A QUANTITATIVE STUDY OF CERTAIN PERTHITIC 

 FELDSPARS. 



By Charles H. Warren. 

 Received, April 26, 1915. 



CONTENTS. 



Page 



Introduction 127 



Summary Description of the Structural Characteristics of the Feld- 

 spars Studied 129 



Quantitative Methods of Study Employed 



Microscopic 134 



Chemical 139 



Discussion of Results 141 



Discussion of the Origin of i'erithitic Intergrowths 143 



Summary 153 



Introduction. — It is the chief purpose of this paper to present the 

 results of a quantitative microscopic and chemical study of certain 

 perthitic feldspars from granite pegmatites. 



The feldspars here considered are all from granite pegmatites from 

 widely separated localities. They show a considerable range in the 

 relative proportions of the two feldspar phases present, and were 

 found upon examination to be satisfactorily free from foreign inclu- 

 sions and from alteration. The number of specimens originally chosen 

 for study was ten. Of these four were unfortunately sent to Europe 

 during the early summer of 1914 for sectioning and have been held 

 there because of the war, so that the results of the investigation 

 can not, at this time, be made quite as comprehensive as was origi- 

 nally planned. However, as it is believed to be unlikely that the 

 results obtained from the study of these four feldspars would have 

 materially modified those obtained from the six remaining prepara- 

 tions, plus the two studied by Makinen, and here included, it has 



