Geology of America. 77 
the beds of our mountain torrents, and the shores of the At- 
lantic, where the rocks have been exposed to the unshielded 
and everlasting concussion of the breakers, and can find no 
attrition that will at all compare with that connected with drift, 
and I am satisfied that to explain it we must resort to some 
otheragency.” He adds, that the work of Agassiz on Glaciers 
has given us the first glimpse of what seems to be a solution 
of this problem. 
He observes, that, in the United States, the striz and fur- 
rows have not been found to radiate in different directions 
from a mountain chain (as they do in the Alps, which Agassiz 
calls a ‘* centre of dispersion’) ; and further, that the northern 
slopes of mountains are grooved, even though very steep, while 
the drift covers the opposite side. We have here the pheno- 
menon of “ Crag and Tail,” so familiarly known in this coun- 
try, and under the same aspect; for the marks of attrition 
here are generally found on the west or north sides of hills. 
New light has been thrown on this subject by Mr Murchison’s 
researches in Russia. He thinks that the striated surfaces 
there cannot be accounted for by the abrasion of glaciers, but 
may rather be ascribed to ¢he action of floating masses of ice, 
armed with stones or gravel adhering to their bottoms, and that 
the mounds and ridges of gravel and clay arise from the ac- 
tion of currents casting up on their flanks masses of ice loaded 
with debris. 
This removes some difficulties; but the question presents 
itself{—why do these furrows, and these ridges or tails of al- 
luvium, generally point north-west and south-east in Russia, 
Sweden, Britain, and the United States, with the crag or bare 
scalp of rock on the north or north-west side, and the tail of 
alluvium on the south or south-east? The phenomena in- 
dicate the motion of masses of water, ice, gravel, and clay, 
from the north or north-west, and may probably be accounted 
for as follows :—While the northern portions of the new and 
old continents were for the greater part under the sea, a broad 
current would set continually eastward, along the regions in- 
cluded in the temperate zone. The grounds which led me to 
form the conclusion, were stated in my work on the geology of 
this district. Let us next assume what Agassiz has proved, that 
