on Quartz Rock. 269 



forms but one great deposit, undergo analogous changes when it 

 approximates to this rock. The transition is in fact more easy and 

 the boundary less definable, since many varieties of quartz rock, 

 formed of felspar and quartz in varying proportions with an occa- 

 sional mixture of mica, only require to assume that crystallized ap- 

 pearance which the vicinity of granite is so apt to produce on the 

 neighbouring rocks, to become undistinguishable from it. The quartz 

 rock in these specimens contains mica, disposed at first in a parallel 

 form, so that it might equally be ranked among the varieties of mi- 

 caceous schistus or of gneiss. By degrees the mica loses its parallel 

 disposition, and at length the whole assumes the aspect of granite. 

 This transition therefore adds one more to those passages into mica 

 slate, clay slate, and graywacke, which I formerly described as oc- 

 curring in quartz rock. Loose specimens are found both at Blair 

 and in Ben Gloe containing drusy cavities, of which the siliceous 

 crystals, though more minute than a pin's head, are perfectly defined. 

 These crystals are evidently of posterior formation to the general 

 mass of the rock, and have resulted from the infiltration of a watery 

 solution of silica into previously formed cavities. They prove no- 

 thing therefore relating to the chemical nature of this rock, of which, 

 mixed with its mechanical formation, there are every where to be 

 found abundant examples, some of which I have formerly enume- 

 rated. 



In Glen Tilt I have taken out specimens from the beds exactly 



similar to those long compressed and smooth cylindrical bodies which 

 are sometimes found in secondary sandstones. This shows another 

 distinct point of agreement between quartz rock and those sandstones. 

 In the same place is found a very interesting variety. It consists 

 of a regular and repeated alternation of quartz with common argil- 

 laceous schist ; the smoothest variety of clay slate. The quartz at 

 Vol. iv. 2 m 



