GLACIAL DRIFT. 333 



present level, following the departure of our part of the ice-sheet. In 

 the first chapter of this volume it was shown that very large amounts of 

 modified drift were excavated by the rivers in deepening their channels. 

 Taking Connecticut and Merrimack rivers as examples, it seemed inex- 

 phcable, if the ocean from being 150 feet higher than now simply fell to 

 its present level, to account for the disappearance of all the modified 

 drift eroded between the upper terraces of these valleys. This explana- 

 tion of the Hudson submarine channel is made more worthy of our 

 belief because it also solves this difficulty. If the ocean was thus de- 

 pressed, the modified drift excavated would be carried by the descending 

 streams beyond our coast-line. w. u. 



Origin of the Blue and Gray Clays. 



In the first chapter (pp. 94, 153-155, and 1 58-161), Mr. Upham has 

 brought together various facts respecting the distribution of the blue 

 and gray clays overlying the till. He finds that the former invariably 

 underlies the latter. This is true throughout New England, but not in 

 the Western States, where alternations of the two kinds have been ob- 

 served. The reason of this invariable order in the east has never been 

 given satisfactorily. In order to contribute to the solution of this prob- 

 lem, I requested Mr. Upham to select for analysis typical specimens of 

 the two tills and clays. Mr. G. W. Hawes presents the following as the 

 result of their chemical examination. Only the iron percentages were 

 determined : 



The tills came from Alton, the clays from Dover. The facts indicated 

 are, — the proportionate increase of the sesquioxide of iron in the upper 

 over the lower till, and in the gray over the blue clay ; also, the clays 

 contain more iron than the till. These facts are sufficient to explain the 

 difference in color. They agree with the theory already propounded for 

 the origin of the two tills, — that the first resulted as a ground-moraine 

 from the pulverization of the rocks, and the second from the dropping 

 VOL. III. 43 



