194 



one, or the other, or of both of them, and is then called pegmatite, 

 or graphic granite. 



4. Quartz and binaxal mica ; which, if the mica is regularly inter- 

 spersed, is called avanturine ; but if it occurs in parallel layers, 

 forms a passage into mica- schist. 



The second class of the author contains those aggregate rocks, 

 into one or more of the component minerals of which magnesia or 

 lime enters as an essential constituent ; and in the first division of 

 this class he places syenite and the other rocks containing some of 

 the ingredients of ternary granite, with the addition of hornblende, 

 on the ground that in all the varieties of the latter mineral a trace 

 has been found of fluoric acid ; and in respect of the presence of this 

 acid the rocks of this division are allied to the rocks of the first class. 



Hornblende contains neither potash, soda, nor lithia ; but it 

 abounds in magnesia and lime. 



The principal granitoid rocks into the composition of which horn- 

 blende enters, are the following : — 



1 . An aggregate of quartz, mica, felspar, and hornblende, or syenitic 

 granite. 



2. An aggregate of quartz, felspar, and hornblende, or syenite. 



3. An aggregate of felspar, mica and hornblende. 



4 An aggregate of felspar and hornblende. Quartz and horn- 

 blende is only an incidental variety. Actinolite or hypersthene 

 sometimes replaces hornblende, and is sometimes superadded to it. 

 Hornblende is the characteristic mineral of the granitoid rocks of 

 Scotland. 



The second division of the second class of the author consists of 

 ternary granite, of which the binaxal mica has been replaced by 

 talc, chlorite, or steatite (which rock has been termed protogine), 

 or by uniaxal mica. The latter aggregate occurs principally among 

 volcanic rocks. 



Neither talc, chlorite, nor steatite contain lithia, nor fluoric acid ; 

 hut the predominant alkalino- earthy ingredient is magnesia. Uniaxal 

 mica contains magnesia, but no lime. 



The third division of the author's second class of granitoid rocks, 

 consists of those into the composition of which tourmaline enters, 

 and this is the characteristic mineral of the granitoid rocks of Corn- 

 wall. With quartz, felspar and mica it forms the schorly granite; 

 and with qviartz and felspar, or with quartz alone, it forms the schist- 

 rock of some mineralogists. 



Tourmaline contains nearly equal quantities of silica and alumina ; 

 and oxide of iron is an ingredient of most of its varieties. It con- 

 tains a trace of one or other, or of both of the alkalies, potash and 

 soda, with a small but variable portion of magnesia, and occasionally 

 a trace of lime. Boracic acid is its characteristic ingredient. 



The author enters into some theoretical views respecting the 

 origin of the various forms of granitoid rock. 



Ternary granite, composed of quartz, felspar (or albite) and 

 binaxal mica, constitutes, according to his view, the lowest accessible 

 rock of the earth's original crust. 



