58 



VARIETIES OF GRANITE. 



Scotland. Mica, readily, separates or divides into thin transparent 

 laminae; and where the plates are very large, as in the Siberian 

 granite, it is used instead of glass for windows. This variety is im- 

 properly called Muscovy talc. Talc resembles mica, but is much 

 softer. When the grains of felspar and other minerals are very mi- 

 nute in granite, it can scarcely be distin2;uished from sandstone. 



Beside the three minerals, quartz, felspar, and mica, which were 

 formerly considered as the essential constituent parts of all true gran- 

 ite, whoever has attentively examined various granitic districts, must 

 have frequently observed, that other minerals occupy the place of 

 mica, either in part or entirely. Thus, near the summit of Mont 

 Blanc, the granite is composed of felspar, quartz, and talc or chlo- 

 rite, the latter mineral supplying the place of mica. To this variety 

 of granite the name of protogene has improperly been given, where- 

 as talcy or chloritic granite would at once convey a distinct idea of 

 its nature. In some instances, hornblende supplies the place of mi- 

 ca, or is intermixed with it. To this rock, the name of sienite was 

 given, because a granitic rock of this kind from Sienna, in Upper 

 Egypt, was much used by the ancients for obelisks. 



The following varieties of granite are often associated in the same 

 granitic mountains, and may be regarded as contemporaneous with 

 it, being, essentially, the same rock, accidentally modified, by an ad- 

 mixture with other simple minerals. 



Common Granite. — The felspar, white or red, composed of quartz, 

 felspar, and mica. 



Porphyritic Granite, in which large crystals of felspar occur in a 

 small-grained granite. The granite near Shap, in Westmoreland, of- 

 fers an excellent type of this. 



Sienite or Sienitic Granite, in which hornblende, either wholly or 

 in part, supplies the place of mica. The granite of Malvern, and of 

 the Charnwood Forest hills affords specimens of this granite. 



Talcy or Chloritic Granite. — Quartz, felspar, and talc or chlo- 

 rite. Many of the granitic mountains, in Savoy, are composed of 

 this granite ; and loose blocks of it are scattered over the valleys and 

 on the sides and summits of the calcareous mountains, in the coun- 

 tries to the north and north-west of the Alps. This granite is by 

 some writers called protogene. 



Felspathic Granite, in which the felspar is the principal ingredi- 

 ent, and the quartz, and particularly the mica, very rare; larger 

 crystals of felspar occur in it. It is, frequently, nearly white. To 

 this variety, Werner has given the name of white stone, and the 

 French, eurite. It occurs in beds, in common granite, in Cornwall. 

 In its most compact form, it becomes a porphyry, and in Auvergne, 

 is closely allied to volcanic rocks. Indeed I observed the common 

 granite of Auvergne to be composed chiefly of felspar and quartz 

 without mica; in some parts, the mica was replaced, by the mineral 

 called pinite. 



