338 M. Mohs on the iDktot^fi) of Useful Minerats. 



example, although the occurrence is comparatively a rare one, 

 that the rocky masses cross over from one side to the other- 

 The longitudinal valleys, therefore, present but little oppor- 

 tunity of making observations ; and hence the last-mentioned 

 points are in general the most important, at least in slate 

 mountains. 



It follows from these considerations that, in the above-men- 

 tioned mountain -masses, the longitudinal valleys, as they 

 neither cut across the stratification, nor the structure of the 

 masses which constitute the district, at considerable angles, 

 are not particularly suited to geological investigations ; and 

 hence, that the openings of the transverse valleys are the 

 places which, in such valleys, are more especially to be inves- 

 tigated. In making this remark, however, we must except 

 particular basin or trough shaped widenings, which extend 

 from the bottoms of such valleys towards their sides, and in 

 which we not unfrequently find peculiar formations, such as 

 coal tracts, &:c. ; these are easily recognised by the rocks of 

 which they are composed, and are with facility distinguished 

 from the mountain-masses which actually form the sides of 

 the valley. 



We must not, however, be understood to mean, that the 

 remaining portions of the longitudinal A'^alleys are to be passed 

 over unexamined. Nothing should remain unexamined, for 

 discoveries are often made where they are not anticipated or 

 surmised ; and although we know that there is but little al- 

 ternation of rocks, and therefore but little opportunity of dis- 

 covering the outcrop of beds, yet we do not know but that veins 

 may run through the longitudinal valley, whose outcrop, as 

 they for the most part cut the structure of the mountain-masses 

 and the direction of the stratification at large angles, is more 

 easily discovered there than in the transverse valleys. It is 

 self-evident how we are to apply what we have already said, 

 and part of what follows, to valleys, the rocks composing which 

 possess no slaty structure. 



Notwithstanding this, in the examination of mountains, or 

 a mountainous district, with the view of discovering available 

 repositories of useful minerals, our attention is more particu- 

 larly directed to the transverse vallej'S, which, at least in longi- 



