Geologists' Association 477 



are rare, the largest bed of this sort lying between Havre and 

 Calais; they appear almost entirely confined to the Channel, and 

 may in some cases be due to the degradation of marine outcrops in 

 situ, though most of them are the result of transport from our shore 

 pebble-reaches. Where a clay bottom is indicated, its origin also is 

 in some cases doubtful. 



Organic Deposits, — In the Channel the shell deposits attain their 

 greatest development as regards British Seas. Here they follow two 

 long, occasionally broken lines, following at a short distance the 

 English and French shores, and forming at the outer mouth of the 

 Channel a vast shell-bank. These deposits actually cross the broad 

 sea valley, partly over and considerably to the west of the spread of 

 bare rock previously mentioned. Elsewhere in the Channel the 

 Gallic side is richer than the English. There are other shell deposits 

 around our coasts, but of less importance. Beyond the ocean valley 

 which lies beyond the Hebrides and the Eock-ball reef, there occurs 

 a fish bank more than three miles in length, affording us an inkling 

 of the manner in which some of our fossil fish beds may originally 

 have been accumulated. 



Fluvio-marine Deposits. — The Thames, Seine, and Tay form mud 

 banks in a sandy sea. The sub-marine delta of the former has the 

 shape of a triangle, of which the apex points seawards ; that of the 

 Seine is also triangular in outline, but the apex points landwards. 

 Such sub-marine deltas can only be recognized when the materials 

 of which they consist are distinct from those forming the prevailing 

 sea-bottom. 



The question of shore deposits was merely alluded to, this subject 

 forming no part of the present paper. Of true chemical deposits it 

 cannot be said that the comparatively shallow seas of Britain offer 

 any good example. 



Although much of the above materials are at present unconsolidated, 

 especially the sands, it is not probable that the larger features of the 

 sea bottoms are liable to important changes, whilst the surrounding 

 geographical conditions remain unaltered. The same agencies which 

 sweep certain spots bare, heap up macerial elsewhere, and the 

 relative form of both covered and uncovered portions of the sea floor 

 is preserved by them. The points of the greatest violence of current 

 action are shown by the bare rock patches, whilst the intermediate 

 stages of agitation are represented by coarse shingle, sandy gravel, 

 sand, muddy sand, and finally patches of mud or clay supervene, 

 which to a certain extent indicate centres of calm. 



2. " On some Specimens of Phosphorite from the Department of 

 the Lot." By F. W. Kudler, F.G.S. 



3. " On a Probable Solution of Mr. Charlesworth's Problem rela- 

 tive to the Boring of Sharks' Teeth." By H. A. Burrows. 



This jDcrforation was coimected with the existence of the foramen 

 for the passage of blood vessels, opposite to which are numerous 

 small holes, as observed to exist in the teeth of recent shaxxs. 



