180 
GEOLOGY OF THE FIRST DISTRICT. 
3. Serpentine, like that at Hoboken, is common in the southern, but rare in the northern 
parts of the island. In speaking of serpentine, we shall make a distinction between that 
variety found in place at Hoboken, and that found at the anthophyllite locality on the island, 
which approximates to serpentine in character, but is harder, and is mixed with limestone. 
These boulders are much more common in Brooklyn than in this city, except perhaps the 
region of Corlear’s hook. This might be expected, taking for granted that they were trans¬ 
ported from Hoboken, which seems to be a necessary conclusion, as no other locality of this 
variety is known in the vicinity. Besides, the direction of the diluvial grooves being from 
northwest to southeast is such as would carry the materials from Hoboken to Corlear’s hook, 
and the east part of Brooklyn. 
4. White or Primary limestone. Boulders of this material were found some years ago in 
cutting away the hills at Corlear’s hook, and in a number of places on the east side of the 
island, between the city and Kingsbridge ; but I have neither seen nor heard of any being 
found on the western side. The principal places where I have found this rock, are on the 
Fourth avenue near 120th-street; at the Seventh avenue, north of the village of Harlem, near 
142d-street; at McComb’s dam, at the northern termination of the Seventh avenue; and in 
a number of places in the valley of the Harlem river, between McComb’s dam and Kings¬ 
bridge. Now the nearest locality where this rock is found in place, is at Kingsbridge, and I 
have no doubt that the boulders in question came from this place ; but had they been trans¬ 
ported in the general direction of the current shown to be northwest and southeast, instead of 
reaching their present location, they would have been carried across the Harlem river, and 
have been deposited in the range of grounds lying about midway between Harlem and West- 
farms. Specimens are in the State Museum. 
5. Granite and gneiss, especially the latter, being abundant in every part of the island 
except the northern extremity, boulders of these might be expected to be more numerous than 
of any other materials ; yet I think greenstone boulders are quite as numerous, take the whole 
island together, as granite and gneiss. The largest boulders I have seen of the latter kind 
measured in diameter twelve and eighteen feet, and are equally common in all parts south of 
the Harlem and Manhattanville valley. The granite, which exists in veins in the gneiss, is 
mostly on the west side of the island; and this when torn from its place and transported by 
a northwest current, would naturally have been distributed over the eastern parts, and even 
as far as Long island. A number of very large ones were excavated between 1825 and 1830, 
in the vicinity of Corlear’s hook, some of which were peculiar in character. One I well 
remember lay many years on the south side of East-Broadway, near its junction with Grand- 
street. It was a grey granite of coarse texture, with tabular crystals of black hornblende three 
or four inches in diameter interspersed throughout. It was eighteen feet long, sixteen broad, 
and eight and a half high. This is the largest boulder I have seen on the island, but those from 
ten to twelve feet diameter are very common. 
A large boulder of granite, eleven feet in diameter, and though somewhat rotund, has a 
rough and angular surface, as though it had suffered little from mechanical violence, rests on 
the gneiss rock on the east side of Bloomingdale road, from a quarter to a half a mile south 
