Dr. Mac Culloch on Quartz Rock', 479 



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 re- 

 mark, that in some cases the same gradation which 1 have before 

 noticed through schistose quartz rock takes place at the line of 

 "ahernation. 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 affcjrds only galena, l)ut it also pro- 

 duces brown blende. 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 circum- 

 stance which, with many others, should caution us against im- • 

 plicitly 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 geogra- 

 phical 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 indispensible) 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- 



