ON ROCKS. 



I. CONSTITUENTS OF ROCKS. 



Eocks are made up of minerals. A few kinds consist of 

 a single mineral alone : as, for example, limestone, which 

 may be either the species calcite or dolomite ; quartzyte 

 (along with much sandstone), which is quartz ; and felsyie, 

 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, 

 bat indefinite mixtures, and hardly less indefinite than the 

 mud of a mud-flat. The limits 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 

 is a 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 Siliceous; (B) the Calcareous. 



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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