Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/490

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Dr. Mac Culloch on Quartz Rock.

to those who may wish to assign its place in a system. I have selected for description all the most prominent varieties which have fallen under my notice, distinguishing their geological and geographical position whenever they were known to me, and marking at the same time the several gradations by which quartz rock appears to pass into mica slate, into clay slate, or into graywacke.

  1. Pure white granular rock, consisting of amorphous grains of quartz, strongly agglutinated:—a perfect granular quartz. Although the grains separately taken are transparent, the mass is necessarily opaque. From Balahulish.
  2. The same rock, but containing angular fragments of white felspar dispersed (although rarely) throughout it. From the same place.
  3. The same rock, with extremely minute amorphous fragments of felspar in abundance. From the same place.

    The rock whence these specimens were taken alternates with a very compact micaceous schistus, and the flat surfaces which have long been exposed to the weather, assume a sort of enamelled appearance, not much inferior to the polish given by the lapidary's wheel.

  4. Pure white quartz, formed of semitransparent amorphous grains, but the mass has a fracture intermediate between the granular and splintery. From Sky, and alternating with micaceous schist.
  5. A similar rock from Balahulish, of a pink colour, but with an aspect more harsh and dry, resembling common secondary sandstone. In the same situation.
  6. Pure granular quartz in the same situation, but the grains of large size, and the fracture consequently very coarse. From Portsoy.