ON MINERAL VEINS IN CARBONIFEROUS LIMESTONE. 363 



same rein samples of "dowks" which, when washed, give almost every 

 conceivable hue of colour, from the most delicate French white to the 

 densest shades of black or indigo, all being due to the different chemical 

 or mineralogical conditions of the matrix. Additional evidence is afforded 

 of the derived nature of the " dowks" by the occasional presence in them of 

 fossil wood. I have found a piece in the Charterhouse Mine, converted 

 into jet, with the cellular portions containing galena ; and another example 

 is recorded from the HudgiU Burn Mine, the exterior of which was covered 

 with galena. Whenever in a given area a series of fissures were open at 

 the same time, and subject to the same conditions of deposition, it is quite 

 possible that certain horizons of infilling might be found in them, though, 

 from the pecuUar circumstances attending them, their recognition would 

 involve close observation. 



Having said thus much of the fissures and their more unprofitable con- 

 tents, I now proceed to consider the views that have been usually enter- 

 tained as to the deposition of the minerals therein. 



Views of Mineral Deposition. — Next to the one which I shall hereafter 

 suggest of a purely marine origin for the minerals, I believe that of sub- 

 limation to have the greatest probability ; but there are, I think, many 

 reasons against this supposed cause. 



It necessarily involves, what was most likely the fact, that the fissures 

 originally continued downwards untU they reached the source of volcanic 

 action, which in some instances must have been seated at an enormous 

 depth. The moment the fissures were opened, of whatever width, if within 

 the influence of the ocean, they must have been filled by its waters, and 

 liable more or less quickly to receive their now varied contents. Their 

 lower depths would naturally be first filled. "Where the rents passed 

 through, as must often have been the ease, soft and yielding beds, tliis might 

 often have occurred quickly, in others only after a great lapse of time. Were 

 the doctrine of sublimation true, we ought, I think, to have greater evidence 

 of the continued passage iipwards of volcanic fluids, in the very lowest depths, 

 instead, as is most often the case, of finding our minerals high up in the veins, 

 or occurriug chiefly on given horizons in them, or even in pockets at their 

 surfaces. It appears to me also that the " dowks " or clays through which 

 60 subtle and potent an agent must have been continually passing, charged with 

 its varied minerals, must have shown much greater evidence of its effects than 

 is now seen in them, and that they would more frequently have tended to 

 precipitate the minerals before they could have passed so far upwards in the 

 vein in which they are now more frequently found. 



The doctrine of segregation, which at this time appears to have most 

 believers, is, I think, open to greater objection than that of sublimation. 

 Its disciples believe that, subsequently to the movements which have caused 

 the fissures, a process has been going on which even now is collecting the 

 various minerals together towards a fixed point, according to their natural 

 afiinities, extracting them from the adjoining rocks, and rcdepositing them 

 in the veins. This view is purely imaginary and incapable of proof, aud 

 seems to me as impossible as the idea I had when a boy, that if you planted 

 a stone, in process of time it might grow into a mountain. There seems to 

 me no possibility bow, under this supposition, any mineral can be with- 

 drawn, and necessarily replaced by another totally different, involving a cir- 

 culatory movement continually in progress in the matrix of some of our 

 most extensive geological formations. Probably, as regards their chemical 

 constituents, there are no more homogeneous rocks than the great series of the 



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