Perry.]_ . 74 [February 28, 
formation of valleys, I would simply observe that, while admitting 
the agencies referred to, I should be unwilling to omit others which 
are perhaps fully as important as those mentioned. Granting that 
many valleys had just such an origin as the author suggests, still the 
conclusion which he states by no means necessarily follows. Indeed, 
I have looked in vain for any stable ground on which to rest his 
“must.” Suppose that the existing river-channels began to be 
scooped out by descending glaciers; the ice masses themselves, as we 
readily infer, might advance for miles beyond the present shores. 
Let these limited glaciers be finally lost in one immense sheet of ice, 
this moving seaward must, in displacing the waters on the shallow 
margins of the ocean, do its legitimate work of erosion. Thus the 
process of excavation, already begun, would still go on; all the shoals 
along the coast might be covered, the icy mass finding its limit only 
as it reached or encroached upon the deep. ‘This being the case, we 
are enabled to see that old depressions would be deepened, while new 
valleys and broad fiords might be readily formed, as well as sub- 
marine river channels, where the shallow sea prevails to-day, even 
though the continent remained at substantially its present height.1 
Or again, if we suppose with Prof. Dana, that the sub-marine 
river channels and fiords were excavated by the action of running 
water, the conclusion is by no means more necessary than on the 
other hypothesis. Whence came the immense ice-sheet that mantled 
the whole region? It seems to be forgotten that, in order to its 
formation, a vast amount of evaporation must take place; that this 
would be, not from the land alone, but. largely, nay mainly, from the 
ocean; that it would necessarily make a large draft upon its re- 
sources; that, in consequence, its waters must have undergone a great 
depression, perhaps one of several hundred feet; that thus it may 
have been far lower than at the present day, possibly lower than at 
any preceding epoch since its introduction upon the face of the 
globe; that, accordingly, the continents were perhaps broadened 
out; and, finally, that the ice-sheet no doubt extended to a con- 
siderable distance over and beyond the shoals now occupied by 
1 Prof. Agassiz once remarked to me, ‘I have seen the polished and grooved 
surfaces of rocks in many fiords of Scotland as far below the present level of 
the ocean as the transparency of the water would allow; also along the shores 
of Lake Superior.” I may add that I have observed similar instances on the 
coast of Maine, as well as on the rocky shores of Lake Champlain and Hae 
George, and at one point on Lake Huron. 
