YORKSHIRE DRIFTS. 
289 
have been other agencies, of which he considered the transportation by 
icebergs to be the most powerful. He further adduced proof of this 
theory from the occurrence of erratic boulders in the Valley of the 
Calder in the neighbourhood of Halifax, and considered that it is in- 
conceivable that these boulders could have been brought over the 
Penine Chain and deposited where they are now found, but that it 
would be easy, if the land were at a considerably lower elevation than 
at present, that they should be brought by floating icebergs from the 
sea and stranded in the narrow inlet which the bed of the river would 
then form. The superficial gravels and sands which form his second 
group, he considered afford very strong evidence of the long-continued 
and comparatively gentle action of water. The third division form- 
ing hillocks and terraces of disturbed material, occur in many of the 
northern valleys, and though they had, until a recent period, escaped 
attention, he believed that they would furnish a clue by which the 
difliculties surrounding the question might be unravelled. Referring 
to the paper recently contributed to the meeting of the British 
Association at Glasgow, by M. Agassiz, in which he endeavoured to 
show that a gTcat part of these terraces and deposits have been pro- 
duced by the action of glaciers, Mr. Clay expressed dissent, and 
considered that M. Agassiz carried his theory to a most unwarrant- 
able extent when he inferred that great sheets of ice resembling those 
now existing in Greenland, once covered all the countries in which 
these unstratified gravels now occur ; but he considered that the 
evidence was sufficient to prove that towards the close of the Tertiary 
era the land was at a lower level than at present, and there was a con- 
siderable preponderance of water. The mountainous districts of Scot- 
land and the Northern Counties of England were not submerged, 
but probably covered with perpetual snow, huge glaciers occupying 
the valleys between, and bringing the spoils derived from the 
rocky summits down to the sea, which, breaking off as icebergs, 
floated away bearing the detritus to some distant localities where, by 
their gTadual dissolution, they gave rise to the unstratified masses of 
clay and boulders. On the re-elevation of the land, the shallower 
parts would approach the surface, and currents would affect the 
upper portion of the deposit, stratifying the debris and forming 
