Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/463

From Wikisource
Jump to navigation Jump to search
This page has been validated.
Dr. Mac Culloch on Quartz Rock.

view of large tracts. In many parts of the shore, however, the disposition of the strata is evidently changed, and they are observed occupying every possible variety of position, from the horizontal to the vertical. The steps by which these changes succeed each other eluded my observation, and perhaps are not to be traced.

I will now describe the principal modifications which this rock assumes, so as to give an idea of its mineralogical character, before I attempt even to conjecture its place in a geological system.

It exhibits the following varieties:

  1. An extremely compact granular stone, consisting of grains of quartz, of unequal sizes, united without cement.
  2. The same, containing grains of clay, which appear to be decomposed felspar.
  3. The same, with more numerous grains of felspar, which appear on examination generally to consist of rounded fragments.

    These rocks are traversed by veins of quartz, of which the aspect is also granular, but they are distinguished from the body of the rock by their snow white colour. They often appear so incorporated with the rock that the line of separation cannot be distinguished.

  4. A similar rock containing angular grains of quartz of half an inch in diameter, and bearing every mark of having been formed from a disintegrated granite, except that it exhibits no traces of mica.
  5. The same rock containing a flattened oval pebble of quartz, perfectly smooth, and having uniform curved surfaces, as if from long attrition. The pebble is two inches in length, and one and a half in breadth. It was so firmly united to the rock, that on one side it has been broken in the attempt to separate it.