QUARTZ-FELDSPAR-PORPHYRY FROM LLANO, TEX. 229 



these analyses it will be seen how close the microscopically estimated 

 chemical composition is to that determined by chemical analysis. 



The higher silica in III shows that the quartz in the rock was 

 underestimated by 1.5 per cent., or that the piece analyzed by Dr. 

 Washington was slightly richer in quartz phenocrysts. 



The following data were determined in the laboratory of the 

 Mineral Survey of the University of Texas: Specific gravity, 2.64; 

 corrected, 2.67. One cubic foot of the rock absorbs 9.47 ounces of 

 water. Crushing strength, 15,300 pounds per square inch of sur- 

 face. 



The alkalies in I and III are remarkably concordant, proving that 

 the determination of the feldspars by optical means was correct; 

 The lime determined in III corresponds to that estimated optically 

 in fluorite and apatite. Fluorine appears only in Analysis I, and is 

 very nearly correct, probably as much so as if determined by chemical 

 means. 



The correspondence between the two oxides of iron in both chemi- 

 cal analyses, II and III, the discrepancy in Anlaysis I, and the pres- 

 ence of a small amount of magnesia in III show that the mica analysis 

 chosen from the Cape Ann rock is not the proper composition for the 

 mica in the porphyry under investigation. The probable composi- 

 tion of this mica may be found by subtracting from Analysis III the 

 chemical constituents of the known minerals — quartz, feldspar, 

 fluorite, and apatite — and reckoning the remainder as mica and the 

 extra quartz already mentioned. The result is as follows: extra 

 quartz, 1.37 per cent.; mica, 8.6 per cent., having the composition {a). 



This is approximately the composition of a lepidomelane like 

 that in the nephelite-syenite {grano-nordmarkose) of Litchfield, Me.,' 



' Loc. cit., mica analysis /. 



