178 



MINERALS AND GEOLOGY 



FIG. 95. 



verses and displaces both Nos. 

 and 2. Granite rocks, by the 

 decomposition of one of their es- 

 sential components, feldspar, have 

 become converted in some dis- 

 tricts into white or light-coloured 

 clays, largely used, under the 

 name of kaolin, in the manufac- 

 ture of porcelain. 



Granite, properly so-called, is composed of three minerals : quartz, 

 feldspar and mica. The feldspar is usually the potash species Ortho- 

 clase (see No. 57, Part II), but is occasionally represented by the Soda 

 species Albite (Part II, No. 58), or by Oligoclase (No. 59). The 

 mica is generally the common potash species Muscovite (Part II, No. 

 77), but is sometimes mixed with, or occasionally replaced by, one of 

 the magnesiaii micas. As a general rule, the quartz, in granite, 

 occurs in vitreous colourless grains ; the feldspar, in red, white, pink, 

 or occasionally green or grey, lamellar masses, which exhibit smooth 

 and somewhat pearly cleavage planes ; and the mica is mostly in 

 small scales, or larger folise, of a pearly-metallic aspect, and silvery 

 white, black, brown, pearl-grey, or greenish in colour. In coarse- 

 grained granites, these component minerals are readily distinguish- 

 able ; but in rocks of fine-grain, they become blended into a common 

 granitic mass. The mica frequently dies out, or is very sparingly 

 present, in which case the rock is sometimes known as Pegmatite, 

 but this name is applied by German lithologists to coarse-grained 

 granites containing a small amount of silvery- white mica in compar- 

 atively large scales or leaves. Occasionally also in these quartzo- 

 feldspathic granites, the quartz is arranged in the form of narrow, 

 irregular crystals in more or less distinct 

 bands, producing, in transverse sections, 

 the appearance of a cuniform or Assy- 

 rian inscription : whence the term "gra- 

 phic granite " sometimes bestowed on this 

 variety. When, again, the quartz and 

 feldspar become intimately blended, so as 

 to possess more or less the appearance of a simple mineral, the rock 

 has been termed Felsite or Petrosilex. Very frequently, through a 



FIG. 



