LETTER TO PROFESSOR PEIRCE. 649 



self rests upon the various rock formations of 

 the district. Now this drift, as I have ascer- 

 tained, formerly extended many miles beyond 

 our present shores, and is still slowly washed 

 away by the action of tides, winds, and cur- 

 rents. Until you know with precision the 

 mineral ogical composition of the drift of the 

 immediate vicinity, so accurately indeed as to 

 be able to recognize it in any new combination 

 into which it may be brought when carried off 

 by the sea, all your examination of soundings 

 may be of little use. Should it, however, be 

 ascertained that the larger amount of loose 

 material spreading over the harbor is derived 

 from some one or other of the drift islands in 

 the bay, the building of sea-walls to stop the 

 denudation may be of greater and more im- 

 mediate use than any other operation. Again, 

 it is geologically certain that all the drift isl- 

 ands of the harbor have been formed by the 

 encroachment of the sea upon a sheet of drift, 

 which once extended in unbroken continuity 

 from Cape Ann to Cape Cod and farther 

 south. This sheet of drift is constantly di- 

 minishing, and in centuries to come, which, 

 notwithstanding the immeasurable duration of 

 geological periods, may be reached, I trust, 

 while the United States still remains a flour- 



