GEOLOGY. 261 



as far as their eastern border is made up of materials from this source. 

 Near Lake Michigan appears a drift deposit as a bluff 12 feet thick, 

 made up of 9 feet of sand over blue clay. This same deposit, where 

 it occurs near Lake Erie, has been considered by geologists as a lake 

 drift. It is important to establish its lacustrine origin from fossils 

 contained in it. Mr. Desor has succeeded in finding in it, on Lake 

 Michigan, eight species of fossil shells, several of them identical with 

 those now living in the adjoining lake. In an underlying stratum were 

 reeds (one of which was from an inch to an inch and a half in diame- 

 ter, extending up into the sand), Equiseta, and a piece of cedar, the 

 I icnlity having probably been occupied by a cedar swamp. There 

 arc- no trees of this species at the present time within many miles of 

 i!ie spot, and the nature of this deposit leads to the supposition of a 

 depression and subsequent elevation of the country. To the south 

 and east of Lake Michigan is a belt of flat prairie of fresh-water origin. 

 The rolling prairies of the West, on the contrary, are of marine ori- 

 gin, and probably antedate the flat, A few boulders are found on the 

 surface of the flat prairie, the presence of which might seem to be 

 incompatible with this theory ; but they may have been deposited by 

 ice, either in the form of icebergs or ground ice. The main drift de- 

 posit over the northern parts of the United States, Mr. Desor attrib- 

 utes to the ocean. On Lakes Superior and Michigan the drift striaj 

 run from northeast to southwest. One set, however, runs due north 

 and south, and is perhaps of a more recent origin. 



Mr. Desor remarked that it was interesting to observe the influence 

 which geological causes have had in fixing the localities of cities on 

 the Mississippi and Ohio. The terraces of the upper Mississippi are 

 very low and exposed to inundations, so that towns can only be built 

 upon the bluff beyond. This limits the location of towns to those 

 bluffs near the river. On the Ohio, on the other hand, the terraces 

 have been elevated to the height of 50, 80, or 100 feet, and the cities 

 are built upon this alluvial foundation, having the drift behind them ; 

 on the Mississippi they are built upon the^ drift itself, so that the 

 banks cf the Ohio offer the most eligible situations for settlements. 

 Proceedings of the Boston Society of Natural History. 



RECENT GEOLOGICAL DEPOSITS. 



MR. DESOR called the attention of the Society to the deposits of ma- 

 rine shells in Maine, on Lake Champlain, and the St. Lawrence, and 

 to the question of their probable origin. These have been referred by 

 geologists to the tertiary deposits. Recently a deposit of fresh- water 

 shells had been found on the borders of Lake Erie, and near Rouse's 

 Point, about fourteen miles from Lake Champlain, he had not long 

 since discovered marine shells of the same species as those found on 

 Lake Champlain and at Montreal, at a height of 310 feet above the 

 sea and 210 above the lake. They were well preserved, most of them 

 having the valves unseparated. It had been contended by some geol- 

 ogists that the shells found at Montreal could not be in situ from the 

 great elevation of the locality ; but here they were evidently so. Sim- 



