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Dr. Mac Culloch on Quartz Rock.
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of purple and of white quartz, the pebbles being from half an inch to two inches in diameter. In Jura, forming part of the series of the purer quartz rock.
- A brecciated mass of felspar and quartz of various colours, compacted by a cement of transparent quartz, forming part of the series at Balahulish.
- Specimens from Arisaig, from Assynt, Mar, Balahulish, Airds, Sky, Schihallien, and numerous other places, which consist of variable mixtures of reddish felspar, and transparent quartz, sometimes appearing to be nearly all quartz, and at others to contain a large proportion of felspar. The grains are of various dimensions, and the texture of the stone is consequently various. At times it has the aspect of a fine reddish sandstone, at others of a coarse grit; and where the grains are very unequal in dimensions it resembles a breccia. The grains have never a defined form.
- A rock consisting of narrow and alternating parallel layers; of fine and coarse granular quartz. Large grains of felspar are found in the coarse layer; but none at all in the line one. From Balahulish.
- A scarcely coherent combination of large grains of quartz and felspar, the interstices either empty or filled with clay. Part of the series at Balahulish.
- Compact extremely fine granular splintery quartz; alternating in thin layers with clay slate. From Cowal.
- Fine sandstones, not to be distinguished from the flœtz sandstones, and like many of them striped in endless alternations by black clay. From the series of Balahulish. These belong to that quartz rock which alternates with clay slate, and show the transition between those two substances.
- Fine grained granular quartz, alternating with layers of
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