Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/277

 of Glen Tilt. It is not an independent vein. So far therefore the geological connection, as well as the identity in mineralogical character of the mass of granite which is approximate to the limestone and schist with the granite mountains of the northern ridge, is indisputable. From this mass, processes or veins of different sizes are seen to issue, until they gradually terminate, as I before said, in a thread. As long as these veins continue of a few inches in breadth, their mineral character remains unaltered. As they diminish, however, the hornblende gradually disappears, although, in the cases in which the vein traverses hornblende schist, this mineral is increased in quantity, and the vein assumes rather a more decided character of that rock which, as I conceive, is inconveniently distinguished by the term syenite. But the vein of mixed quartz and felspar is the most common, and this, as it continues to diminish in size ultimately becomes mere felspar, or else a compound of felspar and quartz so intimate, that the magnifying glass discriminates the particles no longer, and the whole is only distinguishable from common felspar by its peculiar fracture and superior hardness. This circumstance, the varying composition of granite veins, is not unusual, and it may be observed, among many other places, in the Corpach bason of the Caledonian canal. Now, however inaccurate it may be in a mineralogical point of view to designate the substance which constitutes the minuter veins by the term granite, yet for the purpose of geological reasoning it is unavoidable, nay, proper, since the substances have an absolute continuity or geological identity. It would be unjust either by a misapplication of terms, or an undue nicety in their use, to reject any geological argument which might be founded on such a fact as this. The same is true of the siliceous lamina found in the limestone,