TERRACES AND BEACHES AROUND LAKE SUPERIOR. 31 



matter, by mistaking a terrace of rock for one of sand or gravel. There is no 

 danger of such a mistake in New England. 



The terraces and beaches around a considerable part of Lake Superior have been 

 described with great scientific accm-acy by Professor Agassiz, in his work on Lake 

 Superior, and by Messrs. Desor and Whittlesey in the Eeports of Foster and "Whit- 

 ney on the Lake Superior Land District. The latter gentlemen have, also, included 

 a considerable part of the shores of Lake Michigan. 



From the details given by these gentlemen, I judge that surface geology in the 

 regions of these great lakes corresponds essentially to that of New England. Though 

 the different forms assumed by the materials may in their writings often be given 

 under names different from those I have used in this paper, the things described 

 appear to be identical. There is a coarse drift underlying all the other forms of 

 detritus, and above this lie deposits of clay, sand, and loam, overspread in many 

 places and mingled with blocks of various sizes, generally more or less rounded. 

 M. Desor considers the lowest deposit of the clay some 60 feet thick, and those of 

 the sand and gravel above, some 360 feet thick, to belong to the drift, because 

 mixed with, and covered over, with boulders. He divides all the superficial depo- 

 sits into three parts. 1. Drift proper, with the above four subdivisions. 2. Ter- 

 races belonging to a later epoch — a part of the terraces he includes in the drift. 

 3. Alluvial deposits, embracing all those forms of detritus that have accumulated 

 since the continent began to rise from the ocean, such as beaches, terraces, nooks, 

 belts, bars, marshes, flats, and subaqueous ridges. 



As to the number of terraces, M. Desor speaks of as many as seven in some 

 places, and Professor Agassiz says that "six, ten, even fifteen, may be distinguished- 

 on one spot." The number, all agree, varies very much in different parts of the 

 same lake. Professor Agassiz thinks that " these various terraces mark the suc- 

 cessive paroxysms or periods of re-elevation" of the shores of the lake. Desor 

 adopts the same view, certainly so far as to say that the terraces indicate pauses 

 in the vertical movement, which, however, he would make . general over the conti- 

 nent; for he finds the drift deposit at the top of the highest parts of the country 

 around these lakes, not less than 1000 feet above their present level. 



Now it will be seen that while I agree with these gentlemen in regard to the 

 essential facts of surface geology, we differ as to the mode of stating them, and 

 somewhat in the theory of the whole subject. We all agree in supposing the 

 phenomena to require vertical movements in our continent, or its submergence and 

 emergence since the tertiary epoch. But while they suppose that there were pauses 

 in the vertical movement, long enough to form the different terraces, I have been 

 led to suppose that most of them, certainly river terraces, must have been formed 

 without such pauses, and simply by uninterrupted emergence or drainage of the 

 country. We agree as to the occurrence of a deposit of coarse drift at the bottom 

 of the series; but while they regard the superimposed clay and sand as true drift, 

 I suppose them modified drift, and produced almost entirely by water, save that 

 floating icebergs have dropped the large boulders. They, certainly M. Desor, sup- 

 pose the drift period to have terminated when the continent began to emerge, and 

 the alluvial to have then commenced; but I regard drift proper as the result of 



