SIGNS OF PAST GLACiATION. 61 



cock's discovery of boulders on the summit of Mount 

 Washington (over 6,000 feet above the sea), which he was 

 able to identify as derived from the ledges of light grey 

 Bethlehem gneiss, whose nearest outcrop is in Jefferson, 

 several miles to the northwest, and 3,000 or 4,000 feet 

 lower than Mount Washington. However difficult it may 

 be to explain the movement of these boulders by glacial 

 ice, it is not impossible to do so, but the attempt to ac- 

 count for their transportation by floating ice is utterly 

 preposterous. No iceberg could pick up boulders so far 

 beneath the surface of the water, and even if it could ad- 

 vance thus far in its work it could not by any possibility 

 land them afterwards upon the summit of Mount Wash- 

 ington. 



Among the most impressive instances of boulders evi- 

 dently transported by glacial ice, rather than by icebergs, 

 were some which came to my notice when, in company 

 with the late Professor H. Oarvill Lewis, I was tracing the 

 glacial boundary across the State of Pennsylvania. We 

 had reached the elevated plateau (two thousand feet above 

 the sea) which extends westwards and southwards from 

 the peak of Pocono Mountain, in Monroe County. This 

 plateau consists of level strata of sandstone, the southern 

 part of which is characterised by a thin sandy soil, such 

 as is naturally formed by the disintegration of the under- 

 lying rock, and there is no foreign material to be found 

 in it. But, on going northwards to the boundary of Toby- 

 hanna township, we at once struck a large line of accumu- 

 lations, stretching from east to west, and rising to a height 

 of seventy or eighty feet. This was chiefly an accumula- 

 tion of transported boulders, resembling in its structure 

 the terminal moraines which are found at the front of 

 glaciers in the Alps and in Alaska, and indeed wherever 

 active glaciers still remain. But here we were upon the 

 summit of the mountain, where there are no higher levels 

 to the north of us, down which the ice could flow. Be- 



