SALMON — ON THE FOltMATION OF ORE-VEINS. 
429 
metallic ores are generally found in connection wiih eruptive rocks, 
particularly 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 surface, the ore inevitably fails as we 
penetrate deeper into the crystalline rock. This fact is unquestion- 
able, and the arg-ument from it seemed to be unanswerable. The 
Comish 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 
when we penetrate far into it, where, according to the logical result 
of your theory, it ought to be richer ?" 
Prof. Cotta meets this difficulty. He points out that it is only 
where a ra2nd 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 carried them away to 
great depths. This rapid cooling would be expected to occur in 
vein-masses, as porphjTies or " elvans," and in small " stock-masses," 
or on the contact-edges and sm-faces of the larger masses ; and it is 
just in these j30sitions that we do find ore-deposits most abundantly 
developed. 
2. The relation between the varying 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 parallel gToups in the same locality at the same time. 
These would become filled by such minerals as were passing in so- 
lution from the eruptive rock through the veins, and were capable of 
being precipitated at the then state of the temperature and pressure. 
If we suppose a sabscquent change in the direction of the elevating 
force, we would then have a new set of parallel fissures, but vpith 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 pressure then existing. If we suppose some considerable in- 
terval to have occurred between the formation of these two fissure- 
gi'oups, it would follow that a great change of temperature might 
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