214 J^Ji^ American Geologist. April, loco 
these two families, with the exception of the group of quartz- 
diorytes or tonalytes, which contain free silica, correspond 
very nearly in their extreme limits of silica content to those of 
the feldspar groups which have here been chosen as criteria 
to distinguish these families. 
The alkali feldspars, apart from their content of silica, are 
characterized by potash and soda ; the andesine-oligoclase feld- 
spars by soda and lime, the former generally predominating ; 
the labradorite-anorthite feldspars also by soda and lime, but 
wuth the lime preponderating. The same general character- 
istics, W'ith regard to content of potash, soda and lime, are 
found in the analyses of the rocks of the different families 
w'hich these feldspar groups are held to define. In a broad 
way, therefore, the chemical relations of the three chief feld- 
spar groups to one another correspond to the chemical rela- 
tions of the three great families of feldspar rocks to one an- 
other, and so the feldspars constitute the most available single 
basis for classification yet found. 
The feldspar rocks are accordingly divided into three 
families which are defined by the predominance of the three 
principal feldspar groups, above described : First, the granitic 
family, characterized by the alkali feldspars ; second, the dior- 
itic family, characterized by the andesine-oligoclase feldspars ; 
and third, the diabasic family, characterized by the labradorite- 
anorthite feldspars. The term gabbro is dropped altogether, 
since, as strictly defined and generally employed, it is the name 
of a peculiar rock variety consisting essentially of plagioclase 
and diallage*, and when used in the broadest senset is prac- 
tically synonymous with diabase. 
Since rocks form a connected series some mineralogic 
combinations will be found which are exactly between any two 
of the principal families ; to cover these transitions transitional 
families are necessary. Brogger has elevated monzonyte from 
a local designation to a family transitional between the ortho- 
clase and the plagioclase families, and in the writer's classi- 
fication the monzonytes would be established in practically the 
same position, between the alkali feldspar and the andesine- 
*Zirkel, Lehrbuch der Petrographie, Leipzig, 1894, I, 834. 
fMichel-Levy, Structures et Classification des Roches Eruptives, Pa- 
ris, 1889, p. 42. 
