214 



ALBERT JOHANNSEN AND E. A. STEPHENSON 



in percentages may nevertheless give as significant results as if the 

 actual value of d were known." The results of columns VI and 

 VII indicate that d^ cannot be used for calculating the percentage 

 composition when d is the measured width of the mineral grains in 

 the thin section. 



Williams concludes that 



any one of the direct measurements with the microscope does not appear to 

 fulfill even approximately the necessary conditions for a statement of the 

 complete quantitative composition," but that "heavy solutions are a conve- 

 nient and readily applicable method of separating the minerals of a rock and may 



be valuable in determining quantitative relations The accuracy of the 



results will be directly proportional to the coarseness of grain and the readiness 

 with which the minerals can be separated. 



So far as the writers are aware, no other comparisons have been 

 made between the specific gravity method and that of Rosiwal. 

 Contrary opinions to those just quoted are held by many petro- 

 graphers. Thus in Table II we have the comparison of a 



TABLE II 



Quartz 



Orthoclase. . 



AbjAuz 



Biotite 



Hornblende\ 

 Pyroxeije / 

 Magnetite. . 

 Pyrite, etc. . 



Calculated 

 Composition 

 from Analysis 



39 



II 



Rosiwal 



Volume 



Analysis: 



First Specimen 



Weight 



Percentage: 



First Specimen 



•55 

 ■57 

 •47 

 ■77 

 ■44 

 ■37 

 .76 

 .07 



IV 



Rosiwal 



Volume Anal. 



Second 



Specimen 



20 



■97 



.07 



■29 

 ■15 

 .84 

 .07 



•54 

 .02 



V 



Weight 



Percentage: 



Second 



Specimen 



computation from chemical data of mineral composition with 

 measured linear dimensions according to the Rosiwal method 

 made by Cross, Iddings, Pirsson, and Washington' for the Butte 

 "granite." 



In Table II, columns I, III, and V should be compared. The 

 authors state that a small amount of pyroxene was intergrown with 



' Cross, Iddings, Pirsson, and Washington, "A Quantitative Chemico-Mineralogical 

 Classification and Nomenclature of Igneous Rocks," Joiir. Geo!., X (1902), 681-82. 

 Ibid., Quantitative Classification of Igneous Rocks (Chicago, 1903), p. 286. 



