﻿314 MR. A. M. FINLAYSON ON ORE-DEPOSITION [May I9IO, 



rounded or angular fragments of fine-grained galena, together with 

 decomposed granite, cemented by calcite and quartz. The Brow 

 vein at Leadhills shows a similar breccia in places. The Rampgill 

 vein at Nenthead contains sometimes a filling of crushed limestone 

 with both galena and blende, cemented by strings of calcite. 

 Other interesting specimens are seen at the Trecastell Mine, Conway. 

 Some of these show fragments of volcanic ash with a selvage of 

 galena, constituting typical 'ring- ore.' On the other hand, some 

 fragments of brecciated ash contain strings of galena cut off at 

 the edges of the fragments, the whole being cemented by calcite. 

 There is here evidence of ore-deposition both before and after the 

 brecciation of the vein-stone. 



Chalcopyrite generally occurs enclosed in or wrapped round by 

 galena or blende, and has evidently been one of the first sulphides 

 deposited. Pyrite, on the other hand, has generally been later 

 than galena and blende. It is quite a subordinate constituent, 

 and occurs as small crystals in vughs or cavities, deposited on any 

 or all of the other ores. Fluorspar shows variable relations in the 

 field. It commonly occurs as the inner lining of flats and pipes ; 

 at times it forms alternating crusts with galena, and it is again 

 often enveloped by calcite and quartz. The relations of barytes 

 are not very definite. It commonly occurs by itself, unaccompanied 

 by much galena or blende — hence the dictum of the miners at the 

 Leadhills and elsewhere that barytes ' poisons ' the veins for other 

 ores. This relation, which holds good in many districts, strongly 

 suggests that barytes and galena were closely connected in order of 

 deposition, the physical conditions favourable to the deposition of 

 barytes being prejudicial to the formation of the sulphides. 



The examination of thin sections enables the relations of fluorspar 

 to be determined, as has been pointed out in the last section (§ IV) 

 dealing with replacement. As a general rule, fluorspar is observed 

 replacing limestone, and this is succeeded by galena replacing the 

 fluorspar. The priority of the fluorspar is thus clearly indicated. 

 Specimens from the Mill Close Mine show strings of galena pene- 

 trating a mass of crystallized fluorspar, while veinlets of fluorspar 

 are crossed by later veinlets of galena. Other specimens show 

 strings of blende penetrating fluorspar. The relations of fluorspar 

 and chalcopyrite were not directly observed, but it is probable that 

 the chalcopyrite is the older, as the fluorspar has been deposited 

 in intimate relation with the galena and blende. That fluorspar 

 should be one of the earlier minerals formed in the primary 

 deposition appears from the fact that the mobile fluorine in the 

 solutions, released at greater depths from combination with the 

 metals, must have been a very potent constituent, and one of the 

 first to form stable combinations whenever temperature became 

 low enough for its combinations to persist. It may be pointed out 

 in objection that fluorspar often forms the inside crust in flats 

 and pipes, but the two cases are not strictly comparable. We have, 

 in the one case, direct formation of fluorspar by replacement of 

 limestone, and, in the other, crystallization in cavities as the last 



