of the Middle Atlantic Coast Region. 497 



mud at its bottom. Supposing all the mud and artificial 

 obstructions to be removed, the river could stand a lowering 

 of its level of but eleven feet. Judging from surface expo- 

 sures, the rocks at the bottom of the river are frangible or 

 disintegrated gneiss, which is certainly less obdurate than the 

 granite of the Susquehanna gorge, hence we have to conclude 

 that the dislocation here is scarcely one-fourth of that of the 

 Chesapeake Yalley. At the site of the proposed Memorial 

 Bridge, 1000 feet east from Easby's Wharf, rocky bottom is 

 found at a depth of forty-four feet below the surface of the 

 river ; the stratum of mud here is about fifteen feet thick. At 

 the Long Bridge, rock bottom has not been reached by boring 

 or pile driving, and hard bottom in the Washington channel is 

 seventy-one feet below the surface under a layer of sandy mud 

 of sixty-nine feet thickness. The Georgetown channel has no 

 mud at its bottom but runs over a hard bed of gravel and clay. 

 A subsidence along the valley of the Potomac below Wash- 

 ington, inferior to that of Chesapeake Bay, is attested by 

 the bay -like expansion of all the affluents at their mouths. 



Subsidence in Delaware Bajy. — I have not had the necessary 

 time nor data at hand to make a similar inquiry about probable 

 subsidence in Delaware Bay. In fact, we know Delaware 

 River and Bay to have much stronger currents and to carry a 

 greater amount of coarser sediment than Chesapeake Bay, and 

 are prepared to find the traces of a former higher level less dis- 

 tinctly preserved. Nevertheless, we can trace a deep channel 

 from the ocean into the middle of the bay where it is appar- 

 ently choked off by alluvial deposits which fill up the entire 

 upper part of the bay, leaving just enough room for the river 

 channel. This " blind channel " has a depth of from twenty- 

 two to thirteen fathoms, and is separated from the main river 

 channel by shallow banks. The ebb-channel in actual use by 

 the river has but a depth of three and one-quarter fathoms in 

 its shoal est reaches. A comparison of our recent surveys with 

 those made about fifty years ago proves that the high-water line, 

 on the New Jersey side at least, has receded about one-eighth of 

 a mile in the lower bay ; but it would be rash to make subsi- 

 dence responsible for this result. A comparison of the hydro- 

 graphic surveys made about the same respective dates shows 

 that there has been a great deal of shoaling going on in the 

 interval, and it is not impossible that this shoaling has produced 

 a disarrangement of the tidal elements, a retardment accom- 

 panied by an increase in amplitude which would show its effects 

 on the high- water line. 



Time of subsidence. — The evidence of a subsidence of the 

 coast of New Jersey during the past century and yet in 

 progress, collected by the late Professor Cook, must be consid- 



