Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/431

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421
various parts of Scotland.

stone, provided their proportions are not conspicuous, nor their influence on its general aspect remarkable.

But the rocks themselves, to which I am thus desirous of limiting the name of syenite, are so associated in their habits and formation with the rocks of the trap family, and with those porphyries which appear to have originated in similar circumstances, that they have a strong claim on us for one generic term, as well from their geological, as their chemical, or mineralogical relations. If then we consider the term syenite as, founded on this double view of the nature and position of the rocks to which I allude, it will be obviously convenient, if not absolutely necessary, to separate from them the rocks of a granitic character; of these the most simple in its construction is that which consists of quartz, felspar, and hornblende, or of those three ingredients with mica. Such rocks are not of uncommon occurrence, nor is it uncommon to meet a rock in which quartz, felspar and mica are so compounded, as to form a true granite with crystals of hornblende superadded to the compound. Great confusion must follow the separation of these from the granites, and their union to the syenites above described. Their structure and characters are in all cases granitic, nor does the eye readily detect the difference till the darker parts are minutely examined, when the crystals of hornblende may, and that often only with much care, be distinguished from the mica. Their geological affinities associate them also with the granites, and never with the greenstones, since they occur in the formations of the granite æra, as hornblende rock and hornblende slate are known to do. The obvious distinction would be to call these compounds syenitic granites, thus preserving their analogy with the syenitic greenstones, and preventing us from confounding into one mass two formations of widely different characters and epochas, merely from the accidental presence of a single ingredient.