SALMON ON THE FOKMATION OF ORE-VEINS, 



429 



metallic ores are generally found in connection with, eruptive rocks, 

 particulai4y near their junction, yet they comparatively rarely occur 

 abundantly in them, and even when they do, it is found that, al- 

 though productive near the sm^face, the ore inevitably fails as we 

 penetrate deeper into the crystalline rock. This fact is unquestion-> 

 able, and the argument fi'om it seemed to be unanswerable. The 

 Cornish miners have asked, " If the metals come from below, out of 

 the granite, how is it that this same granite is invariably poor for ores 

 wben we penetrate far into it, where, according to the logical result 

 of your theory, it ought to be richer r" 



Prof. Cotta meets this difficulty. He points out that it is only 

 where a ro^pid cooling of the eruptive mass has occurred that we 

 ought to expect metallic ores ; where the mass has cooled slowly the 

 gi'eater specific gravity of the metals has caiTied them away to 

 great depths. This rapid cooling would be expected to occui' in 

 vein-masses, as porphyries or "elvans," and in small "stock-masses," 

 or on the contact-edges and sui'faces of the larger masses ; and it is 

 just in these positions that we do find ore-deposits most abundantly 

 developed. 



2. The relation between the vai'ying contents of veins and their varying 

 direction is also explained by the distinctive veins being in fact 

 " nothing else than the products of unequal stadii of cooling of one 

 and the same vein-forming process." Even from the little we know 

 of the dynamics of geology we can at least understand that the up- 

 heaving force to which fissures are due must have acted so as to 

 produce nearly pai^allel groups in the same locality at the same time. 

 These would become filled by such minerals as were passing in so- 

 lution fi'om the eruptive rock through the veins, and were capable of 

 being precipitated at the then state of the temperatm-e and pressure. 

 If we suppose a subsequent change in the direction of the ele^'atiug 

 force, we would then have a new set of parallel fissures, but ^vith a 

 difierent direction. These would in their turn become similarly 

 filled with such minerals as were then in circulation through the 

 vein-region, and were capable of being deposited at the temperature 

 and pressui^e then existing. If we suppose some considerable in- 

 terval to have occurred between the fonnation of these two fissm^e- 

 groups, it would follow that a great change of temperatm-e might 

 VOL. II. p p 



