S. V. Wood, Jim. — On Glacial Deposits. 353 



vast ice-slieet that filled the North Sea, urged that the remains of 

 Mollusca occurring in the unstratified clay or Till of Caithness were 

 due to the j)loughing out of a pre-existing sea-bed, by which such re- 

 mains became incorporated in the unstratified formation produced by 

 this ice-sheet. Now the proof that unstratified Till was deposited 

 under the sea appears to me simple and convincing. When Sir Charles 

 Lyell visited Holderness in 1869, in company with his nephew Mr. 

 Leonard Lyell, and Mr, Thomas M. Hughes of the Geological 

 Survey, they found in the midst of the unstratified chalky clay (or 

 Till, as the Scotch would call it) of the lower part of Dimliugton 

 Cliff, a thin streak of greenish sand embedded in the clay, which, 

 according to the description of Mr. L. Lyell sent me by Sir Charles, 

 was " crammed ivitJi perfect specimens of Nucula Cobboldics." Some 

 of these, Mr. Lyell adds, had he believes the two valves adherent, 

 and certainly an Astarte was found in that condition. 



Now here we have an unequivocal instance of a colony of Mollusca 

 that must have established itself on a sea-bottom formed of unstrati- 

 fied glacial clay, and been afterwards tranquilly covered over with 

 similar material, just in the same way in which with stratified deposits 

 a band of shells is covered by a succeeding stratum of sand or mud. 

 Can it be questioned in the face of this that unstratified clay or Till 

 has been deposited under the sea, which is my first proposition ? 

 Further, if this cannot be questioned, then why refer any of this 

 unstratified clay or Till to a deposit on a terrestrial surface ? for it is 

 quite a rare exception to find Mollusca in any part of it, no instance 

 of the kind being known to have occurred in all the wide expanse 

 of this chalky clay except at Dimlington. 



I have elsewhere pointed out that this wholly unstratified deposit, 

 undistinguishable in its physical character from that Scotch Till 

 which is held to be the terrestrial deposit of land-ice, spreads evenly- 

 over large areas of stratified sands that yield Mollusca in places ; 

 and that such clay not unfrequently passes down vertically into 

 these sands by a few feet of sandy clay obscurely stratified. What- 

 ever therefore be the cause of the anomaly, I do not see how it can 

 be questioned that by some means a vast mass of material, which is 

 admitted by all to be a product of land glaciation, has been spread 

 out under the sea over an extensive area in a wholly unstratified 

 condition. 



This being so, the second, third, and fourth propositions seem to me to 

 be answered by necessary implication. It will not be denied by Mr. 

 Geikie, or any one, that the material was generated by the action of 

 the land-ice. It could not get out to sea in this vast volume unless 

 its motion was incessantly outwards from beneath the generating ice- 

 sheet ; and it could not thus travel outwards without the material 

 generated at the commencement of this action haviug all found its 

 way into the sea before its termination. If, for instance, we assume 

 the period as 100,000 years — it was probably far more — the material 

 generated in the first 10,000 would have all found its way to the sea 

 before the end of the second 10,000 years, and so on throughout the 

 entire period of 100,000 years. 



VOL. IX. — NO. XCVIII. 23 



