1872. 115 [Perry. 
Now “ice,” to quote Professor Agassiz, “‘is transparent to heat.”1 Ac- 
cordingly heat would be transmitted to the underlying rocks. These 
becoming warm, the directly overlying ice must waste. Such boulders, 
therefore, as were beneath the melting portion, could not fail to be laid 
bare, while those upon the back of the ice-mass, and immediately 
above the summits were likely, as the thawing took place, to be laid 
down upon them. But more important deposits were to follow. As 
rear portions of the ice-sheet moved forward, they might occasionally 
force along into the open space an underlying rounded boulder. Thus 
advancing, successive parts of the ice-mass would be constantly forced 
upon the edge of the bared summits; each of these parts must in due 
time melt, and drop such far-travelled angular erratics as chanced to 
rest upon them. And this operation was likely to continue, until the 
eradual wasting of the sheet of ice reduced the level of its surface, be- 
neath that of each mountain peak. When the ice contained an erratic, 
as the upper portion of the continental mass must have occasionally 
enclosed a strageler derived from more northern heights, and when it 
was so situated as to be carried directly upon a given height, we see 
that it might be readily perched in the isolated place in which it is 
now found. By this process, if it were long enough protracted, many 
boulders must at last find themselves collected together, and placed in 
the strange attitude to which reference has been made. Thus erratic 
after erratic would be left permanently fixed, the ice not being able 
again to pass over the summit, and sweep away the accumulated de- 
posits of the many preceding years.? 
Travelled rocks of this kind are occasionally met with in various 
parts of the country, sometimes isolated, sometimes thrown together 
in great profusion. One of extraordinary size and of vast weight 
may be seen in Whitingham, Yt., on the summit of a hill lying on the 
east of the valley of the Deerfield, and 500 feet above the river. 
This huge boulder was evidently transported across the deep valley, 
and left in its present elevated position by the slowly wasting field of 
ice. There is another of like character on,Hoosac Mountain, 1300 
feet above the depression on the west, which it clearly crossed, prob- 
ably borne along on the surface of the great ice-mass or enclosed in 
1See Systeme Glaciaire. 
2«These perched boulders,” as Professor Agassiz informs me, ‘‘may be seen in 
the Alps in such proximity to the present glaciers as to leave no doubt of their 
mode of deposition.” 
