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NEW YORK STATE MUSEUM 



OUTLINE OF THE GEOLOGY OF TTESTPORT 

 The southern part of Westport is mainly gneiss, but the northern is 

 is all anorthosite and gabbro. The anorthosites have an extended 

 development in Split Rock Mountain, and also appear in the south- 

 east. The gabbro is especially important in the central portion. The 

 sedimentary rocks mark the southeastern lake shore. The Potsdam, 

 Calciferous, Chazy and Trenton are all well shown. 



PETROGRAPHY 



Petrography of the gneisses. There are several varieties of 

 gneiss as shown by the thin sections. These occur, forming the 

 larger portion of what is regarded as the basal series. Gneisses are 

 also found as immediate associates with the undoubted anorthosites, 

 and such are doubtful forms for stratigraphic classification. The 

 endeavor has been made to differentiate the certain and well marked 

 varieties in each series from the intermediate or doubtful ones. 



The most characteristic gneiss in the undoubted exposures of the 

 basal series, is formed by an aggregate of quartz and microperthitic 

 orthoclase in largest amount, with which are orthoclase, plagioclase 

 and brown biotite in less degree. The rock shows no features at all 

 remarkable among gneisses. The accompanying figure (Fig. 1) 



Fig. 1.— Common biotite-gneiss, Bulwagga Mountain, Moriah. O is orthoclase; M, micro 

 perthite; Q, quartz; B, biotite; P, plagioclase. 



