498 A. Lindenkohl — Post-glacial Subsidence, etc. 



ered as the main support of the theory which accounts for the 

 existence of the submarine channel of the Hudson by sub- 

 mergence. In order to approximate the time of commence- 

 ment of subsidence we have to take the evidence afforded by 

 the latest Quaternary deposits. According to Mr. McGree* the 

 clay terraces on which the city of Washington is built and 

 which are supposed to be cotemporary with the first glacial 

 invasion, indicate a submergence of about 150 feet during the 

 period of their deposition. Hence it appears that the Lower 

 Potomac and Chesapeake Bay with their depressed channels are 

 of more recent origin. The borings at Fishing Battery cited 

 above, which went to a depth of 140 feet and brought nothing 

 to light older than alluvium, teach us that the deep channel of 

 the Chesapeake must be of more recent date than any of the 

 Tertiary and Quaternary deposits about the head of the bay. 



The submarine border of the coast. — Returning to the 

 subject of submarine channels, it has to be stated that diligent 

 search has thus far failed to discover indications of such for 

 either Chesapeake Bay or Delaware Bay, with the exception of 

 a deep cul-de-sac of 396 fathoms inside of the bathymetric 

 line of 100 fathoms, occupying nearly the same relative posi- 

 tion to Delaware Bay as the canon described above does to the 

 Hudson. 



In studying the geological changes in the sea bottom off the 

 Middle Atlantic States, a remarkable fact should not be lost 

 sight of. The sea bottom intervening between the submarine 

 Hudson river channel and the coast of Long Island is charac- 

 terized by its great regularity and smoothness, which can best 

 be explained by assuming a gradual subsidence or an adjust- 

 ment by superficial deposits. The bottom between the channel 

 and the New Jersey coast, on the contrary, is distinguished 

 for its ruggedness ; great irregularities in the soundings give 

 indications of shallow ridges and of cross channels, which go 

 to prove that there was a periodical retrogression of the coast 

 line, and that the sea keeps the conquered territory in very 

 much the same condition in which it was found. 



Greensand at the sea bottom. — The specimens of bottom 

 collected during the recent survey of the approaches to New 

 York, of which there are several hundreds at hand, show con- 

 siderable quantities of black grains, described black specks on 

 the charts, mixed up with the sand and mud of the entire region 

 from Cape May to beyond Montauk Point ; it is only in the 

 mud of the gorge and of the deeper part of the continental slope 

 that they are either scarce or missing. They are of spherical 

 shape, of jet black luster, of brown color when fractured, and 

 vary in size, with the fineness of the sand or mud, from the 



* This Journal, vol. xxxv, May, 1888. 



