270 Dr. Mac Culloch on the Geology of Glen Tilt, 



as well as of the detached pieces and small points, since they are ab- 

 solutely identical in composition with the smaller reticulations. 



With respect to the limestone, it is subject to a variation of aspect 

 where it approximates to the granite which it is interesting and im- 

 portant to notice. The colour of the great limestone mass is lead 

 blue of various intensity, and its texture is almost universally 

 large grained and highly crystalline. It is true that in some few 

 instances it preserves this aspect in the situations described, but, 

 in the greater number of cases, particularly in those where it is pe* 

 netrated by the smaller veins and where the beds have undergone 

 an elongation or a contortion, the crystalline texture disappears. 

 It then assumes an appearance exactly resembling that of hornstone 

 or com.pact felspar, having a smooth texture, with a thin-edged 

 fracture intermediate between the splintery and flat conchoidal. 

 I may as well add here that the schist is often of an argil- 

 laceous, but highly indurated, character, and also that it often 

 passes into hornblende slate, and this (it is worthy of notice) 

 the more frequently as it approximates nearer to the granite. 



So much has been said with respect to the origin of granite veins, 

 and on the mode by which their intrusion into the schistose rocks 

 which generally accompany them has been effected, that it is superflu- 

 ous to repeat it, since nothing new can be offered on the subject. By 

 whatever mode it has been produced, it is obvious that the same ex- 

 planation will apply to the case of the limestone which is in* 

 terstratified with the schist, as far at least as the veins are con- 

 cerned, although the fact itself be a new one. But how has the 

 alternation of laminae of granite with those of limestone been 

 effected ? 



It is necessary here to anticipate that which will be fully 

 described, and, I trust, proved hereafter in a more proper place, the 



