1 24 TJic American. Geologist. February, i899 
it does. The absence of certain knowledge on this point is 
the more unfortunate because, were we sure that the intrusi-y^e 
(Hd not penetrate the gneiss, it would go far toward establish- 
ing the age of this group of intrusions as post-Calciferous and 
pre-IIudson River. As it is, the relations of the pegmatyte 
sheets to the major and minor folds of the locality point to- 
ward the conclusion that the intrusion took place toward the 
close of the period of disturbance of the crystalline series. 
As was stated in an earlier part of this paper, the material? 
passed over in following the general line of Hawthorne street 
southward to the Harlem river are, in order, after the Haw- 
thorne St. limestone exposure, — gneiss, drift, a second outcrop 
of gneiss, and then a second outcrop of limestone. These last 
outcrops are in such a position that a determination of their 
dips is a matter of difficulty. I believe, however, that the 
gneiss nearest the river dips steeply to the east, and that the 
limestone adjacent to it is conformable. The gneiss is a black, 
highly garnetifefous, biotite gneissoid schist; its stratigra- 
])hical position leaves no doubt as to its being an outlying un- 
eroded portion of the Manhattan schist. 
These facts, taken together with the dips observed at the 
northerly limestone outcrop, would indicate that the two for- 
mations-, limestone and gneiss, are here folded into a syncline. 
trending northeast and southwest, and crossfolded so as to 
give a pitch toward the southwest. This synclinal is over- 
turned toward the west, as is shown by dips on both sides of 
its axis being in the same direction. My reasons for consid- 
ering that this locality is on the southern border of the trough, 
and near the end of the syncline, are stated in an earlier part 
of this paper. 
Throughout this region, in addition to difficulties arising 
from the extreme metamorphism and the complicated series 
of folds and cross-folds, relations are obscured by the extent 
to which the softer formations have been eroded, by both 
water and ice; and by the great depth of the terraced drift. 
I^'urthermore, buildings or other artificial structures may fre- 
(|uently be found covering critical sections. The grow'th of 
the city, especially in this direction, is so rapid that informa- 
tion regarding the geological structure of Manhattan island, 
imlcss collected soon, will be forever unobtainable. 
