Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/465

From Wikisource
Jump to navigation Jump to search
This page has been validated.
455
Dr. Mac Culloch on Quartz Rock.
  1. Coarse graywacke slate.
  2. Fine ditto.
  3. Perfectly fine and uniform clay slate, of a dark blue colour.

It is important to remark, that the beds which are found at the foot of the mountain, are of a coarser texture than those at the summit, and these are probably the uppermost beds of the deposit. Many of them are nearly black from the quantity of clay they contain; in others are found grains of mica, and in some there are imbedded large fragments of clay slate and chlorite slate.

In hand specimens a gradation may be traced from the finest and most compact quartz rock, down to a perfect breccia, containing fragments of slate, although the ground does not admit of our tracing the sequence of the beds.

If therefore we consider the circumstances which I have described as existing in the finest quartz rock, the rounded pebbles of quartz which it contains, and the gradation that may be traced in it through all the series which I have above noticed, and which are generally confounded under the term graywacke, we need not hesitate to conclude that the quartz rock of Jura is a mechanical deposit, or a rock recomposed from the fragments of older ones. I know that authors have talked of primitive sandstone, and even of primitive breccia, but the awkward nature of this compound renders it desirable that we should, if possible, discard a phraseology which involves a contradiction in terms. It is perfectly true that many of the beds in Jura contain large tracts of a granular quartz, often very pure and compact, and which from its crystalline texture might in the hand be supposed a primitive and chemical deposit; but the occurrence of blunted fragments, and of rolled pebbles, suffice to shew, that like many other rocks it possesses at least the compound structure both of a chemical and of a mechanical deposit. Neither