40 M. Mohs' Summary of Geognostical Phenomena. 



of this difference. The phenomena which, according to what 

 has been formerly said, can be regarded as nothing else but 

 original productions, must be well distinguished from those, 

 which are consequences of changes that have taken place in 

 the original condition of the repositories themselves and of 

 their minerals. To the former belong the more or less consider- 

 able beds of a clayey nature, in which only here and there, either 

 detached imperfectly-formed portions of the rock, or at least of 

 the structure of the mountain-masses in which they occur, can be 

 recognised ; and there are vein-like repositories, pretty much of 

 the same description, which seem to be filled with, as it were, 

 softened and compressed fragments of the neighbouring rock, 

 and which arc generally regarded as such like upfillings. Al- 

 though the connection in which they stand with the pheno- 

 mena already mentioned points to a different origin, we find, 

 almost exactly as was there shewn, repositories of both kinds, 

 which consist chiefly of clayey materials, from which here and 

 there more compact masses have as it were been separated, 

 which are occasionally metalliferous, and sometimes, indeed, 

 very rich. It is very easy to judge incorrectly regarding the 

 true nature of such formations, to regard them as mechanical 

 productions, and thus to be deterred from prosecuting their 

 farther investigation ; and hence, also, these require a careful 

 study. 



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Surface relations of the district to be examined. — Since the 

 operations directed to the search for valuable repositories of 

 useful minerals can only be prosecuted on the surface of rocks, 

 that surface, in which such investigations are to be carried on, 

 becomes an important object of consideration ; and is even the 

 very object with which the whole business is to commence. 

 The general relations of the surface of mountain-groups are 

 very easily made out. They are chiefly founded on the posi- 

 tion and characters of the valleys and the elevations included 

 between them. Most valleys, or at least the most important 

 ones, take their origin in the highest parts of the mountain- 

 groups, frequently in the numerous ravines, and their direc- 

 tion follows the declivity of the group, until an obstacle oc- 

 curs, produced, either by the nature of the rock, or by the 



