Dr. Mac Culloch on the Geology of Glen Tilt. 307 



phical Survey of Scotland exists on which rehance can be placed. 

 The errors are In fact beyond eniurveration. It would otherwise 

 be a desirable object to trace the real line of direction taken up by 

 this elevated edge of limestone through the long course I have 

 pointed out : but it must be reserved for future examination. 



Resuming now the left hand ridge of Glen Tilt at the point where 

 riie limestone terminates, the remainder of the task of description is 

 easy. The whole mountain of Ben Gloe is one mass of quartz rock, 

 with scarcely any notable quantity of schist interstratified with it. 

 It is the immediate boundary therefore of the limestone bed, and 

 from the relative position of both, it must consequently be consi- 

 dered as lying above the limestone. The extent of this rock to- 

 wards the east is considerable, since it stretches into Mar, and occu- 

 pies a great part of Glen Shee. To the west it soon disappears, or 

 at least its continuity is broken, since on the west side of the Garry 

 it is only found in alternation with schist. To the south of Ben 

 Gloe this rock is once more seen alternating with limestone, which 

 is again succeeded by the mica slate, and this without further 

 changes, except a gradation into clay slate, terminates at the well 

 known boundary of the secondary strata near Dunkeld and Blair- 

 gowrie. 



The contortions and the conglomerate aspect of some of the beds 

 are the most remarkable circumstances respecting the quartz rock 

 of Ben Gloe ; the former proving that quartz rock, like the schists 

 with which it is associated, has been in a flexible state, and the latter 

 proving beyond doubt the partially mechanical nature of this de- 

 posit. Besides these it produces three coloured varieties of quartz, 

 pink, amber brown, and blueish grey ; the former equalling in 

 colour the most beautiful varieties of that well known substance. 



I have reserved to this place what remains to be said concerning 



2q2 



