236 



mountains it reappears, and as far as their eastern border is 

 made up of materials from this source. 



Mr. Desor spoke of a deposit considered by geologists as a 

 Lake drift, particularly noticeable at Lake Erie. He had con- 

 sidered it important if possible to establish its lacustrine origin 

 from fossils contained in it. On Lake Michigan it appears as a 

 bluff twelve feet thick, made up of nine feet of sand over blue 

 clay. He succeeded in finding in this deposit eight species of 

 fossil shells, several of them identical with those now living in 

 the adjoining Lake, such as Cyclas, Paludina, Melajiia, and 

 Unio. In the underlying stratum were reeds, one of which was 

 from an inch to an inch and a half in diameter, extending up into 

 the sand, equiseta, and a piece of cedar, the locality having 

 probably been occupied by a cedar swamp. There are no 

 trees of this species at the present time within many miles of 

 the spot. 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 this flat prairie of 

 fresh water origin. The rolling prairies of the West on the con- 

 trary are of marine origin 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 bergs or ground ice. The main drift deposit over the 

 northern parts of the United States, Mr. Desor attributed to 

 the ocean. 



Mr. Edward Cabot mentioned, in connection with Mr. Desor's 

 observations, having found on the bank of the Illinois River a 

 red cedar trunk rounded and worn, twenty-five feet long ; there 

 being no living specimens of this tree in that part of the country. 



Prof. Agassiz said that he had shown, at a former meet- 

 ing, that in studying the relations of different stages in the 

 embryonic development to permanent forms of insects, a 

 better idea of their natural classification could be obtained 

 than in any other way, and he now proposed to continue 

 his observations, and to show that this view might be still 

 further carried out, even to the fixing of the relative posi- 

 tions of the different families. 



