1872.] 123 [Perry. 
the glacial mass would be accompanied by important results. That 
the thawing of the ice would lay bare what is properly known as 
typical drift has been already seen. But another effect of no less 
consequence requires special attention. From the southern border 
of the wasting ice-fields floods of water must flow, doing a work, part 
of which still remains to tell its story. While the bared drift was in 
some instances largely left in its actual condition, in others it was to 
a greater or less extent worked over, especially in its superficial por- 
tions, and in those parts known as moraines, by the waters from the 
wasting ice. The detrital matter, thus subjected to a new process 
and in a measure remodelled, took some one or several of the forms 
under which it is usually known as modified Drift, 2. e., Drift which 
has undergone changes of a more or less marked character. The 
material of the morainic masses, and of the Drift generally, where 
exposed, for a longer or shorter time, to the action of running water, 
would be sifted. The water itself, issuing from beneath the ice- 
mass, and passing over finely comminuted matter, must be muddy ; 
moving southward, especially if it came to a stand-still in some de- 
pression, it would lay down its burden as a mud-deposit. Having 
taken up the finest material, it no doubt usually deposited, after flow- 
ing to a considerable distance, what is now known as clay. The next 
finest material might not be transported so far, and ordinarily found 
its place as half-stratified beds of sand; the coarser being borne a less 
distance was left in an assorted condition; while ordinarily the 
coarsest of all could be moved only a very little way, and there al- 
lowed to lie in a rough, uncouth state nearly as we now find it. 
Accordingly clay was likely to be carried furthest, and usually to 
occupy the more distant depressions; meanwhile the arenaceous por- 
tion would undergo less transportation; still even this, in connection 
with extraordinary floods, might be borne to remote basins, and de- 
posited above the clays. Thus the waters from the wasting ice, con- 
tinuing to operate for a long while, could not fail to modify the sur- 
face, and in most valleys and over broad plains to overlay the typical 
Drift with one or several of the manifold forms of Drift as modified, 
to some of which reference has been made. These constitute in large 
part the fresh-water terraces, which usually slope with the streams 
along which they for the most part occur. Such is the rule in regard 
to these beds of stratified or semi-stratified clays and sands. 
But in addition to this view of the subject, which is all important 
to a right understanding of the Terraces, there is another point, 
