1893.] NEW YORK ACiUDEMY OF SCIENCES. 203 



PETllOGEAPHY OF THE GNEISSES OF THE TOAVN OF 



GOUVEKNEUR, N. Y. 



BY C. H. S^n'TH, JR. 

 INTRODUCTION . 



In a jDrevious conimniiication* tbe writer has given a brief 

 descrijition of tbe main geological features of a portion of tbe 

 towns of Gouverneur and Fowler, St. Lawrence County, New 

 York. It is tbe aim of tbe present paper to setfortb briefly tbe 

 petrograpbic cbaracters of tbe formations tbere discussed. 

 Rocks, wbicb, like tbe limestone and sandstone, tbougb of great 

 stratigrapbic importance, are comparatively simple and uniform 

 in comjDosition, receive little attention ; wbile considerable 

 space is sometimes given to rocks of limited extent, but of more 

 interest from a petrograpbic standpoint. It is boped tbat tbe 

 facts bere recorded may be of value as a basis for comparison 

 in future work on tbe geology of tbe Adirondack region. Tbe 

 writer is indebted to tbe Geological Department of Columbia 

 College for tbe grinding of all rock-sections used in tbe 

 prei^aration of tbis paper, and takes j)leasure in exj^ressing bis 

 appreciation of tbis material assistance. 



GNEISS. 



Wbile as a wbole tbe gneiss presents a great sameness over 

 wide areas, still local variations are so numerous and diverse 

 tbat tbe limits of tbis paper would bardly suffice for tbeir 

 description. Therefore, only tbe general character of tbe 

 gneiss, together with a few of its more important modifications, 

 will be described. 



Tbe color of tbe unweatbered gneiss is generally gray, less 

 often red, and varies from tbis to black, tbe latter color being 

 confined to distinct layers and lenticular patches. The banding 

 and foliation vary greatly, being sometimes marked and again 

 wholly absent. Thus considerable areas in tbe gneiss have all 

 tbe cbaracters of granite, and tbis is commonly the case with 

 band specimens. 



*"A (iPolopical EreoDDoippance in the Vicinity of Gouverneur." N. Y. 

 Trans. N. Y. Acad. Sci., XII., p. 97. 



