208 
PHYSICAL HISTORY OF THE 
there cannot, in my opinion, be any doubt; 
thirdly, on the fact that this fresh-water basin 
must have been closed against the sea by some 
powerful barrier, the removal of which would 
naturally give an outlet to the waters, and 
cause the extraordinary denudations, the evi¬ 
dences of which meet us everywhere through¬ 
out the valley. 
On a smaller scale, phenomena of this kind 
have long been familiar to us. In the present 
lakes of Northern Italy, in those of Switzer¬ 
land, Norway, and Sweden, as well as in those 
of New England, especially in the State of 
Maine, the waters are held back in their basins 
unstratified clay deposit of Rio and its vicinity is genuine 
glacial drift, resulting from the grinding of the loose mate¬ 
rials interposed between the glacier and the solid rock in 
place, and retaining to this day the position in which it was 
left by the ice. Like all such accumulations, it is totally 
free from stratification. If this be so, it is evident, on com¬ 
paring the two formations, that, the ochraceous sandy clay 
of the Valley of the Amazons has been deposited under dif¬ 
ferent circumstances ; that, while it owes its resemblance 
to the Rio drift to the fact that, its materials were origi¬ 
nally ground by glaciers in the upper part of the valley, 
these materials have subsequently been spread throughout 
the whole basin and actually deposited under the agency of 
water. 
