232 M. Am^dee Burat on the Variations of 



this enormous quantity of phosphoric acid, not found else- 

 where, either in the other minerals of the vein, or in the 

 bounding rocks, have been derived ? Is it not more logical 

 to admit that the phosphates have been formed at the same 

 time, and in the same way as the galena, and that they are 

 principally condensed near the surface, perhaps because they 

 were more volatile than the galena which occupied the deeper 

 parts? 



Among cupriferous veins, thatof Rheinbreitenbach, which 

 we have described in the Etudes sur les Mines^ will serve 

 as an example of variations of composition in depth. This 

 beautiful vein, composed of compact quartz, has, for its 

 normal mineral, at a level of 120 yards, an intimate mixture 

 of variegated copper and pyriteous copper, while, through- 

 out all the upper part, it was the phosphate of copper that 

 predominated. 



These phosphates of the superior regions were mingled 

 with some accidental combinations, such as arseniates, mala- 

 chite, native copper, and red copper-ore. As we descend, the 

 sulphuretted minerals mix with the phosphated minerals, 

 and at last exclude them. We may have recourse to the hy- 

 pothesis of a spontaneous decomposition of the sulphurets of 

 the superior region, in the cupriferous repositories of Siberia, 

 where malachite is their substitute; and in the repositories of 

 Santiago de Cuba, where it is native copper and red copper 

 ore ; but how can we explain, in this instance, the intrusion 

 of an immense quantity of phosphoric acid into a vein whose 

 composition is so simple ? 



The quartz which serves as a veinstone {gangue) to the 

 phosphated minerals, as well as to the sulphuretted ones, 

 likewise undergoes some variations, which may throw light 

 on these theoretical questions. In the whole region of the 

 phosphates andoxides, this quartz presents calcedonous drusy 

 cavities ; and it is in the latter that we find arborescent na- 

 tive copper, and those beautiful red capillary oxides, so much 

 sought after by mineralogists. In the deeper parts, after 

 the sulphuretted minerals have excluded the phosphates, the 

 calcedonous character is eliminated, there are no longer drusy 

 cavities, and the whole filling up consists solely of compact 



