THE FELDSPAR METHOD 633 



alteration tends to produce feldspars of varied composition from 

 one particular feldspar, nor conversely to change a wide feldspar 

 range into a narrow one. 



The purpose of the work of this thesis was to determine the 

 efficacy and validity of the hypothesis as above stated, namely, 

 that metamorphic rocks having a narrow range of feldspar composi- 

 tion are probably igneous in origin, whereas those having a wide 

 range of feldspar composition are more likely of sedimentary 

 origin. To test the validity of this hypothesis it was first neces- 

 sary to get some idea as to the abundance of feldspars in various 

 sediments and also to determine the range in composition of these 

 feldspars. This involved a study of sediments both in the uncon- 

 solidated and consolidated form. It was then further necessary to 

 study metamorphic rocks of known sedimentary and igneous 

 origin in order to note whether the feldspar composition was such 

 as would have characterized the original sedimentary or igneous 

 equivalent. The methods used in this study and the results 

 obtained are presented in this paper. 



The writer wishes to acknowledge his indebtedness to 

 Dr. Edward Steidtmann, of the University of Wisconsin, for 

 suggesting the fundamental idea upon which the feldspar method 

 is based, and to Professors A. N. Winchell and C. K. Leith for 

 suggestions and criticisms. 



2. Methods used to determine feldspars.- — In the determination 

 of the feldspars two distinct methods were used depending upon 

 the character of the material to be examined. Where thin sections 

 were available and the rock was fairly coarse grained the Fouque 

 method was found very serviceable. 



When thin sections were not available and the material was so 

 fine grained as not to be adapted to the Fouque method, the 

 material was studied in powdered form and the feldspars deter- 

 mined by immersion in a series of hquids of known index. With 

 the Hquids either the Becke or inclined illumination method can 

 be used. The determination of feldspars from rock powders in 

 this manner is especially valuable in cases where the feldspars are 

 partly altered, where the rock is fine grained, when the feldspar 

 content is low, and for all unconsolidated sediments. 



