62 SURFACE GEOLOGY. 



of the terraces was subsequent. They overlie the grooved and rounded rocks" (p. 

 103). Yet, if I understand Prof. Agassiz, he ascribes these vertical movements to 

 the injections of trap veins, so common along the shores. "This process of inter- 

 section, these successive injections of different materials (in the veins), have evi- 

 dently modified at various epochs, the relative level of the lake and land, and 

 probably also occasioned the modification which we notice in the deposition of the 

 shore drift, and the successive amphitheatric terraces, which border, at various 

 heights, its shores" (p. 424). 



Now, with so little personal knowledge of lake terraces, it may be presumption 

 in me to call in question any of these conclusions. But a few suggestions may not 

 be improper. 



Were Lake Superior, itself an ocean, alone concerned, we might have less diffi- 

 culty in admitting these views, and in supposing that its terraces mark the pauses 

 in the uplifts of its shores. But I apprehend that scarcely a lake exists in our 

 country that does not show distinct terraces, nay even ponds, covering only a few 

 hundred acres, exhibit them distinctly. I know of some such in New England. 

 Now surely we cannot suppose that the shores of each of these smaller lakes and 

 ponds have undergone any such elevation since the drift period : I mean to say that 

 they have been elevated only as a part of the continent, and not by a local move- 

 ment, as must have been the case if the shores are raised above the waters. So 

 that if we could dispose of the Lake Superior terraces in this manner, those of other 

 lakes would still remain unaccounted for. Moreover, as to the cause assigned for 

 this rise of the shores, viz., trap dykes, I do not see how these could have been 

 concerned in the last movement which produced the terraces. For the surface of 

 these dykes is smoothed and striated by the drift agency, which shows them to 

 have been injected long before the drift period, whereas the terraces have all been 

 formed since. 



I agree with Professor Agassiz in the opinion, that subsequent to the drift 

 period, our continent has been beneath the ocean, and has subsequently risen. 

 But it seems to me that it came up bodily, or as a whole ; at any rate, I have not 

 met with any evidence of local elevations. Supposing it was the ocean that spread 

 over all our continent; as that was gradually raised, the waters might have left 

 evidence of their recession, and of their successive pauses (if any prefer that view), 

 in the form of terraces around all our lakes. I think that a rise of the land, 

 unattended by paroxysms and pauses, may more easily show us why the number 

 and height of terraces differ so much on different bodies of water, and that the 

 unequal number which we find on the same lake, or river, may thus be more satis- 

 factorily explained. For if there were such pauses to any great extent, I do not 

 see why the number and height of the principal terraces should not correspond 

 everywhere, even though we leave out of the account the irregularities of the 

 minor terraces. Yet I admit the occurrence, occasionally, of such pauses. I 

 could not, for instance, look on the Parallel Pioads of Lochaber, in Scotland, with- 

 out feeling that probably they mark paroxysmal movements of the waters. But 

 it cannot be denied that men, even geologists, are too prone to resort to paroxysms 

 and irregular action to explain phenomena; and I look upon the labors of Sir 



