184 GRANITES AND GRANITIC VEIN-STONES. [XI. 



crystalline in texture, and consisting essentially of orthoclase- 

 u-'ldspar ami quartz, with an admixture of mica, and frequently 

 of a triclinic feldspar, either oligoclase or albite. This is the 

 definition of granite given by most writers on lithology, and 

 applies to a great portion of what are commonly called granitic 

 rocks; there are, however, crystalline granite-like aggregates 

 in which the mica is replaced by a dark colored hornblende or 

 ainpliibole, and to such a compound rock many authors have 

 given the name of syenite, while to those in which mica and 

 hornblende coexist the name of syenitic granite is applied. 

 It is observed that in certain of these hornblendic granites the 

 quartz becomes less in amount than in ordinary granites, and 

 finally disappears altogether, giving rise to a rock composed of 

 orthoclase and hornblende only. To such a binary aggregate 

 Von Cotta and Zirkel would restrict the term " syenite," which 

 was already defined by D'Omalius d'Halloy to be a crystalline 

 aggregate of hornblende and feldspar ; by which orthoclase- 

 feldspar may be understood, since he describes varieties of 

 syenite as passing into diorite, a name by most modern 

 lithologists restricted to a compound of albite, or some more 

 basic triclinic feldspar, with hornblende. It is apparently by 

 f tiling to appreciate the distinction between orthoclase and 

 triclinic feldspar, in this connection, that Haughton has lately 

 described, under the name of syenite, rocks which are composed 



; ystalline labradorite and hornblende. 



2. Naumann, regarding orthoclase and quartz as the essen- 

 tial constituents of granite, designates those aggregates which 

 contain mica as mica-granites, and thus distinguishes them 

 from hornblende-granites, in which the mica is replaced by 

 hornblende. These definitions seem the more desirable, as the 

 name of granite is popularly applied both to the hornblendic 

 and the micaceous aggregates of orthoclase and quartz. There 

 are not wanting examples of well-defined rocks of this kind in 

 which both mica and hornblende are almost or altogether want- 

 ing. Such rocks have been designated binary granites, a term 

 which it will be well to retain. Chloritic and talcose granites, 

 into the composition of which chlorite and talc enter, need 



