GEOLOGY. 81 



PRIMARY SERIES. 



Crystalline, or Unstratijied. 



Granite. 



Greenstone. 



Basalt. 



Porphyry. 



Amygdaloid. 



Lava. 



Pumice. 



Sedimentary, or Stratified. 



Gneiss. 



Mica Slate, or Schist. 



Clay-Slate. 



Chlorite Slate. 



Quartz Rock. 



Primary Marble, or 



Limestone 



Of the Crystalline or Unstratijied Rocks. 



GRANITE. This rock not only forms, as it were, the 

 floor or foundation for the various superincumbent strata, 

 but, by its disintegration, it has furnished materials 

 for various other rocks. Granite is universally diffused, 

 many of the highest mountains in the different quarters 

 of the globe being composed wholly, or in part, of this 

 mineral. There is a granitic mountain near the Cape of 

 Good Hope, '100 feet high, and half a mile in circum- 

 ference, consisting of a single block of granite. Veins 

 of granite are sometimes found to pass upward, through 

 other strata, to a very considerable extent. As granite 

 was apparently formed by the action of heat, it is sup- 

 posed that, in many cases, it was in a state of fusion 

 subsequent to the deposition of several posterior forma- 

 tions, so that it has been injected into fissures and open- 

 ings by some expansive force. 



Granite is evidently not a simple substance, but is 

 composed of various substances, intimately mixed and 

 cohering firmly together : these substances are generally 

 quartz, felspar, and mica ; and sometimes hornblende. 

 It occasionally consists of only two substances ; of 

 quartz and felspar, or of felspar and hornblende. The 

 quartz may be distinguished by its glassy appearance ; 

 the felspar is smooth, and of somewhat a different 

 colour from quartz ; the mica is brilliant and sparkling ; 

 and the hornblende dark. 



Granite has been much used for architectural pur- 

 poses both by the ancients and the moderns 3 and, from 



