270 Dr. Mac Culloch's Supplementary Observations 



the same time is compact and crystalline, differing in no respect from 

 the most common specimens of this substance. The alternations are 

 such and so frequent that a cross fracture of this rock may almost 

 be compared to the striped leaf of arundo colorata. 



In another situation I observed specimens consisting of pure quartz 

 without any such mixture of clay, but so fissile as to scale off in 

 leaves as thin as paper. 



Although garnets abound so much in mica slate, I have only met 

 with one instance in which they occur in quartz rock; this is at 

 the west end of Mar forest near the Dee. The garnets however are 

 very incomplete, although large in size ; they occupy only the in- 

 tervals between the layers of the stone, and on splitting it are found 

 as if compressed between the surfaces. 



Another remarkable variety of this rock also occurs in Glen Tilt. 

 It resembles precisely the schistose sandstones which accompany the 

 coal strata, and is found in distinct laminae from an eighth to a 

 quarter of an inch in thickness, detached from each other and sepa- 

 rated by thinner laminae of loose mica or clay. It offers another ex- 

 ample of the striking resemblance between quartz rock and the 

 secondary sandstones ; an agreement much more remarkable than 

 that of the mica slate which generally accompanies it, with the slate 

 clay which is the associate of those sandstones. It would seem as if 

 the quartz rock from its greater simplicity of materials, a simplicity 

 less liable to chemical changes, had undergone fewer alterations 

 during the progress of time and of those actions by which its present 

 form was produced, than the more compound schist with which 

 it was originally associated. In those varieties of quartz rock which 

 are, like that last described, formed of distinct laminae, natural joints 

 occur resembling those of clay slate and producing on fracture, 



