Dr. Mac Culloch on the Geology of Glen Tilt. 269 



of Glen Tilt. It is not an Independent vein. So far therefore the 

 geological connection, as well as the identity In mineralogical cha- 

 ract;pr of the mass of granite which is approximate to the limestone 

 and schist with the granite mountains of the northern ridge, is in- 

 disputable. 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 dimi- 

 nish, however, the hornblende gradually disappears, although, in 

 the cases in which the vein traverses hornblende schist, this mine- 

 ral is increased in quantity, and the vein assumes rather a more de- 

 cided character of that rock which, as I conceive, is inconveni- 

 ently 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 com- 

 pound of felspar and quartz so intimate, that the magnifying glass 

 discriminates the particles no longer, and the whole is only distin- 

 guishable 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, how- 

 ever inaccurate it may be in a mineralogical point of view to de- 

 signate the substance which constitutes the minuter veins by the term 

 granite, yet for the purpose of geological reasoning it is unavoid- 

 able, nay, proper, since the substances have an absolute continuity 

 or geological identity. It would be unjust either by a misapplica- 

 tion of terms, or an undue nicety in their use, to reject any geo- 

 logical argument which might be founded on such a fact as this. 

 The same is true of the siliceous lamlnse found In the limestone, 



