J 2 NEW YORK STATE MUSEUM 



There are a number of anorthosite exposures in the vicinity of 

 Keeseville, from which building material has been taken for local 

 structures and also for shipment in years past. The stone passed in 

 the trade as Ausable granite. The Prospect Hill quarries, just south 

 of Keeseville, are described in Smock's reports as having been 

 operated between the years 1888 and 1890. 



Southeastern New York. Massive igneous rocks play a sub- 

 ordinate part in the structure of the Highlands region. Local 

 intrusions of granite, diorite and syenite in the form of dikes, sills 

 and bosses occur, however, in various sections, and afford a fairly 

 varied assortment of quarry materials. Among the more extensive 

 bodies which have gained some prominence as sources of construc- 

 tional stone may be mentioned the Peekskill or Lake Mohegan 

 granite, the granites near New Rochelle and Garrisons, and the Pine 

 Island bosses in Orange county. An area of somewhat gneissoid 

 diorite, called the Harrison diorite, is found in eastern Westchester 

 county, as an offshoot from the large intrusions of the same rock 

 in Connecticut. 



The very basic intrusives are represented by the Cortlandt series 

 of gabbros, having a large boss just south of Peekskill, but showing 

 such variability of composition and appearance as to be of little 

 value for quarry purposes. In this class also belong the serpentines 

 of Westchester and Richmond counties. The great sill of diabase 

 which forms the lines of vertical cliffs known as the Palisades ex- 

 tending along the west side of the Hudson river south from the 

 Highlands has been a prolific source of material for crushed stone 

 of the best quality. 



The gneisses which are the most important element in the geology 

 of this section have a composite character, including both igneous 

 and sedimentary derivatives. Some types in the northern and cen- 

 tral Highlands appear to be only slightly modified granites, as 

 exemplified by the exposures on Storm King, Crow's Nest and 

 Breakneck mountains at the portal of the Hudson gorge. They 

 have been employed for dimension stone, but mainly for rough 

 work, concrete and road material. Much of the gneiss in the 

 central Highlands is of so variable a nature through injection of 

 igneous material and inclusions of different character as to admit 

 of no extensive application. 



In Westchester county the Yonkers gneiss is of considerable im- 

 portance for local construction purposes. It is a fairly uniform, 

 though distinctly foliated, biotite gneiss of blue or reddish color. 



