M. Mohs ^n the Discovery of Usefnl Minercds. 2^1 



an investigation must go into detail ; but, at the same time, 

 it must be undertaken in such a mannei* that a general result 

 shall follow : that is, we must not only be careful that nothing 

 of any consequence escapes us, but also that every thing shall 

 be brought into proper connection. A well imagined plan 

 •will render very essential aid, at all events in saving time and 

 expense ; but this cannot be executed without an accurate and 

 special knowledge of the district imder observation, and this 

 can only be acquired by the mode which I have pointed out. 

 In general, it is most advantageous to begin with a longitu- 

 dinal valley, and to select the largest and most extensive ; 

 one also which cuts through the mountains, and in which the 

 transverse valleys terminate that present themselves on the 

 declivities of these mountains. But here there are two dif- 

 ficulties in the way, which, in many cases, are not without 

 importance, and which must be attended to. The bottoms 

 of these broad, and open valleys are, for the most part, co- 

 vered by cultivation, and the river and roads occupy a con- 

 siderable portion of their surface. Natural and artificial 

 causes have also, frequently, so altered the acclivities of 

 'these valleys, that they present but little opportunity of 

 making observations ; and the only point of view in which 

 we can make use of them, under such circumstances, con- 

 sists in this, that we collect information regarding disco- 

 veries which have been made accidentally during the construc- 

 tion of roads, houses, fortifications, or wells, during quarrying 

 operations, &c., or in consequence of great natural occun-ences. ' 

 Not one of the quarries of the district should remain unex- 

 amined ; but the information collected must be used with 

 caution, and we must endeavour to convince ourselves of its 

 truth or probability, by weighing carefully the circumstances 

 and relations which we can ourselves observe. The second 

 difficulty is of much greater moment ; the longitudinal valleys, 

 in slaty mountains, frequently run in one and the same moun- 

 tain mass, and follow its direction without cutting its struc- 

 ture at considerable angles; or, what is always more frequent, 

 they run along the boundary of two mountain-masses, in such 

 a manner that one of the declivities consists of the one moun- 

 tain-mass, and the opposite of the other. It is not without 



