PERTHITIC FELDSPARS. 



139 



The figures given for the various feldspars, of course, refer strictly 

 speaking only to the particular fragment from which the thin-sections 

 were cut, as do also the figures given for the chemical composition. 

 No doubt, there exists some variation in the relative amounts of the 

 two feldspars present in difi^erent feldspar fragments taken from the 

 same pegmatite and even between different parts of the same large 

 crystal. However, from the WTiter's observation on other smaller 

 sections of these feldspars, as well as upon perthites in general, he 

 believes that the samples here studied are quite representative of the 



TABLE II. 



Chemical analyses of alkali-feldspar intergrowths, Nos. 1-6 by Warren; 

 Nos. 7 and 8, Eero Makinen; No. 9, analysis of albite separated from pertbite 

 by Warren. 



run of the perthites or microperthites from granite pegmatites in 

 general, and that, therefore, the results here given are equally repre- 

 sentative. 



Chemical. — The entire fragment remaining after the thin-sections 

 had been cut was first very carefully crushed in a large flat-bottomed, 

 chilled steel mortar, loss by the flying away of particles being guarded 

 against. This material was then slowly ground in a screened, mechani- 

 cally operated agate mortar. From this material the final sample 



9 Estimated to contain about 3 to 4 per cent attached raicrocline. 



