340 M. Mohs on the Discovery of Useful Minerals. 



gin the actual search, which often requires a considerable 

 time if it is to be decisive. 



When the bottom of such a valley is not obscured by culti- 

 vation near its mouth, or generally in its lower portions, and 

 when the river allows of its channel being examined, such are 

 the most advisable points with which to make a beginning 

 in our examination. In particular, the loose masses in the 

 streams are, as it were, an index of the composition of the 

 valley ; for, not only do we ascertain from them what rocks 

 are to be expected, but we have also our attention directed to 

 the occurrence of particular repositories, when we find frag- 

 ments and blocks which consist entirely or in part of ores, or 

 of minerals which accompany them in their beds or veins ; and 

 the last are often the most important, inasmuch as they probably 

 indicate in what rocks the repositories occur from which the 

 ores are derived. We may at present pass over the conclu- 

 sions to be drawn from the sharp or blunt angles of the frag- 

 ments, and the more or less complete rounding of the masses, 

 for evei'y thing upon this topic which is not in itself self evi- 

 dent, may be found in all books which treat of this subject ; 

 but it must be remarked, that although the fragments and 

 masses give no indication of the occurrence of ores and useful 

 minerals, we must not, from this cause alone, unconditionally 

 assume their absence, or be deterred from proseci;ting the 

 search before it has actually commenced, for a number of 

 causes may prevent the appearance of fragments or boulders 

 of such minerals in the beds or on the banks of rivers, although 

 repositories of importance and value may probably exist in the 

 . district, 

 r Sometimes there occur in the valleys and on their decUvi- 

 ties, also on the ridges of the mountain-ranges, fragments and 

 blocks which are not derived from the district, but have been 

 transported from remote places. The mode of distinguishing 

 these from the others, and the judgment to be formed from 

 their presence, may be ascertained from their composition, 

 and from a comparison of their nature with that of the mineral 

 masses occurring in the district under examination. 



With the consideration of the fragments and the rolled 

 .ra&riJ moit «-«cs) r:i.j*^- S-n-o-.-c- ?,:t ti' r n; 



