45 
knowi]. From these, aqueous agencies form the Aqueous rocks by 
erosion and deposit, or by chemical precipitation. The latter in 
turn may be changed to Metamorphic rocks by dynamical and 
chemical agencies, which, however, do not usually destroy the 
lines of stratification. 
Cases 1, 4, 5, and part of 6 . — Eruptive rocks. The clas- 
sification which has been adopted for these is based upon the fol- 
lowing plan: 
First, an arrangement according to the percentage of silica. 
The highest in silica, or acidic rocks, are placed at the beginning 
of the series, then those having lower percentages and, last, the 
lowest, or basic rocks. 
Thus, beginning with the granites, which have from 8o to 65 
per cent, of silica, we pass among the coarse-grained rocks to the 
diorites, which have between 65 and 55 per cent., then toA^e 
gabbros and diabases, having usually more than 45 per cent, 
and end with the peridotites, having below 45 per cent. A corre- 
sponding series begins with the syenites, and ends with the neph- 
eline rocks. 
Second, under the divisions representing different percent- 
ages of silica, a vertical arrangement is adopted by which the 
coarse-grained or holocrystalline rocks are placed first, then those 
of finer grain or those having a porphyritic structure and, last, the 
amorphous rocks. Thus among rocks having from 80 to 65 per 
cent of silica, the granites, being coarse-grained, are placed first in 
the vertical order, the granite porphyries second, and rhyolite, 
nevadite, obsidian, etc. , which are amorphous, last. 
Case 1. — First granite audits varieties, such. SiS granitite , 
graphic-granite , etc. These are rocks having quartz, potash feld- 
spar, and one or more minerals of the mica, amphibole, or 
pyroxene groups as essential constituents. 
Second row, granite-porphyry , quartz-porphyry , vitrophyre, 
felsophyre, etc. Like the preceding in composition, but more or 
less porphyritically developed. 
Third row, rhyolitOy nevadite, pumice, obsidian, etc. These 
are amorphous volcanic rocks, having high percentages of silica, 
usually more than 70 per cent. 
The order of the series now passes on to Case 5, described on 
page 47. 
