Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/282

 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 laminæ 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 character 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 considerably greater dimension in one direction than in another, from I 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 varying position of the quartz grains which form it. The most crystallized and pure specimens of quartz rock afford this variety, and examples of it are to be found every where among the more compact beds of that rock. We may therefore conclude that the avanturine, 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.