102 



BULLETIN OF THE 



of this mctamorphism and of a proper understanding of its methods and 

 effects, will be apparent when it is recollected that to it is due all the 

 economical value the beds possess. The beds as originally formed prob- 

 ably contained the elements of its minerals, together with its copper and 

 silver, more or less disseminated through their mass, as much so re- 

 mains till the present day, or else they were so contained — at least in 

 part — in overlying rocks, and in this form they could have been of no 



economic value ; nor could any process taking place at that time have 

 concentrated the minerals in the manner in which they now occur." He 

 opposes the theory that the copper was deposited in its present position 

 by igneous action. 



The metamorphic action is thought to have resulted from the agency 

 of percolating waters. *' All the phenomena tend to prove that it is by 



r 



naeans of some such chemical actions as these, continued through long 

 periods of time, that the mctamorphism of these beds has been effected. 

 It is such metamorphism which has developed the amygdaloidal mela- 

 phyrs, formed segregations, modified and filled the veins and amygdules, 

 placing in them their minerals in the present relative positions ; and, in 

 the general process, the copper, like the other ingredients, was selected 

 from its disseminated and therefore useless condition, and concentrated 

 in veins, amygdaloids, and conglomerates till it reached a percentage of 

 richness that gives to the deposits an economical importance. This 

 action has taken place certainly not at a high temperature, and possibly 

 at a temperature no greater than that of the beds at present, while it 

 may have been largely aided by that electric action which chemistry 

 almost always induces, and which is known to be active at the pi'esent 

 day. In fact the presence of the latter is proof that chemical action is 



even yet going on 



Where observed, the hanging-walls of the sandstones were generally 



m 



smooth and gently undulating, but occasionally quite uneven, while the 

 upper two to twelve inches w^ere somewhat changed, being harder or 



softer, or lighter or darker-colored than the mass of the bed The 



foot-walls are sometimes smooth and undulating ; the surface of the 

 underlying bed, when not an amygdaloid, as was sometimes the case, 

 seeming to have been worn smooth, as if by attrition; or else the sand- 



The 



ti 



In one 



stone seemed to fill inequalities in the underlying amygdaloid, 

 sandstones were not observed to be changed near the foot-wall, 

 remarkable instance, a crack or fissure was observed extending down 

 into a melaphyr, which was filled by the overlying sandstone. The con- 

 clusion Is inevitable, that the melaphyr had formed, hardened, and 

 cracked before the sandstone was deposited 



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