272 Dr. Mac Cul loch's Supplementary Observations 



is often hyaline, while the mica is disseminated through it in scales 

 of which the tendency, notwithstanding the crystalline state of the 

 quartz, is parallel either to the lamina: or to the beds of the stone. 

 These specimens when polished exhibit the characters of the most 

 perfect avanturine, but their colours are only white or greyish. 

 I have never yet met with the most esteemed, the yellow variety. 

 As the quartz rock approaches more nearly to mica slate, the cha- 

 racter of the specimens which resemble avanturine changes, until 

 they resemble the variety of this ornamental mineral found at 

 Ekaterineberg. In many cases the scales of mica have a con- 

 siderably greater dimension in one direction than in another, from 

 which the stone acquires a fibrous aspect. This variety, of a 

 fine blue grey colour, occurs in Glen Fernat in large beds, and 

 when polished does not yield to some of the most beautiful foreign 

 specimens of avanturine. There is yet one other modification of 

 this mineral, of which the splendour results merely from the vary- 

 ing position of the quartz grains which form it. The most crystal- 

 lized and pure specimens of quartz rock afford this variety, and 

 examples of it are to be found every where among the more com- 

 pact beds of that rock. We may therefore conclude that the avan- 

 turine, so much esteemed and long so ill understood, is a variety of 

 quartz rock ; a circumstance likely to give this rock that importance 

 among collectors of specimens, which I have attempted to claim 

 for it among geologists. 



