Vol. 54.] AND THE COKALLINE CRAG. 343 



D being afterwards deposited in a comparatively deep and tranquil 

 sea, which, during period E, is supposed to have attained a depth 

 of from 500 to 1000 feet. Beds F & G were believed by Prestwich 

 to indicate a gradual upheaval, the latter having originated in 

 water sufficiently shallow to allow of the denudation of the older 

 portion of the Crag by current-action, and the heaping-up of the 

 resulting material in the form of submarine banks. 



In the first place, I would point out that in no part of the 

 Coralline Crag have we any indication of deep-water conditions. 

 Sir John Murray states that along the continental shores which 

 face the great oceans the sea-bottom is generally covered with mud 

 below the 100-f athom line,^ and Prof. Herdman finds this mud-zone 

 in the Irish Sea at 50 fathoms.' Such deposits are known to all 

 geologists, as for example, in the Argiles bleues of Antibes and 

 the Ligurian coast, where f ossiliferous beds have quietly accumulated 

 in deep water near the old shore-line. They are not distinctly 

 stratified as is the Coralline Crag, and the shells are not arranged 

 in layers, but occur here and there at diff'erent levels. 



Coming nearer home, we have in Belgium the Isocardia co?'-beds, 

 contemporaneous with the Coralline Crag, which, though not repre- 

 senting deep-sea conditions, are of a character similar to those just 

 mentioned. In them, as M. Van den Broeck informs me, the lamelli- 

 branchs are always found with the two valves united, and never 

 arranged in layers. The enormous preponderance of the specimens 

 found in the Coralline Crag consists, on the contrary, of the drifted 

 and stratified remains of dead animals. A few bivalves occur in it 

 double, and some, as at Gomer, in the position of growth ; but there 

 is no reason, on the one hand, why lamellibranchs may not occa- 

 sionally be found living in sheltered spots on banks of dead shells, 

 nor why, on the other, living shells should not sometimes be carried 

 along the bottom by currents with other debris, and be buried 

 with them. Constant reference is made in the British Association 

 Reports on Dredging to the occurrence of dead mollusca with both 

 valves united,^ and my son, Dr. S. F. Harmer, informs me that such 

 an occurrence is by no means unusual. 



The difference between the /socarcZia-beds of Belgium and the 

 Coralline Crag is most marked and very instructive. In the one 

 case, we have an ancient sea-bottom, with the mollusca in situ, as 

 they lived -, in the other, masses of dead shells accumulated by the 

 action of currents. The beds of Prestwich's zone E, which he con- 

 siders to be indicative of deep-sea conditions, are of the same drifted 

 and stratified character as the rest of the formation.^ 



The mollusca found in the Coralline Crag cannot be regarded as 



^ Challenger Eeports, Summary, vol. ii. p. 1433. 



2 Eep. Brit. Assoc. (Ipswich) 1895, p. 703. 



' See, for example, Rep. Brit. Assoc. (Dublin) 1857, List of Shells from 

 Turbot Bank, p. 230. 



•i As to this, see also Clement Eeid, Mem. Geol. Surv. 1890, ' Plioc. Deposits 

 of Britain,' p. 36, who says: — 'I have been unable to recognize in Prof. Prest- 

 wich's division e a single stratum unaffected by current-bedding.' 



