1870.] T. S. HUNT — ON GRANITIC ROCKS. 389 



binary aggregate von Cotta and Zirkel would restrict the term 

 syenite, which was ah-eady defined by d'Omalius d'Halloy to be a 

 crystalline aggregate of hornblende and feldspar, by which ortho- 

 clase-feldspar may be understood, since he describes varieties of 

 syenite, as passing into diorite ; a name by most modern lithologlsts 

 restricted to a compound of albite or some more basic triclinic 

 feldspar with hornblende. It is apparently by failing to 

 appreciate the distinction between orthoclase and triclinic feldspars, 

 in this connection, that Haughton has lately described under the 

 name of syenite rocks composed of crystalline labradorite and 

 hornblende. 



§ 2. Naumann, regarding orthoclase and quartz as the essential 

 constituents of granite, designates those aggregates which contain 

 mica as mica-granites, and thus distinguishes them from horn- 

 blende-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 ag- 

 gregates of orthoclase and quartz. There are not wanting ex- 

 amples of well-defined rocks of this kind in which both mica and 

 hornblende are almost or altogether wanting. 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 only be mentioned in this con- 

 nection. The name of syenite, so often given to hornblendic 

 granites, will, in accordance with the views already expressed, be 

 restricted to rocks destitute of quartz. While the disappearance 

 of this mineral from hornblendic granites is held to give rise to a 

 true syenite, the same process with micaceous granites afi'ords a 

 quartzless rock consisting of orthoclase and mica, for which we 

 have no name. Great masses of an eruptive rock, granite-like in 

 structure, and consisting of crystalline orthoclase or sanidin, with- 

 out any quartz, occur in the province of Quebec. This rock con- 

 tains in some cases a small admixture of black mica, and in others 

 an equally small proportion of black hornblende. The latter 

 variety might be described as syenite, but for the former we have 

 no distinctive name, and I have described both of these by the 

 name of granitoid trachytes, a term which I adopted the more 

 willingly on account of the peculiar composition of the feldspar ; 

 and also because compact and finely granular rocks in the same 

 region, having a similar chemical composition, present all the 

 characters of typical trachytes, and apparently graduate into the 



