XL] GRANITES AND GRANITIC VEIN-STONES. 185 



only be mentioned in this connection. 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 horn- 

 blendic granites is held to give rise to a true syenite, the same 

 process with micaceous granites affords a quartzless rock con- 

 sisting 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, without any 

 quartz, occur in the province of Quebec. This rock contains 

 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, pre- 

 sent all the characters of typical trachytes, and apparently 

 graduate into the granitoid rocks just noticed.* In all at- 

 tempts to define and classify compound rocks, it should be 

 borne in mind that they are not definite lithological species, 

 but admixtures of two or more mineralogical species, and can 

 only be arbitrarily defined and limited. 



3. Having thus defined the mineral composition of granitic 

 rocks, we proceed to notice their structure. Gneiss has the 

 same mineral elements as granite, but is distinguished by the 

 more or less stratified and parallel arrangement of its constitu- 

 ents ; and lithologists are aware that in certain varieties of 

 gneiss this structure is scarcely evident, except on a large 

 scale ; so that the distinction between gneiss and granite rests 

 rather on geognostical than on lithological grounds. To the 

 lithologist, in fact, the granitoid gneisses are simply more or 

 less stratiform granites, while it belongs to the geologist to 

 consider whether this structure has resulted from a sedimentary 



* American Jcnivnal of Science (2), XXXVIII. 95. See also Zirkel, Petro- 

 graphie, II. 179. 



