Classification of Rocks. — Spurr. 2 1 3 
feldspar class, while the systematic niineralogic grouping in 
each class is retained. General designations for chemically 
equivalent but mineralogically different groups should be for- 
mulated, the designation being written at the side of the table 
and the groups embraced under a single designation being 
separated from the others by heavy horizontal lines. 
The principles upon which this scheme of classification is 
founded are, therefore, in the order of importance: (i) Miner- 
alogy, (2) Structure, (3) Chemical equivalence. 
Class of Feldspar Rocks. 
In the general systematic classification, as above stated, 
rocks are divided into great classes, according to a prevailing 
mineral or mineral series, which runs throughout, being con- 
spicuously persistent and significant as others are unmeaning 
and varying. Of these various classes (the feldspar class, 
nepheline-leucite class, the olivine class, etc.), only the most 
important one, that of the feldspar rocks, is here elaborated. 
These rocks are divided, according to the nature of their feld- 
spars, into three families, the first characterized by alkali feld- 
spars, the second by oligoclase-andesine feldspars, and the 
third by labradorite-anorthite feldspars. The alkali feldspars 
contain about 63 to 68 per cent, of silica, the oligoclase-ande- 
sine feldspars about 56 to 63 per cent., and the labradorite- 
anorthite feldspars from 48 to 56 per cent. The rocks char- 
acterized by these feldspars also vary in regard to their silica 
content, generally in the same direction as the contained feld- 
spars, although, of course, the percentage of silica in a rock 
is not necessarily even approximately that of the enclosed feld- 
spars, but is only occasionally so. Thus KeniD" makes the 
average silica content of gabbros (essentially equivalent to the 
writer's diabase family, which is characterized by labradorite- 
anorthite feldspars) 45 to 55 per cent; that of the diorytes (ap- 
proximately the same as the writer's dioryte family — excluding 
the quartz-diorytes or tonalytes — which is characterized b\- 
andesine-oligoclase feldspars) 50 to 65 per centf. Therefore 
* Handbook of Rocks, New York, 1896, p. 18. 
tDiorytes which do not contain quartz cannot exceed 63 per cent, 
of silica, since this is the highest silica content of any feldspar contain- 
ing lime loligoclase-albite). The analysis showing 65 per cent, of silica 
is therefore not strictly a siinj^le dioryte hut a more acid rock. 
