S>44 M. Mohs on the Discbveri/ of Useful Minerals. 



The surface of superposition itself: if it presents itself as 

 a real rock-separation, that is, a true distinct concretion 

 surface {zusamtnensetzungs fldche) ; or if the rocks of the 

 masses on both sides are sharply and strongly united Avith each 

 other ; or if they pass into each other, and in what way they 

 do so ? Thirdly, If the appearance of another rock is combined 

 with these passages ? We know that, on the boundaries of 

 two mountain-masses, conglomerates often occur, by means 

 of which the passage takes from the one to the other ; or, 

 that, in place of these, layers of clay, &c. present themselves. 

 But we also know that repositories of useful minerals often 

 occur between two diiferent rocky masses ; and hence, although 

 we may not find such minerals themselves, yet we must take 

 care to ascertain if Avhat we actually discover may not be con- 

 nected in some way or other with repositories of the descrip- 

 tion mentioned. It is plain that, for this purpose, a very exact 

 knowledge is requisite of these repositories, especially of their 

 barren masses (" ihrer taiiben viitteV) and otherwise unpro- 

 ductive portions which generally remain unnoticed ; and it is 

 likewise evident that, when we believe ourselves in some de- 

 gree justified in suspecting something, we must endeavour to 

 ascertain as much, antl to inform ourselves as far as is possible, 

 Avithout actually beginning the operations of a search. Here, 

 also, we ought to notice the shoots of one mountain-mass into 

 another, and also the frequent alternation of small beds, ere 

 the succeeding mass appears in full size. Fotirthh/, The con- 

 dition of freshness or weathering of the rocky masses on one 

 or on both sides of the surface of superposition ; and, lastly, 

 Fifthly, The position and direction of the six. face of super- 

 position itself This surface is rarely an even one, and indeed 

 often deviates considerably from it. We must not, therefore, 

 at least every where, take for granted the regular bed-like 

 form of the mountain-masses, even though we should find 

 them in a perfectly conformable position, that is, the struc- 

 ture of the rock and the mountain-masses (the last being 

 Avhat is termed stratification) parallel to the surface of su- 

 perposition, but we must wait for further observations, and 

 combine them properly with one another, in order at last to 



