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M. Mohs on the Discovery of Useful Minerals. 358 



lated to lead us to a certain result. Hence it is adA'isable to 

 follow the outgoing as far as possible (and it is a good symp- 

 tom if it permits us to pursue it for several hundred fathoms), 

 in order to enable us to observe it at a variety of places. If 

 there are alterations of the mountain-masses, we must particu- 

 larly endeavour to trace the outgoing into these different 

 masses ; for it has been often observed that veins in the dif- 

 ferent mountain-masses alter their nature so much, that, un- 

 less we were aware of the existing connection from observa- 

 tions made at different points, we should not be inclined to 

 view them as the same repositories. It is therefore possible 

 that a doubtful, that is an unpromising outgoing, may be of 

 such a nature in other mountain-masses, that not only the un- 

 certainty has disappeared, but that favourable prospects have 

 actually taken its place. But we cannot trust implicitly to this 

 consideration, for we know well that veins sometimes traverse 

 various mountain-masses without undergoing the slightest 

 change in their nature. 



If, from these, and probably from other observations and 

 phenomena which, from their diversity, cannot be all intro- 

 duced here, but which can easily be determined at the locality, 

 we have convinced ourselves that we have to do with a true 

 outgoing of a vein, we endeavour to trace it by the usual 

 means to a place where the repository can be examined in re- 

 ference to its mining value. It has been remarked in another 

 place, that in this investigation we ought especially to avail 

 ourselves of the assistance of trial-shafts, and the reason of 

 this is, that veins frequently extend downwards, for ten or 

 twenty fathoms, or even more, without exhibiting traces of 

 ore,, or indications shewing that the vein is worthy of the 

 miner"'s attention, while at gi'cater depths it opens out and 

 becomes valuable. However, it can.be easily understood that 

 the employment of shafts depends on the deviation from the 

 perpendicular of the surface of the rock ; for it is evident that 

 even when this is considerable, galleries for exposing or trying 

 the vein can lend useful assistance, and, in the selection of a 

 point for investigation, we must take all this into consideration. 



What has previously ))Ceu .said of the mode of procedure to 

 be followed for the discovery of repositories of useful mineral.? 



