228 



JOSEPH P. IDDINGS 



From the microscopical measurements of the minerals and the 

 optical characters of the feldspars it is possible to estimate approxi- 

 mately the chemical composition of the rock. The feldspars appear 

 to be albite and orthoclase (potash microcline) in almost equal pro- 

 portions in the groundmass, and the phenocrysts appear to have these 

 molecules in nearly the same" proportions. In assuming a chemical 

 compositon for the brownish-green mica, the analysis of that in the 

 soda granite (grano-liparose) of Cape Ann, Mass.,^ was chosen. 



On this basis the chemical composition of the rock was calculated 

 to be that given in column I. This was done before chemical analyses 

 of the rock were made, and the result is of great interest as showing how 

 far this method of estimation may be relied on in favorable cases. If 

 the microscopical measurements had been made to include the mag- 

 netite and zircon, the result would have been still more elaborate. 

 Subsequently Analysis II was made by Mr. S. H. Worrell, of the 

 University of Texas Mineral Survey, on a sample of the rock from 

 the land of Mr. H. C. Harned, near Llano, Tex. As the alkalies 

 were not separately determined in this analysis. Dr. H. S. Washing- 

 ton very generously undertook to analyze material from the specimen 

 studied microscopically. The result is given in Analysis III. .. From 



^ See Table XIV in Quantitative Classification of Igneous Rocks (Chicago, 1903), 

 mica analysis e. 



