(79) 
been exposed for a thickness of at least 20 feet, by the exca- 
vations, for the new Williamsbridge sewer. The river passes 
along the foot of the ridge on the east side and divides the 
gravel terrace in two, so as to leave a small remnant on the east- 
ern gneisses. It then runs against a westerly spur of the ridge 
and cuts through it, in a pronounced gorge, diagonally across 
the foliation of the rock, which is a hard micaceous gneiss. 
From the entrance to the gorge a swampy depression ex- 
tends westward to the railroad and has all the characteristics 
of an abandoned channel. The railroad has crossed it by an 
embankment and culvert. Just east of the culvert there is 
gneiss but a few feet below the soil, and at this point the old 
stream evidently surmounted a reef. The depression con- 
tinues southward just west of the track, as far as Morrisania ; 
it is then crossed by the track, which, traversing a low divide 
into the next limestone valley to the west, follows this to Mott 
Haven, and then passes over the river to Manhattan Island. 
The depression runs south from Morrisania and enters the 
East River opposite Randail’s Island. In all its extent there 
is no natural barrier, although many streets have been filled 
in across it. At its highest point, near Bedford Park Station, 
it is not more than ten or twelve feet above the present sur- 
face of the Bronx River at the gorge, probably less. The 
river is about five feet deep at this point. All these relations 
are shown on the accompanying map, which is reproduced 
from the Harlem sheet of the United States Geological Sur- 
vey. The excess of streets and railways on the original map 
mask the contours, which can only be traced on it with diffi- 
culty. Accordingly everything but the contours, the river 
and one or two railway lines have been omitted in redrawing, 
and for the same reason no attempt has been made to put in 
the geology. The authorities of the Botanical Garden have 
a map of its area on a very large scale and with five foot 
contours. The writer has used this also in the preparation 
of the paper, although all the material points are well illus- 
trated on the smaller scale. 
By observing the map it will be seen that both the river 
. 
