certain MetalHferotis Bepositories in Depth. 229 



Such are the general characters of the variations, of which 

 examples are to be found in all the metalliferous districts. 

 Cornwall, the Vosges, Belgium, the Rhenish provinces, Sax- 

 ony, the Oural, the metalliferous district of Santiago de 

 Cuba, kc, present numerous types of these modified reposi- 

 tories. The pacos and argentiferous colorados of South 

 America likewise afford examples of metalliferous gossan, 

 and repositories becoming sulphuretted as they increase in 

 depth. 



Having had occasion to study a very considerable number 

 of these variable repositories, we were led to doubt whether 

 the differences of composition in the superior parts were 

 really owing to alterations posterior to the formation of these 

 repositories ; and we have come to the opinion, after many re- 

 searches, that, in a great number of cases, these differences re- 

 sult from circumstances contemporaneous rvith the formation of 

 the repositories themselves. 



If decompositions induced by atmospheric agents, or by 

 the subterranean waters generally found in empty veins and 

 fissures, which facilitate their circulation by the greater elec- 

 trical conducting power of veins — if these decompositions 

 originated in the particular state we have pointed out, this 

 particular state would be as general as the causes to which 

 it is attributed. Such, however, is by no means the case; 

 and we can instance entire districts where these decomposi- 

 tions have not reached the metallic sulphurets. In the dis- 

 tricts where these superior regions of certain repositories 

 are very much changed, we observe a great number of others 

 which are not so ; we even find this divergency of characters 

 in veins which are parallel, placed in juxtaposition, and sub- 

 jected to conditions absolutely identical. 



Thus, for example, Algeria has presented us with a great 

 number of veins, the minerals being either copper pyrites, 

 or grey arseniferous and antimoniferous copper. At Mou- 

 zaia, where these veins form, at the surface, very salient 

 walls, these spontaneous decompositions have scarcely reached 

 the cleared parts ; the first strokes of the hammer are suffi- 

 cient to lay bare the sound and metallic portions. In the 

 valley of Oued Boukandah, near Tenes, as well as in that 6f 



VOL. XLVI. NO. XCII. — APRIL 1849. Q 



