Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/464

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Dr. Mac Culloch on Quartz Rock.
  1. Grey quartzy rock, with a basis of a compact splintery appearance, containing imbedded grains of transparent quartz.
  2. Small grained breccia, of which the basis is an earthy-looking mixture of felspar and quartz, resembling clay stone, or compact felspar, and uniting angular and rounded fragments of quartz.
  3. An uniform aggregate of large grains of felspar and of transparent quartz, without the aspect of granite, since all the grains appear to have been mechanically rounded and re-united.

This quartz rock in the several varieties now described, although as I have already remarked, it forms the essential and fundamental part of the island, does not occupy it to the exclusion all others. Beds of a rock resembling both mica slate and graywacke, of common graywacke, of finer graywacke slate, and of perfectly fine and uniform clay slate, together with beds of chlorite slate, appear in various places, all difficult to trace through their whole bearings, yet all apparently superimposed on the quartz rock. It is necessary to describe these rocks somewhat more particularly, as they are intimately connected with the quartz rock, and serve to illustrate its history. The following are the most remarkable varieties:

  1. A mixture of quartz in grains, with mica slate, of a character intermediate between quartz rock and mica slate, or rather resembling some of the varieties of that graywacke which I have described in my accounts of Abenfoyle (p. 447.)
  2. The same, of a much larger grain, with distinct scraps of mica slate.
  3. The same, with the mica slate so predominans that the compound forms a dark rock, in which the grains of felspar and quartz bear a small proportion to the slate;
  4. The mica slate still increasing, and the texture still granular.
  5. The same, with a slate fracture.