History of the Geology of New York Island. 119 



now be mentioned. The first is a cross-fracture at Tnbby Hook, 

 extending from the Hudson River to the lowlands of the Har- 

 lem River ; the second great one is at Manhattanville, extend- 

 ing by the upper part of. the Central Park, through Harlem 

 Creek to the East River ; a third one is along the creek crossed 

 by 2d avenue at E. 50th st. ; a fourth along the creek, near E. 

 20th st. ; and probably there was a fifth rivalling the second, 

 crossing the island, along Canal and Centre streets, by the old 

 Collect Pond and Ferry st. to the East River ; the sixth sepa- 

 rates our island from Governor's Island. 



At the fracture in E. 50th st., and some other localities, a 

 northward incline of the rocks is visible, but the general incline 

 is southward from 3° to 8°. This incline can be readily seen 

 at the lower tunnel, and Yorkville Tunnel of the Harlem Rail- 

 road. It is this southward incline of the rock mass, as well as . 

 its abrasion, that carries the rocks from a height of 180 feet at 

 the highest northern point, to a level below high tide at Canal 

 street. 



Under what previous classification of American Rocks does 

 the New York group arrange itself? 



So far as my reading extends, it was Nuttall (vide his account 

 of Franklin, N. J., in Silliman's Journal, Vol. V., a.d. 1822), 

 who first considered the rocks flanking the Highlands as meta- 

 morphic. To the late Prof. Emmons belongs the honor of 

 developing his Taconic system. These he always considered a 

 part of it. Mather seems at first to have coincided with him, 

 but afterwards changed his mind and called them metamor- 

 phosed lower Silurian. 



If we look at the lower sedimentary rocks flanking the great 

 primary (Laurentian) centres, we shall find that each of them 

 has its own peculiar system of sandstones, slates, and limestones, 

 or one or two of them. 



New York group differs from the true type of the Taconic, 

 in the absence of the lower members. These would, in their 

 regular sequence, not reach our island but underlie Long 



