I50 NEW YORK STATE MUSEUM 



ite and syenite. The various rock types arc the differentiated 

 products of a basic magma which was intruded in late Paleozoic 

 time. It is thought that the Mohegan granite may represent the 

 acid extreme of the series, although occupying a rather isolated 

 position to the northeast of the basic intrusives. At any rate the 

 granite has the same relations to the surrounding formations -which 

 consist principally of Manhattan schist. 



There are no active quarries within the area and the only min- 

 eral product now worked is emery, which is found in small lenses 

 and pockets near the borders. The rocks are too heavy and dark in 

 color for building stone. It would appear, however, from observa- 

 tions by the writer that there are numerous opportunities for the 

 quarrying of road material of good quality. The fine-grained 

 gabbros and norites particularly seem well adapted for the purpose, 

 being closely knit, tough materials, very similar to diabase in their 

 composition. The best ledges, however, are found in the interior 

 at some distance from the railroad and the Hudson river. The 

 rocks in places are quite heavily charged with pyritic minerals 

 as indicated by their rapid weathering with the formation of a 

 reddish clayey soil. The pyritic zones are probably localized and 

 do not seriously affect the quality of the material as a whole. 



Analyses of representative types of the Cortlandt gabbros and 

 norites are given herewith. No. 1 is gabbro, southeast of Salt Hill, 

 H. T. Vulte, analyst. No. 2 is norite, i J / 2 miles south of Peekskill. 

 S. S. Rogers, analyst. 



1 2 



Si0 2 54.72 51.49 



AloOs 17-79 20.72 



Fe 2 3 2.08 1.80 



FeO 6.03 7.28 



MgO 5.85 3.82 



CaO 6.84 6.71 



Na 2 3-02 3.70 



K 2 3-0i 2.14 



H 2 0+ .31 



H 2 0- .10 



CO2 trace 



Ti0 2 2.26 



P 2 0; .15 



S .11 



MnO .13 



99.34 100.72 



