M. Mohs on the Discovert/ of Useful Minerals. 343 



frequently first rendered aware by this circumstance of the 

 change of rock. The fragments of the rocks on the two sides 

 of the position indicate where we are Hkely to discover them. 

 They are then to be sought for accordingly in the declivity. 

 If we are not so fortunate as to meet with exposed points, we 

 still can find out the places where the surface of superposi- 

 tion can be uncovered with least difficulty. We are to follow 

 this mode of procedure at as many points as may seem neces- 

 sary to convince us that we have become sufficiently acquainted 

 with them, and ascertained theii* natm'e. The circumstances 

 and relations to be attended to are chiefly the following: — 

 First, The position of the structure of the rock masses on both 

 sides, on the great and small scale, relatively to the surface of 

 superposition ; if it is parallel to it, or if one or the other in- 

 tersects it ; if it is bent ; if it approaches the other, or if it un- 

 dergoes any kind of dei'angement ;* likewise the condition of 

 the rocks in the neighbourhood : if they are unaltered, or if 

 they are altered, and in what respect ; if they contain mine- 

 rals which were not previously found in them, &c.t Secondly, 



* In certain kinds of mountain-masses, a state of disorder is assumed to 

 be almost universal, in respect to tbe structure of the slaty rocks -with which 

 tLcy come in contact ; although the observations made upon them seldom 

 contain any thing of the kind, and indeed only in cases where the phenomena 

 can be explained by structure alone. 



t It is customary to attribute the presence of such minerals, for example 

 garnet and some others, to the plutonic influence of certain granular moun- 

 tain-masses, that are in contact with the slates in which these minerals aie 

 found (sometimes, however, also in the gi-anular rocks). It is doubtful, how- 

 ever, if such an explanation can be applied to ores, pyrites, glances, and 

 blendes, which are enimicrated as occurring in such situations, and the ap- 

 pearance of these minerals, under such circumstances, may, of itself,contribute 

 to diminish the probability which wo might otjierwise bo persuaded to assign 

 to this explanation, in so far as regards garnet, analcime, &c. On the other 

 hand, the occurrence of these, especially wlien they are found in both rocks, 

 belongs to tiie proofs of simultaneous formation, of which some have been al- 

 ready given in tlie previous part of this treatise. (Lyell, in his Principles of 

 Geology, 3d edition, vol. iv. p. 286, says, " It is also an important circum- 

 stance, that, near the point of contact, both the granite and the eecondciry 

 rocks become metalliferous, and^'contain nests and small veins of blende, 

 galena, iroji and pppper pyrites.") 



