DRIFT, BOULDERS, OtC. 
425 
same agent which performed this work must also have excavated to some extent the shales 
then overlying the trenton limestone which forms a part of the floor of Lake Ontario. After¬ 
wards we have the marine deposit; and then the ocean must have occupied the present floor 
of the lake, and widened the breach already existing in these soft shales. 
I have now answered the inquiries I proposed ; 
1. In regard to the transportation of the northern boulders ; the principal agents, I have 
stated, were icebergs. 
2. The surface previous to the boulder period, I have stated, must have been depressed, 
and according to hypothesis, may have been overflowed by wide shallow rivers. 
3. The scorings of the rocks took place previous to the boulder era ; or at least it is a work 
which may have been done by rapid currents bearing along gravel, sand, and ice occasionally 
loaded with stones ; but it is a phenomenon which, in the main, is independent of the action 
of icebergs. 
4. I have stated that the valleys now containing our great northern and western lakes, were 
excavated partly by broad shallow rivers, but probably widened by the action of the sea at 
the period of the deposit of the tertiary; during which period, icebergs floated from the north, 
and transported most of the boulders which we now find upon this section. 
In the preceding remarks, I had in view the country which forms the Second district, and 
the hypothesis I have proposed accords best with the phenomena I have observed in this 
region. I can, by no other hypothesis, explain the phenomena so satisfactorily as by the pre¬ 
ceding. The fact of its submergence for a time, or during the deposit of the tertiary, does 
not make provision for the scorings and groovings of rocks in the mode we now find them ; and 
hence the hypothesis must go back farther, to a time when I have supposed a condition of the 
surface favorable to the existence of broad shallow rivers, which frequently changed their beds 
and channels, in consequence of the oscillations produced by disturbing forces. 
In the progress of the Survey, I have felt the necessity of modifying previous views in rela¬ 
tion to the cause or causes which have scored the surface of rocks so widely and extensively; 
and instead of seeking one cause, by which this phenomenon may have been produced, and 
which may be applied to the whole country, or to the entire northern hemisphere, I have 
found several; adopting in this respect the same rule of interpretation that I have in relation 
to the distribution of the boulders, drift, and all the loose materials ; for these materials can 
by no means have been all distributed by one single agent. So the scratches upon rocks may 
have been produced, 1st, by the currents of river bearing along gravel and sand, and at times 
ice and boulders; 2dly, by glaciers ;* and 3dly, by icebergs with implanted boulders, though 
not generally. The latter were the true agents in the distribution of northern boulders, while 
the northern part of New-York was submerged. And here I take the opportunity of saying, 
* Essays of Venetz, Chaepentier and Agassiz. Not that I can apply the glacier theory to the northern district. I men¬ 
tion it here as a cause, which, in its own field, produces these effects. 
Geol. 2d List. 54 
