420 Dr. Mac Culloch on the Geology of 



is a term pretty generally admitted. We may, however, venture 

 to question the propriety of applying to rocks of this nature, a term 

 borrowed from the ancient naturalists, when it seems certain that 

 the rock of Syene was, in fact, a true granite, containing hornblende 

 merely as an occasional and accidental ingredient. Yet, as we are in 

 want of a generic term for the various rocks which consist of horn- 

 blende and felspar, and which are not considered as greenstones, it 

 may be advisable to admit of its use for this purpose, and to sacrifice 

 a little of our classical accuracy for the sake of convenience, rather 

 than by the introduction of another new term, to introduce new 

 confusion into that which already reigns In the denominations of 

 rocks. I hold it necessary however that the word syenite, if it is 

 to be used as a generic term, should be limited, as rigidly as the ever 

 varying and mingling composition of rocks will admit, to those 

 compound rocks which are akin to greenstone in their chemical and 

 essential characters, or consist of certain notable and distinct propor- 

 tions of felspar and hornblende, but in which the felspar is either 

 the predominating ingredient, or is crystallized in masses so large 

 and distinct as to remove the compound from those similar compo- 

 sitions now generally known by the name of greenstone. When 

 the condition of the stone is of that intermediate nature which ren- 

 ders its precise place uncertain, the modified term of syenitic green- 

 stone is easily applied. In difficult cases where these rocks are still 

 more varied in their composition, and in all others where a multi- 

 plication of terms would lead to an unbounded and unnecessary 

 nomenclature, it will even be preferable to distinguish the variation 

 from the more rigid form by a detailed description rather than to 

 encumber the science with a useless neology. It is easily under- 

 stood that no doubts respecting the place of these rocks in a system, 

 need arise from the intermixture of quartz, or even of mica in the 



