Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/345

 consists in the use of the term greenstone, which has been applied by them to some of the dark varieties of the syenitic granite. It has appeared to me, as I have remarked in the body of the paper, that these varieties of syenitic granite were distinguishable from true greenstone by the presence of quartz, and that however minute their texture, and apparently uniform the white ingredient, it always contained that mineral in conspicuous proportion, while in greenstone, a common member of the trap family, it was rarely present, and might, when present, be considered as accidental. If I have been mistaken in this observation, I shall consider it unfortunate that no criterion of a mineralogical nature can be found to distinguish these rocks, if indeed the presence of the occasional minerals, mica, epidote, or sphene, minerals in general of a more ancient origin, cannot lend us some assistance. It is so necessary to prevent rocks distinct in their geological relations from being confounded, that the terms which are assigned to them, ought if possible to be equally distinct, since the mere effect of terms, as we all experience, has in every science a powerful effect in misleading or influencing our judgments. If there really be no such distinction as I have here thought probable between certain syenitic granites and the greenstones of the trap family, it will then be necessary to have recourse to the same expedient as we use in distinguishing the limestones, the addition of the terms, primary, and secondary; an expedient however, which in this case it would be desirable to avoid, since many geologists are inclined already to distinguish by the addition of these very terms the real trap rocks which are found connected with primary or with secondary strata.