8. V. Wood,jun. — American and British Surf ace-Geology. 485 
passed over it ? The shells and shell-fragments, more or less wora, 
which occur in these morainic clays, owe their presence in my opinion 
to the alternate advance and recession (such as now in Greenland 
occurs from Century to Century) of the glacier-ice along the sub- 
marine valleys or fiörds, through which its escape to the sea took 
place ; by means of which the sea-bottom of these fiörds 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 wom 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 Nucula Cobboldia which is 
present in the midst of the morainic clay near the foot of Dimliugton 
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 were 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 clay, and packed 
with perfect valves of Nncida Cobboldice and other shells, some of 
them having the valves United. The position of this thread appears 
to have been below, but near to, the beds of sand occupying hollows 
in the chalky clay forming the lower part of Dimlington Cliff, 
and which were distinguished by Mr. Rome and ruyself in our sec- 
tions 1 by the letter b, and which are overlain by the purple clay, 
also full of chalk debris in its lower part, but getting less and less 
upwards tili such debris disappears altogether from the clay. These 
sand beds, b, are very irregulär, 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. Rome and myself under the letter b, speaks as though 
we regarded them as marking an interval in which the chalky 
1 Quart. Joura. Geol. Soc. vol. xxiv. p. 148 ; and Palsontographical Society’s 
volume for 1871 ; Introduction to Crag Mollusca, Supplement, p. xxv. They are 
also shown in the section which accompanies the sequel of the present paper 
under the letter b. 
