Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/430

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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 hornblende 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 proportions 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 compositions now generally known by the name of greenstone. When the condition of the stone is of that intermediate nature which renders its precise place uncertain, the modified term of syenitic greenstone is easily applied. In difficult cases where these rocks are still more varied in their composition, and in all others where a multiplication 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 understood 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