T. S. Hunt on Granitic JRocks. 83 



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 this binary aggregate von Cotta and 

 Zirkel would restrict the term syenite, which was already de- 

 fined 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 failing to appreciate the dis- 

 tinction between orthoclase and triclinic feldspar in this con- 

 nection, that Haughton has lately described under the name of 

 syenite rocks composed of crystalline labradorite and horn- 



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

 tial constituents of granite, designates those aggregates which 

 contain mica as mica-gi'anites, 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 

 wanting. Such rocks have been designated binary gi-anites, 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 connection. The name of 

 syenite, so often given to hornblendic granites, will, in accord- 

 ance 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 aflbrds a quartzless 

 rock, consisting of orthoclase and mica, for which we have no 

 name. Great masses of an eruptive rock, granite-like in struc- 

 ture, and consisting of crystalline orthoclase or sanidin, without 

 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 

 bj 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, pre- 

 sent all the characters of typical trachytes, and apparently grad- 

 uate into the granitoid rocks just noticed.* In all attempts to 

 * This Journal, II, ixxviii, 95. See also Zirkel, i 



