ON ROCKS. 
I. CONSTITUENTS OF ROCKS. 
Rocks are made up of minerals. A few kinds consist of 
a single mineral alone: as, for example, dimestone, which 
may be either the species ‘calcite or dolomite ; quartzyte 
(along with much sandstone), which is quartz ; and felsyte, 
which is orthoclase. But even these simple kinds are sel- 
dom free from other ingredients, and often contain visibly 
other minerals. Nearly all kinds of rocks are combinations 
of two or more minerals. They are not definite compounds, 
but indefinite mixtures, and hardly less indefinite than the 
mud of a mud-flat. The lmits between kinds of rocks 
are consequently ill-defined. Granite graduates insensibly 
into gneiss, and gneiss as insensibly into mica schist and 
quartzyte, syenyte into granite, mica schist into hornblende 
schist, granite also into a compact porphyry-like rock, and 
trachyte ; and so it is with many other kinds. ‘The fact 
isa chief source of the difficulty in studying and defining 
rocks, and especially the crystalline kinds. The different 
rocks are not species in the sense in which this word is used 
in science, but only kinds of rocks. 
The minerals which are the chief constituents of rocks 
are of two classes: (A) the Stliceous; (B) the Culcareous. 
A. The siliceous are as follows : 
1. Quartz, which probably makes up one-third of the 
rocky material of the crust of the globe. 
2. The Feldspars (p. 272); of which orthoclase (with 
microcline) is most abundant; next to it, oligoclase and 
labradorite ; and next albite, andesite, and anorthite. 
3. The Micas (p. 265): muscovite and biotite, of equal 
prominence, the others much less common. 
4. Amphibole and Pyroxene species (p. 245, and beyond) : 
especially hornblende or black amphibole, and augite or 
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