Classification of Rocks. — Spurr. 227 
The principle above suggested has the great advantage that 
it estabhshes a definite division of the soda-lime feldspar rocks, 
which was before vv'anting. It has certainly been a mistake, 
as several waiters have remarked, to consider mineral species 
so varied in composition and habit as the soda-lime feldspars 
as a unit for the purpose of rock classification. The division 
of the feldspars into three groups instead of two is more even, 
in respect to content of silica and to other chemical peculiar- 
ities. All schools of petrography recognize that the com- 
parative silica content is important in rock classification, yet, 
according to the first division of the feldspars for the purpose 
of classification, into orthoclase and plagioclase, we have, in 
orthoclase division, a range of from 62 to 66 per cent, of silica* 
or only 4 per cent., while in the triclinic or plagioclase feld- 
spars there is a range of from 43 to 69 per cent., or 26 per cent. 
Between the alkali feldspars and the soda-lime feldspars (the 
division adopted in the later classification) there is not quite 
so much difference ; the content of silica in the alkali feldspar 
rocks, including albite, ranging from 58 to 69 per cent., or 11 
per cent., while the range in the soda-lime feldspars is from 43 
to 63 per cent., or 20 per cent. Even in this latter case, how- 
ever, the soda-lime feldspars have twice the range of silica that 
the alkali feldspars have. When the feldspars are divided into 
three groups, as above suggested, we have the orthoclase-albite 
group, ranging from 58 to 69 per cent., with a difference of 1 1 
per cent. ; the andesine-oligoclase group ranging from 55 to 63 
per cent, with a difference of 8 per cent., and the labradorite- 
anorthite group, ranging from 43 to 53 per cent., with a dif- 
ference of 10 per cent. In this division, therefore, we have a 
much more equable distribution of the silica content. 
The three chief families of feldspar rocks being thus de- 
termined by the predominance of the three classes of feldspars, 
the two intermediate families need no excuse, since, as rocks 
form a continuous series, there will be many forms which are 
transitional. For this class of transitional rocks Brogger has 
already emphasized the need, and his monzonyte family is 
transitional between the orthoclase rocks and the plagioclase 
rocks. The division of the plagioclase rocks in the present 
*Figures taken from Dana's Mineralogy, i8q6. 
