Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/489

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479
Dr. Mac Culloch on Quartz Rock.

quartz rock is sufficiently various in different parts to establish its identity in the principal leading features, with the more extended masses which I have already described. It is also worthy of remark, that in some cases the same gradation which I have before noticed through schistose quartz rock takes place at the line of alternation. This instance therefore offers a proof easily inspected, and quite satisfactory, of its forming part of the same system of deposits as mica slate, and establishes its rank among the rocks of this class.

The mine of Tyndrum has long been known as a lead mine. Of the ores of this metal it affords only galena, but it also produces brown blends. It is in the quartz rock that the ore is at present found, the mine being wrought by open levels, lighted and aired from above by small shafts. Thus we see that this rock is also entitled to a place among the metalliferous ones, a circumstance which, with many others, should caution us against implicitly adopting those general rules respecting metalliferous rocks, which have been too decidedly laid down.

The nature of this paper has compelled me to chuse a geographical form of detail, which I have further preferred, that others may have an opportunity of examining the evidence on which these conclusions are grounded. But it has necessarily led to an account of the circumstances attending this rock, so detailed and divided, that it will not be useless for the purpose of mineralogical discrimination, to bring under one general view a description of all its leading varieties, and for the purpose of geological science, all the leading facts connected with its history. From the former (perhaps indispensable) details, future observers will be enabled to satisfy themselves of the truth of these fundamental remarks, and from the latter, opportunity will be afforded for a more easy ge-