Facts at Variance xmth the Igneous Hypothesis of Geologists. 339 



direction of the veins containing similar substances, in any 

 given district, to admit of their being referred to the con- 

 traction of the rocks which inclose them. The large " elvan 

 courses," or porphyritic dykes, which abound in Cornwall, are 

 even more remaikable than the veins for a considerable de- 

 gree of parallelism ; and their inclination from the perpendi- 

 cular in descending is greater and more uniform, it being 

 mostly towards the N.W. 



Mineral veins having, however, the closest affinity to each 

 other, as it respects their contents and horizontal bearing, 

 are very commonly found to separate widely in descending at 

 angles of 30^ or 40^, and upwards; whereas other veins which 

 cross them at large angles at the surface, and consequently in 

 their descent also, for the most part differ entirely in their 

 contents. These well-ascei'tained points are scarcely less op- 

 posed to the hypothesis of veins having originated from fissures 

 resulting from the shrinking of the rocks, which would in- 

 volve their contemporaneous formation in the same rock, than 

 the indisputable fact I have heretofore referred to, — that the 

 contents of veins change with the rocks they traverse. It may, 

 moreover, be well to mention that veins commonly possess the 

 same general appearances in valleys as in the contiguous hills ; 

 and not only do they not exhibit symptoms of having over- 

 flowed, but in both situations the metalliferous veins are in 

 general equally furnished with ^'■gossan" or other foreign 

 matter overlying the ore*. 



It has been urged, that mineriil substances do not suffer de- 

 composition or vitrification by heat when under great pres- 

 sure. It is not necessary to inquire whether there be sufficient 

 proof to establish the correctness of this conclusion, because 

 it can hardly be asserted that such great pressure could have 

 existed at or near the surface, or in the fissures resulting from 

 the contraction of the rocks. 



Open fissures, or cavities, are frequently found in some me- 

 talliferous veins, and very rarely in others of equal or greater 

 thickness. 



The yellow sulphuret of copper, crystallized oxide of tin, 

 and other metallic, as well as earthy combinations, which are 

 found in these cavities, and are easily afiectcd by heat, give no 

 indications of its having ever existed even in a slight degree. 



Other facts and arguments might be adduced. But are not 

 those I have alludeil to sufficiently decided to show that the 

 hypothesis of the igneous origin of rocks cannot be maintained 



• How does this fact accord with the assumed denudation of the valleys ? 

 fis not this (|«ery too general ? Surely it is dcmuimlrahle that some valleys 

 h-.ive been formed by the process of denudation, however true it may be 

 that that iiujile of foruiatioii has been ascribed to others by asbuniplion 

 merely.— Edit. ] 



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