116 



DE. J. W. EVANS ON THE ROCKS OF THE [Feb. I906, 



A number of traverses were made through a microsection, in 

 order to estimate the volumetric composition of the rock by the 

 method of Rosiwal, 1 who showed that the volumes of the different 

 minerals are proportional to the sums of their intercepts on any 

 line or lines drawn through the rock, if the number of minerals 

 traversed be sufficient. 2 It was found that, out of a total length of 

 52-26 millimetres, the sums of the intercepts of the different 

 minerals were as follows : — 



Quartz 20-09 



Orthoclase 1337 1 3 



Microcline 9-35J 



Albite 6-22 



Biotite 314 



Apatite (estimated) 004 



Fluor (estimated) 0-05 



52-26 



These figures represent, therefore, volumetric proportions of the 

 minerals. Classing microcline with orthoclase, and allowing for 

 anorthite in the albite, the minerals are reduced to those shown in 

 the first column of the next table. The figures in the second 

 column represent the volumetric proportions in which they are 

 present. Multiplying these by the densities (taking that of ortho- 

 clase as unity) given in the third column, we obtain the amounts 

 in the fourth column, which represent the proportions by weight of 

 the same minerals. In the last column these are recalculated as 

 percentages. 



Volumetric Density Gravimetric Percentage 



proportions. {orthoclase— \). proportions. Min. Compos. 



Orthoclase ... 22'72 1-00 22-72 42-19 



Albite 5-78 1-03 5-95 11-05 



Anorthite...... 0'44 1-08 0'48 0"89 



Quartz 20-09 1-04 2089 38'79 



Biotite 3-14 1-18 371 689 



Fluor 004 1-24 0-05 009 



Apatite 005 1'23 006 011 



Totals 52-26 53-86 100-01 



The chemical composition of the rock may now be calculated from 

 that of the minerals. The composition of the biotite is assumed to 

 be that of the mineral from El Capitan, Yosemite Y alley, the analysis 



1 Verhandl. d. Geol. Reichsanst. 1898, pp. 143 et seqq. ; ' The Quantitative 

 Classification of Igneous Rocks ' 1903, p. 204 ; J. P. Iddings, Journal of 

 Geology, vol. xii (1904) p. 225; and Ira A. Williams, 'American Geologist' 

 vol. xxxv (1905) p. 34. 



2 The line must, I need scarcely say, not be so traced as to pass through one 

 class of constituents rather than another. 



3 These contain, in some cases, microperthitic inclusions of albite. 



