34 The American Geologist. January, i905 
elation for the Advancement of Science, in England. The 
writer also attended the Congress at Bern and visited many 
museums on the continent and at London, examining espec- 
ially the fossil turtles. 
THE COMPARATIVE ACCURACY OF THE METHODS FOR 
DETERMINING THE PERCENTAGES OF THE SEVERAL 
COMPONENTS OF AN IGNEOUS ROCK. 
By Ira A. Williams, Ames, Iowa. 
The object of this paper is to present the results of a 
series of determinations of the quantitative mineral compo- 
sition of an igneous rock. These determinations were made 
according to the various methods that have heretofore been 
employed by different investigators in estimating the rela- 
tive proportions of the constituents of igneous rocks. The 
work was done at Columbia University where the writer was 
at the time Fellow in Geology. Not all of the methods em- 
ployed, however, are equally applicable to both the thoroly 
crystalline and the porphyritic rocks ; while with the glasses 
and felsytes no accurate estimate of composition can be 
made by any means except chemical analysis. 
The methods used may be listed : (a) calculation of min- 
eral composition from the chemical analysis of the rock; 
(b) separation of the mineral grains of the crushed sample 
according to their specific gravity by Thoulet's solution; 
(c) the method based on the microscopic measurement of the 
diameters of grains in thin sections ; (d) that based on the 
microscopic measurement of the areas of the grains in thin 
sections ; (e) photographing microscopic fields, then cutting 
out and weighing from prints thus secured the areas of the 
various constituents ; (f ) calculation of volume percentages 
from the data obtained by the foregoing microscopic 
methods. 
In order to obtain results admitting of comparison, it 
was necessary at the outset to select a rock to which each 
of these several methods could be applied. The rock must 
therefore be a holocrystalline one. The coarser the grain, 
moreover, and the more clear cut and distinct the boundaries 
