464 



SPECIAL GEOLOGY. 



Playfair, Macculloch, and others ; Mr. "Wallace,* divides the 

 granitic rocka into two classes ; the first comprises granites, 



FIG 303. Vein of Granite, penetrating a mass of granite older than itself, in the 

 vicinity of Carlsbad. 



in the composition of which the alkaline earths form no 

 essential part ; whilst in the second class, they are essential 

 ingredients. The first class is divided into two orders ; the 

 first, called perfect granite, is a ternary compound of quartz, 

 felspar, and di-axial mica, universally diffused, and generally 

 coarse-grained, when found in masses; the second called im- 

 perfect granite, including the compounds of felspar and mica, 

 without quartz, and of quartz and felspar, without mica. 

 There is no binary compound of quartz and mica, as this 

 would be incapable of assuming the granitic structure. The 

 second class includes three orders : 1st, hornblendic granite ; 

 2nd,taZc0seynm&te,orprotogine; and ^T^scJiorly granite. Each 

 of these orders includes several varieties. Of these granites, 

 the author regards the ternary, composed of quartz, felspar, 

 and di-axial mica, as the lowest accessible rock of the earth's 

 original crust, uplifted and protruded through sedimentary 

 strata at different periods, from the earliest to the latest age 

 of igneous disturbance. The fine-grained varieties of ternary 



* Memoir read before the Geological Society. 



