MINERALOGICAL CLASSIFICATION OF IGNEOUS ROCKS 67 



biotite, and hornblende, and was called a syenite. One naturally 

 would suppose from the name that andesine and quartz were of 

 subordinate importance, yet an examination of many thin sections 

 showed 20 per cent quartz and 30 per cent each of orthoclase and 

 andesine, a rock which is a quartz-monzonite Brogger. One rock 

 found to be thus incorrectly named raises doubts as to the accuracy 

 of the determinations of all other rocks in the same report. 



Fi k 6 n & 7 K%& 



Figs. 3-8. — Various proportions of dark minerals in a rock 



During the past few years the writer has required his students, 

 in their rock descriptions, to give the percentages of the different 

 constituents, 1 and he has invariably found that the estimates 

 of the less abundant minerals, such as the dark constituents in leuco- 

 cratic rocks or the light constituents in those that are melanocratic, 

 are entirely too high, and that the' first summation of all the con- 

 stituents runs between 80 and 95 per cent. The reader may test 

 for himself, before reading farther, his ability to estimate per- 

 centages by examining Figs. 3 to 8, which were made by pasting 



1 For a specimen card showing percentages see Albert Johannsen, A Manual of 

 Petrographic Methods (New York, 1914), p. 614. 



