ORIGIN" AND DEVELOPMENT OF ORE-DEPOSITS. 61 



The knowledge thus afforded as to the constitution of the 

 outer portion of our globe is not inconsiderable, for it would seem 

 that the existence of distinct gold districts, silver districts, tin 

 districts, and the like must indicate local subterranean di:ffer- 

 ences, whether these metals have been derived from the country 

 rocks by lateral secretion or from below by ascension ; though 

 we must not lose sight of the fact that they tell us nothing, or 

 next to nothing as to the condition of the great mass of the 

 earth's interior. 



3. It has, perhaps, been sufficiently urged in the foregoing 

 sections that the valuable contents of the workable ore-deposits, 

 together with most of their accompanying "vein-stones" 

 have often been derived by processes of solution from the 

 adjacent rock-masses, and deposited after more or less of 

 wandering and transference ; sometimes without change, at other 

 times in entirely new forms ; as natural concentrations, in veins, 

 cavities, and shrinkage-cracks ; or as pseudomorphous replace- 

 ments.* But, whenever the openings favouring such concentra- 

 trations extend downwards far enough to communicate with the 

 zone of pseudo-fusion, they must at once become channels through 

 which other mineral matters can arise, and it is evident that 

 many of our most important deposits have been formed by this 

 more direct method of ascension. In particular, it would seem 

 that the greater part of the tin, tourma'ine, and fluor-spar, as 

 well as most of the metallic sulphides found in the veins of the 

 West of England, have come up through the weakened zones of 

 rock produced by the granitic and felsitic intrusions.! 



The deposits just referred to were, indeed, more directly 

 ascensional than some others, but not more really so. For the 

 ore-charged intrusive rocks which have yielded useful concen- 

 trations come from below by hypothesis ; and any ore-charged 

 sedimentary rocks which may have existed before those rocks 



* Credner lias remarked that the mineral matter of the veins in granite in 

 Saxony is not (immediately) derived from deep seated sources, but from the 

 partial decomposition of the adjacent rocks by the infiltration of water. See 

 " Die Granit Gange," Zeit d. Deutsch. Geol. Gesells., 1875. 



1 1 have a suspicion that some of the killas was already stanniferous before 

 the granitic (or at least the felsitic) eruptions began — as in the case of some of 

 the stock works already described, 



