GEOLOGY. 13 



added or substituted, as it often is, for mica, it is then denomi- 

 nated SiENiTE — the form which it assumes chiefly on the 

 borders of the Upper Nile, and of which many of the grandest 

 ruins in Egypt consist. The mica sparkles like gold, and 

 exists sometimes in crystals, of more than a foot square, when 

 it is split up into thin plates, and used as a substitute for glass. 

 Some granites are binary, consisting only of two minerals, 

 felspar with quartz or hornblende, and when polished break 

 into irregular lines resembhng Arabic letters, on which account 

 it has been called graphic granite. A vein of this rock tra- 

 verses the district about a mile east of the town of Portsoy in 

 Banffshire, in connexion with mica-slate and a bed of lustrous 

 marble of great celebrity. 



The rock which immediately overlies the granite is Gneiss, 

 of which there are three varieties, each composed of felspar, 

 quartz, and mica, and only distinguishable by the size, form, 

 and arrangement of the crystals that constitute the mass. 

 Gneiss is essentially, therefore, a granite in its component parts, 

 but differs from granite in being always stratified, and in pre- 

 senting none of the phenomena that accompany the agency of 

 fire. It is indisputably admitted to be of aqueous origin, 

 formed by precipitation in water, and afterwards indurated by 

 chemical action or mechanical pressure. It consists of a series 

 of thin lenticular plates, which give it a ribbon-like appearance, 

 and which, according to the predominance of one of the ingre- 

 dients, causes the rock to assume the slaty, granular, or 

 ay-oreo-ate structure. Talc, hornblende, chlorite, actinolite, as 

 in granite, are not un frequently diffused through the substance 

 of gneiss, whence particular names have been adopted to dis- 

 tinguish the varieties in which they occur. Thus, when talc 

 or chlorite is mixed in the substance, it is termed protogine 

 by the French geologists ; when the crystals of felspar and 

 quartz are very minute, the rock is named whitestonc or lep- 

 t'mite ; when the hornblende and felspar predominate, mixed 

 with actinolite, it graduates into a primitive greenstone; and 



