448 LOUIS AGASSIZ. 



understand the mode of formation of the sub- 

 marine dikes bordering the coast at various 

 distances, which would be oesars were they 

 elevated ; with the aid of the dredge I sat- 

 isfied myself of their identity. With these 

 facts before me I cannot doubt that the oesars 

 of the United States consist essentially of 

 glacial material remodeled by the sea ; while 

 farther inland, though here and there reach- 

 ing the sea-coast, we have unchanged glacial 

 drift deposit. At some points the alteration 

 is so slight as to denote only a momentary rise 

 of the sea. Under these circumstances one 

 would naturally look for fossils in the drift, 

 and Mr. Desor, in company with Mr. de Pour- 

 tales, was the first to find them, at Brooklyn, 

 in Long Island, which lies to the south of 

 New York. They were imbedded in a glacial 

 clay deposit, having all the ordinary charac- 

 ter of such deposits, with only slight traces 

 of stratified sand. It is true that the greater 

 number of these fossils (all belonging to spe- 

 cies now living on the coast) were broken into 

 angular fragments, not excepting even the 

 thick tests of the Venus mercenaria. 



But a few days ago, as I have said, a wider 

 range was given to the field of our researches 

 by an observation of mine. The suburb of 



