PEGMATITE, SILEXITE, AND APLITE OF NORTHERN 



NEW YORK^ 



WILLIAM J. MILLER 



Smith College, Northampton, Massachusetts 



INTRODUCTION 



During twelve seasons of field work in the Adirondack Moun- 

 tain region of New York, the writer has made many hundreds of 

 observations on bodies of pegmatite, quartz (silexite), and aplite. 

 Certain of the phenomena long proved to be very puzzling, and 

 more recently it has become evident that some generally accepted 

 interpretations of such phenomena are not satisfactory when 

 applied to many of the occurrences in the Adirondack region. 

 While engaged in the detailed geologic mapping of the Lyon Moun- 

 tain quadrangle, the writer was soon impressed by the exceptional 

 opportunity for a careful study of the pegmatite, quartz (silexite), 

 and aplite bodies which are there so wonderfully exhibited in 

 countless numbers. Many detailed notes were taken and sketches 

 were made. The North Creek quadrangle, surveyed by the writer 

 some years ago,^ was also revisited for the purpose of making 

 further observations on certain pegmatites there so well exhibited. 

 In this paper the discussions deal almost entirely with the Lyon 

 Mountain and North Creek quadrangles. 



PEGMATITE, SILEXITE, AND APLITE IN THE GRANITES OF THE LYON 

 MOUNTAIN QUADRANGLE 



The granites. — Several varieties of granite and granitic syenite 

 constitute the main bulk of the rocks of this district. Among 

 these the variety of chief interest in the present discussion is a 

 fine to medium-grained, usually pink rock varying from a true 

 granite through granitic syenite to quartz syenite or even quartz 

 diorite. In the more typical specimens of granite dark minerals, 



^ Published by permission of the state geologist of New York. 

 = W. J. Miller, N.Y. State Mus., Bull. 170. 



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