QUARRY MATERIALS OF XEW YORK I49 



(Adirondack branch). The dike is notable for its continuity along 

 the strike, although its thickness is nowhere very great, being about 

 60 feet from wall to wall in the quarry opening. It can be traced 

 northward beyond the railroad by occasional exposures for over 

 one-half of a mile and finally branches into two or three smaller 

 dikes. The section south of the railroad is fully as long. The dike 

 stands nearly vertical and cuts through a garnetiferous schist. 

 The openings are just south of the railroad and east of the north- 

 south highway. An examination of the diabase under the micro- 

 scope shows that the mineral constituents are pyroxene, feldspar and 

 magnetite in the order of their importance. The minerals are 

 somewhat decomposed by weathering, though in hand specimen the 

 rock appears hard and has a metallic ring. 



FORT ANN, WASHINGTON COUNTY 



Several dikes of trap are found on the ridge east of the canal, 

 near Fort Ann. They are of small size, though their occurrence 

 so near shipping facilities has given them economic interest and 

 led to active quarrying in one case. The Champlain Stone and 

 Sand Co. operated a crushing plant for a short time about 1907. 

 The dikes are the usual diabase, with pyroxene, feldspar and 

 magnetite as the principal constituents. Specimens examined by 

 the writer showed slight decomposition but not sufficient probably 

 to affect materially the wearing quality of the ^tone for road uses. 



THE CORTLANDT BASIC ROCKS 



A great boss of igneous rocks, mainly of the dark basic kinds, 

 is found in northern Westchester county, just south of Peekskill. 

 It covers a large part of the town of Cortlandt, having an area of 

 about 25 square miles, rounded in outline and extending along the 

 Hudson river for some distance on its western border. The in- 

 trusion has been described at length by J. D. Dana and G. H. Wil- 

 liams. More recently G. Sherburne Rogers^ has published a very 

 detailed account of the geology and petrography of the rock series, 

 with many chemical analyses and a map showing the distribution 

 of the different types. 



According to Rogers's investigations, the intrusives consist of a 

 complex of rocks of which the largest element is the norites, but 

 including also gabbro, pyroxenite, peridotite, hornblendite, dior- 



1 Geology of the Cortlandt Series and Its Emery Deposits. N. Y. Acad. 

 Sci. Annals, v. xxi, 191 1. 



