M. Mohs on the Discovery of Useful Minerals. 359 



made in the upper rocky masses do\\Ti to the lower oues, if 

 we wish to be assured that no repositories of smaller extent 

 have been passed over. ■ ■ m 



If, as is the case with the coal-formation, we know the 

 rocky masses in which the useful minerals must occur, in so 

 far as these are present, and above and under which we would 

 seek in vain, it is plain that we only pierce these, and that we 

 do so in the same manner as in reference to a vei'y extensive 

 bed. 



If the stratification is trough or basin shaped, the only dif- 

 ference to be attended to is as to the direction of the lines in 

 which the bores are made, for these must converge towards 

 the middle line, or towards the centre. The shortening of 

 those operations, which have been here described to their full 

 extent, can be effected according to circumstances, and to the 

 opinion we may form, since we select from the parallel or 

 converging rows of bores one or several, which we consider 

 sufficient for our object. We thus become acquainted with in- 

 dividual sections : and although these can afford no perfectly 

 conclusive result, yet thej' are more to be trusted than sepa- 

 rate sections in the primitive and transition classes of rocks, 

 inasmuch as, Mith the exception of certain cases on the one 

 side and the other, the arrangement of these is seldom so re- 

 gular as that of the secondary class. 



By the mode now described, we acquire information as to 

 the presence or absence of repositories of useful minerals, but 

 in the case of the former, we obtain no satisfactory data for 

 judging of their value. In order to acquire this, and at the 

 same time to becoiue acquainted with the phenomena of the 

 repository at a point suited to future undertakings, Ave look out 

 for one point, or, according to cu'cumstances, several points, in 

 which we find the necessary requisites for mining operations, 

 and the fewest drawbacks which might prove troublesome. 

 At these points we investigate more closely the nature of the 

 repository, not only in reference to its products, but also as 

 to its extent in all directions, and its thickness, and we must 

 do this by means of bores, which must penetrate down to the 

 bottonj. The smallest number that can be employed is three ; 

 for, by means of three points no| lying in a straight line, the 



