Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/280

 the same time is compact and crystalline, differing in no respect from the most common specimens of this substance. The alterations 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 intervals 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 laminæ from an eighth to a quarter of an inch in thickness, detached from each other and separated by thinner laminæ of loose mica or clay. It offers another example 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 laminæ, natural joints occur resembling those of clay slate and producing on fracture,