DECOMPOSITION OF ROCK BY CORROSIVE ACTION 55 



proved by driven piles, reached, here, twenty-two feet. The 

 bed of this formerly great swamp is now crossed by Daly 

 Avenue, Honeywell Avenue, Southern Boulevard, Mapes Avenue, 

 and Prospect Avenue. At Daly Avenue near Tremont Avenue, 

 the depth of the peat was such that it was found necessary 

 for foundations to drive piles forty-five feet in length. Not 

 only were the lower grounds so filled up, especially the long 

 valley depressions, such as those of the Bronx River, Eastchester 

 Creek, Tibbit's Creek, etc., but thin local sheets seem to have 

 rested in the hollows among the rounded hummocks of the glaciated 

 upland; these are in part still represented by little marshes, or the 

 ponds in the various parks. It appears but a moderate estimate to 

 assert that at least one-third of the surface of this region was once 

 covered by an almost continuous sheet of fresh-water bog, out of 

 which the higher elevations protruded as knobs of forest-covered 

 rock. Along the adjoining coast at Hunt's Point, Bartow, etc., 

 these ancient bogs have been since overlaid, during the subsidence 

 now in progress, by a sheet of salt meadow, surrounding a large num- 

 ber of small scattered islets of now bare outcrops of gneiss and 

 granite. 



Further evidence of the early and long activity of organic acids 

 in solution, removal, concentration, and deposit of iron oxide from 

 the surface of these rocks is afforded by numerous accumulations 

 of bog iron ore once found throughout this region as well as over 

 Manhattan Island. Though generally small, some of these were 

 of sufficient volume to be of economic importance and use two 

 hundred years ago. 



Escape of decayed schist from removal by the glacier. — There was 

 here no knob or eminence on the northwest for the protection of 

 the softened schists from the scour of the ice moving from that 

 direction. On the contrary, a low valley lies on that side, which 

 we presume was occupied by the peat-bog. The pegmatite itself, 

 though softened, probably served long as the main protection of 

 the schist, in connection with the pegmatitic branches and seams 

 intercalated in the schist in this part of the tract. The next result- 

 ing condition was apparently the erosion of this surface of the 

 schist in an inclined plane, tending to lift the edge of the ice-sheet 



