518 
GEOLOGY OF THE FIRST DISTRICT. 
On Long island, rocks of this class were seen along the shore near Hurlgate, at the Hog’s- 
back, and along the shore for half a mile, and in the village of Halletscove. They are mostly 
hornblende slate and hornblendic gneiss, in vertical strata, intersected in some places by veins 
of granite. Some of the strata are nearly pure hornblende. In the gneissoid hornblende 
and hornblende slate, the hornblende forms layers, and gives the rock a highly striped 
appearance. The layers and strata of hornblende are from one-twentieth of an inch to one 
foot in thickness. This rock, even-in hand specimens, illustrates the striped aspect of gneiss 
in a striking and beautiful manner. 
The island of New-York is underlaid by rocks of gneiss, micaceous gneiss, hornblendic 
gneiss, hornblende slate, hornblende rock, limestone, the anthophyllite rock, and granite. The 
rock in place crops out in almost every square acre on the hills in the north part of the island, 
and in many places along the shore -of the Hudson and of the East river. The micaceous 
gneiss is the prevailing rock. The strata are nearly vertical, some dipping eastward, and 
some westward. The strike is also variable from northeast to northwest, but north-northeast 
is the prevailing direction.- Granite in numerous beds and veinsj some of which are very 
tortuous, were observed. A good example of this may be seen on the sides of the rock 
excavation for the approaches to the tunnel, on the railroad from New-York to Harlem. Very 
many interesting localities were observed where excavations had been made in quarrying, 
grading the streets, and where the rock was exposed on the surface ; but- the localities were 
not noted, as the detailed geology of the island was committed to Prof. L, D. Gale.* At 
Hurlgate ferry, at the east end of Eightysixth-street, the strata are vertical, and are of gneiss 
more or less micaceous and hornblendic, interlaminated with beds and irregular masses of 
granite. The strata are frequently bent, where these irregular masses of granite were 
observed. Pyrites are common in this gneiss, and in some places sulphate of iron is formed. 
The following sketch by Prof. Gale, will give an idea of the general character of the rocks, 
their distribution, strike, dip, etc. on the island of New-York : 
The island of New-York “is about twelve miles long, and from a half of a mile to two 
miles and a quarter broad. 
“ The basis rock of the island is gneiss, if we except about one mile in length of the northern 
extremity, which is limestone. The middle and northern portions are rough and broken from 
the almost constant exposure of the rock above the surface ; while the southern, although 
consisting of the same material, is every where covered, and in some places to great depths, 
with alluvial and diluvial deposits.! The gneiss varies considerably in character in different 
portions of the island. For example, the northern part abounds largely in limestone, and the 
* It is to be regretted that Prof. Gale did not give more details of so interesting a region as the island of New-York. 
where excavations of the rocks exposed so many important facts, and where so many persons would be led to examine 
them. He has given us many details on the alluvion and drift, and it is to be hoped that he will at some future time give 
to the world the detailed observations he has made on the primary rocks. 
t These have been described on pages 19, 134 to 139, and 179 to 183, under the heads of Alluvion, Quaternary znd Drift 
deposits. 
