228 PREHISTORIC EUROPE. 



It is interesting to note that in Cornwall, where the " head " 

 is well developed, we encounter at the base of certain post- 

 glacial deposits, to be described in the sequel, torrential gravels 

 which are well known from the fact that they have been long 

 worked to get at the lumps of tin-ore which they contain. 

 This tin-bearing accumulation is composed of a confused mass 

 of sand, gravel, pebbles, blocks, and boulders, some of which 

 have weighed as much as 200 lbs. and upwards. It is just 

 such a deposit as might have been formed by torrents more or 

 less suddenly discharged from melting snow and ice. Mr. 

 Ussher has shown that it is posterior in age to certain ancient 

 raised beaches upon which the "head" reposes, and it is 

 therefore of approximately the same age as the " head " itself. 

 The latter, in short, will represent the angular debris moved 

 forward by the action of frost, melting snows, etc., while the 

 stanniferous gravels will denote that portion of the subaerial 

 waste which was swept into gullies and stream-courses, and 

 hurried along by the tumultuous waters of spring and summer. 



With the cause or causes that induced the Glacial Period 

 we are not at present concerned, but we may at least conclude 

 from the facts so briefly set forth in this and the preceding 

 chapter, that whatever the origin of the glacial climate may 

 have been, it certainly cannot be attributed to any mere 

 elevation of the land. The phenomena are much too general 

 to be thus accounted for. Not only has Europe passed through 

 its Glacial Period, but abundant evidence is forthcoming to show 

 that North America has experienced similar climatic conditions. 

 A great ice -sheet covered all the northern regions of that 

 continent and flowed as far south as the latitude of New Jersey, 

 and still farther south in Ohio, Indiana, Illinois, and Missouri. 

 At the same time the valleys of the Eocky Mountains and the 

 Sierra Nevada were filled with gigantic glaciers, compared with 



Cornwall, Devon, and West Somerset; and "W. A. E. Ussher's papers on the 

 ''Recent Geology of Cornwall" (Geological Magazine, 1879), in which copious 

 references will be found to other sources of information. The same writer gives a 

 further account of "head " in his Post- Tertiary Geology of Cornwall. 



