22 THIRD REPORT 1833. 



The agency of sublimation has lately been advocated by Pro- 

 fessor Necker of Geneva, in a paper read before the Geological 

 Society of London * ; and he has extended an ingenious hypo- 

 thesis of Dr. Boud, who would bring under a general law the 

 relation of metalliferous veins and deposits to those crystalline 

 rocks which, by the majority of modern geologists, are consi- 

 dered to have been produced by fire ; and thus to lead to the 

 inference that the metals were deposited in the former by sub- 

 limation from the latter. 



M. Necker inqviires, 1. Whether there is near each of the 

 known metalliferous deposits any unstratified rock i 



2. If none is to be found in the immediate vicinity, is there 

 no evidence which would lead to the belief that an unstratified 

 rock may extend under the metalliferous district ? 



3. Do there exist metalliferous deposits entirely disconnected 

 from unstratified rocks ? 



Professor Necker answers these questions by showing that in 

 various countries there are such relations as he supposes, and 

 admits, in reply to the last, that there are cases where the depo- 

 sits seem to be unconnected with any trace of unstratified rock. 



If metalliferous deposits are commonly in crystalline rocks 

 which are attributed to igneous origin, it must be allowed also 

 tliat there are others abundantly rich where no apparent con- 

 nexion is to be traced. M. Necker mentions the mountain 

 limestone as such ; but he does not seem aware of the extent 

 of those deposits, which, with the beds of grit and shale which 

 alternate with it, present numberless regular veins abounding 

 with certain ores. 



As this fact is indisputable, it seems necessary to show not 

 only that unstratified rocks may be under them — which there is 

 little doubt about, — but that there should be some connexion 

 between the veins which contain the metals and similar chan- 

 nels or passages in the rocks below. No such evidence, I be- 

 lieve, at present exists ; and I am not aware of any veins having 

 yet been found to penetrate from the stratified rocks into those 

 upon which they rest. 



This supposition must therefore, like many others, be taken 

 as a mere probability to account for some appearances in certain 

 places, but not to explain all the phaenomena. 



There is one point which, before I conclude, I would endea- 

 vour to press on the attention and consideration of future ob- 

 servers, because, in the first place, it does not appear to have 

 been much regarded by writers on the subject ; and next, be- 



• March 28th, 1832. 



