90 Sir Charles Lyell [April 27, 



mulated in certain cases under water, we are at liberty to assume 

 the same in all others, where such an explanation will best accord 

 with the facts. 



The Saxicava rugosa, Mya truncata, Natica clausa, and other recent 

 species of mollusca, are common to the drift of North America and 

 Europe, and constitute part of a fauna, characteristic of a climate 

 somewhat colder than that of the latitudes where the fossils are 

 now met with. The Caplan also, or Mallotus villosus, a fish swarm- 

 ing in the seas of Greenland, Labrador, and wherever ice abounds in 

 the North Atlantic, is found fossil in clayey concretions or clay stones, 

 in the drift of Maine and Canada, as well as in Greenland. It 

 appears that in the glacial period, as now, the isothermal lines, or 

 rather the lines of equal winter temperature bent many degrees 

 farther south on the west than on the east side of the Atlantic, so 

 that the monuments of glacial action extend some eight or ten 

 degrees farther south in North America than they do in Western 

 Europe. Large erratics and glacial furrows are rare south of lati- 

 tude 48^ or 50° in Europe, and are seldom seen south of 40^ in 

 North America ; but where mountain chains like the Alps or Hima- 

 laya have formed independent sources of cold, we find exceptions. 

 In Madeira and the Canary Islands, between latitudes 28^ and 33*^ 

 N., Sir C. Lyell searched in vain for glacial striae, and other con- 

 comitant phenomena, although some of the mountains there are of 

 great height. In the southern hemisphere all the manifestations 

 of the agency of ice, which are wanting in the equatorial zone, 

 reappear in full force, when we reach Chiloe, Patagonia, and Tierra 

 del Fuego. 



If ice-islands, running aground on the bed of a sea or on a 

 a coast, can smooth and furrow the subjacent floor of the ocean by 

 pushing before them or pressing down under them loose sand, 

 pebbles, and stones, the size of such islands is certainly sufficient to 

 afford as much friction and mechanical force as we require. Some 

 of them, measured in the southern hemisphere, exceed 10 miles 

 in diameter, and their height out of the water is from 100 to 200 

 feet, the mass of ice below being about eight times in volume that 

 rising above the sea-level ; if such masses when they run aground 

 are moving only at the rate of one or two miles an hour, their 

 momentum must be enormous. 



The area now subject to the action just alluded to in both hemi- 

 spheres, is many times greater than that over which terrestrial 

 glaciers descend. In like manner, in the period of the " Northern 

 drift,*' the submarine were far more extensive than the supra- 

 marine glacial operations ; and since we have evidence of much sea 

 having been converted into land since the glacial period, we must 

 expect to find more space bearing the imprint of subaqueous ice 

 than of space exhibiting evidence of the movement of ice over dry 

 land. 



In conclusion, Sir C. Lyell observed, that as the great majority 



