thames, in its present form, had not at that time, I consider, come 

 into existence. 



^ Mr. Kendall reminds me that valves of Pecten, etc., encrusted with adnate 

 polyzoa, occur chiefly in those parts of the Crag where the reef-building forms 

 are found. These also could exist only when the currents passing over that 

 portion of the area were free from sediment. 



Q.J.G. S. No. 215. 2 c 



