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On the Variations of certain Metalliferous depositories in 

 Depth. By M. Amedee Burat. 



In a previous memoir,* we have proved the real continuity 

 of mineral veins in depth, in the different classes of metalli- 

 ferous repositories, at least in regard to the zones accessible 

 to our mining operations. We have obtained this proof by 

 shewing, that in the greater part of cases in which mines 

 have been declared exhausted in depth, this alleged ex- 

 haustion was owing to a wrong interpretation of slight acci- 

 dents, and insufficient examination — to the increasing diffi- 

 culty with which subterranean works are carried on in pro- 

 portion to their depth ; and, finally, to the frequent changes 

 that take place in the mineralogical nature of the substances. 



Many metalliferous repositories, regular and irregular, 

 present us with a composition in their upper parts which be- 

 comes modified in depth ; those variations which refer at 

 once to the veinstones {jgangues) and minerals, have been as- 

 cribed, in the majority of cases, either to spontaneous altera- 

 tions or to molecular transportations posterior to their for- 

 mation. 



Accordingly, in all mining countries we observe the fact, 

 of such frequent occurrence, of the alteration of the minerals 

 at the surface, to which the Germans give the name of Cha- 

 peaux defer, and which in Cornwall are termed Gossan. The 

 most striking feature in this alteration is the colouration of 

 the mass by ochreous tints owing to the decomposition of 

 pyrites, and a general softening of the repository whose ar- 

 gillaceous veinstones {jgangues) are rotten^ according to the 

 expression of miners, and the quartzy veinstones {jgangues) 

 are carious and full of holes. 



These modifications are not confined to the surface, but 

 extend to variable depths, often beyond 50, and even in cer- 

 tain cases beyond 100 yards. In these upper regions, the 

 characteristic minerals of repositories are likewise in mine- 

 ralogical conditions altogether peculiar. Lead veins, for ex- 



* See vol. xlv., p. 346, of this Journal. 



