364 Gen. Totten on the Disappearance of Ice in Northern Lakes. 
invasion. These lines of boulders are to be seen in many places, 
registering accurately, not the work of the preceding year, but 
the greatest effort of any previous year. 
e circumstances of some deep-lying boulders may be such 
that they are rarely embraced, acted on, or moved, and such 
may long, by fits, continue to be erratic, though finally to joi 
the general shore parade 
The force of these moving fields is very great, even when the 
decomposing process is much advanced. I have seen a timber 
wharf, which was about thirty feet square, ten or fourteen feet 
high, and filled solidly with earth and stones, shoved along the 
bottom about thirty feet, by a single continuous push of a great 
field of ice just ready to be resolved into its prismatic elements. 
The motion was very slow, only to be seen, indeed, by close ob- 
servation, while the ice was broken at the edge of contact into 
innumerable fragments, piling themselves, with a tinkling sound, 
high upon the wharf and following ice. 
A simple and effectual guard against this danger to wharf or 
pier has been found to be, the giving to the exposed face a cer- 
tain talus (about one of base to two of height, I think), which 
turns the ice upwards to the top of the structure, where its frag- 
ments accumulate, sometimes to a considerable height. This 
easy diversion of so great a force is due, of course, to the pe- 
culiar crystalline structure of the ice, the degree to which it has 
n decomposed, and the consequent brittleness against a trans- 
verse strain. Should there be an unfrozen margin to permit 
this motion of large fields of ice, before the solution of conti- 
uity in the crystalline arrangement, nothing but the solid earth 
could stand before it. 
These remarks have extended further than I intended, and I 
fear much beyond what was required by the state of knowledge 
on the subject. But I venture, nevertheless, in reference to the 
portion of these remarks, one further observation—namely, 
that nature seems to have especially provided, in the structure 
of these wintry coverings of water surfaces, for their prompt Te 
moval, when their existence would retard the advancing year. 
Kes 
