﻿S. V. Wood,jun. — American and British Surface- Geohgy. 485 



passed over it ? The shells and shell-fragments, more or less worn, 

 which occur in these morainic clays, owe their presence in my opinion 

 to the alternate advance and I'ecession (such as now in Greenland 

 occurs from century to century) of the glacier-ice along the sub- 

 marine valleys or fiords, through which its escape to the sea took 

 place ; by means of which the sea-bottom of these fiords was 

 ploughed out and became mixed up with the moraine on the 

 advance of the ice, and, thus intermixed, was deposited where it is. 



To the same action, I conceive, was due all that part of the shell- 

 bed contained in the upper layers of the Middle Glacial sands just 

 referred to which consists of w^orn shells and shell-fragments ; these 

 having been carried away by the currents when the ice advanced on 

 the submarine valleys, and been deposited in these sands in asso- 

 ciation with organisms fallen from floating objects. When the 

 glacier-ice receded, this ploughing-out ceased, and the moraine ac- 

 cordingly ceased to be intermixed with shell-fragments, portions 

 of it being carried away and dropped over these sands, while 

 the bulk of it remained as extruded during recession on the bottom 

 upon which the glacier had rested. 



Different, however, to this must have been the conditions to which 

 the sand thread full of perfect valves of Nucida Cobboldics which is 

 present in the midst of the morainic clay near the foot of Dimlington 

 Cliff owed its origin ; for in that case the only explanation seems to 

 be that the shells established themselves in a thin bed of sand 

 deposited on the submarine moraine, and Avere afterwards killed 

 and buried by the descent upon them of a sheet of the moraine 

 lifted from some other place. This bed or thread of sand with 

 shells was discovered by Prof. Hughes and Mr. Leonard Lyell, 

 during a visit made by them to the Holderness coast, in company 

 with Sir Charles Lyell ; and in a memorandum sent to me with 

 the shells by Sir Charles, the thread of sand was described as 

 intercalated in the mass of the unstratified chalky claj^, and packed 

 with perfect valves of Nucula Cohholdice and other sJiells, some of 

 them having the valves united. The position of this thread apjDears 

 to have been below, but near to, the beds of sand occupying hollows 

 in the chalk}'" clay forming the lower part of Dimlington Cliff, 

 and which were distinguished by Mi\ Eome and mj'self in our sec- 

 tions ^ by the letter h, and which are overlain bj^ the purple clay, 

 also full of chalk debris in its lower part, but getting less and less 

 upwards till such debris disappears altogether from the clay. These 

 sand beds, b, are very irregular, and in some places they are inter- 

 mixed with, and in others replaced by sheets of morainic clay, 

 which present the distinct appearance of having been dropped 

 successively. Mr. Geikie, in referring to the beds thus described 

 by Mr. Eome and myself under the letter b, speaks as though 

 we regarded them as marking an interval in which the chalky 



' Quart. Joimi. Geol. Soc. vol. xxiv. p. 148 ; and Palceontographical Society's 

 volume for 1871 ; Introduction to Crag MoUusca, Supplement, p. xxt. They are 

 also shown in the section which accompanies the sequel of the present paper 

 xmder the letter d. 



