364 J. D. Dana— Geological Relations of the 
As stated, these limestone areas follow the courses of the 
rivers mentioned ; but the historical fact in each case is this, 
that the river follows the course of the imestone, the softness 
of the latter rock causing it to yield easily to eroding or denud- 
ing agents. 
Area 4.—In this area, ledges of limestone show themselves 
just above the point of junction of the Harlem an 
railroads, and were cut through in grading for the tracks. The 
valley of the Bronx has here a flat marshy bottom and this 
continues, with the same bordering schist northward to West 
Mt. Vernon and southward to and below Williams Bridge, 
indicating the probable presence there of the limestone, as in- 
dicated on the ma was informed at Williams Bridge of 
the former existence of an outcrop of limestone visible at low 
water, on me river just below the bridge, but failed to find any 
no The strike of the mica cone adjoining the belt on the 
feet is N. 20°-29° K., and an ke °—15° Ke ; and west of the 
areas are nealy on the same line of strike. There is no 
essential difference in the schist adjoining them. The dip of 
the rocks around the latter afford evidence that it is situated 
along the axis of a local anticlinal. From Stamford westward 
to the Harlem the dip is in general westward; but at Port- 
chester on the way toward Rye the dip changes to sear 
and then becomes westward again west of the area. The sym- 
bols indicate the wrenching the beds underwent in the making 
of the fold, and show that the fold was steepest and narrowest 
to the southwest, and had its axis inclined to the south-south- 
westward. — 
vicinity of = Harlem railroad, the rock chang to a hard 
It appears that upon these high see the earthy material or 
drift left by the glacier has remained almost undisturbed, 
instead of being cut through to the rocks as on the slopes that 
