38 M. Mobs 1 Summary of Geognostical Phenomena, 



consist of small veins (gangtriimmern), and contributes gene- 

 rally to the ascertainment of the near and uninterrupted con- 

 nection which subsists among the whole series of repositories 

 of useful minerals. 



As veins are not confined to a particular mountain-mass and 

 its structure, they may, as already stated, be continued from 

 one mountain-mass into another ; and, as experience shews, may 

 actually do so several times. It is at the same time remark- 

 able, that, in the different rocks through which they pass, they 

 not only exhibit a change of character and of thickness, but 

 likewise of the minerals they contain, so that they might be 

 regarded as different repositories in one mountain-mass from 

 .what they are in another, were we not able to follow them from 

 the one into the other. But it must be added, that this is 

 not always the case, but that several veins are known which 

 traverse different rock-masses, and that without undergoing the 

 slightest change. There is another phenomenon connected with 

 this subject which deserves especial attention in a search after 

 veins. It consists in this, that veins often occur near the boun- 

 dary between different mountain-masses, partly in such a man- 

 ner that they only occur in one or the other of these mountain- 

 masses ; and partly in such a manner, that they pass from the 

 one into the other, and thus exhibit the appearances just before 

 mentioned. There are several remarkable instances of this kind 

 which are well known, and more might be discovei'ed if the 

 relations of superposition and of mountain-masses were better 

 investigated and better understood. 



Changes in Vein-stones and in neighbouring rocks. — There 

 are various important peculiarities of repositories of useful 

 minerals which have not hitherto been mentioned in this essay, 

 and which may remain unconsidered in this place. One 

 of these, however, requires to be shortly discussed, as it may 

 have an important influence on the discovery of such reposi- 

 tories, and more especially of veins. This consists in the 

 changes that the minerals, of which the vein masses are com- 

 posed, and even the mountain-rocks in their vicinity, undergo. 



There are often found in veins, impressions of crystals of mine- 

 rals which no longer exist, in other minerals that still occur in 

 the original placp of their production. There occur minerals 



