Structure of Slate-Bocks, 7 



in the usnal cases, the masses of the slate rocks are just M 

 little disturbed by the granite as if that rock differed from the 

 slaty mass only in structure. 



Structure of Slate-Bocks. — The various relations exhibited 

 in the arrangement of the beds of the slate-rocks, are the most 

 important which can be ascertained, in order to aid the search 

 for useful minerals ; for these are the rocks which chiefly con- 

 tain such repositories. 



These relations, when viewed on the large scale, consist 

 more especially in the following : — That the various masses, 

 which constitute the slate rocks, generally preserve through- 

 out the whole mountainous tract a uniform direction (a si- 

 milar strike) ; and that the deviations from this direction, 

 which are frequently met with, and which often amount to 

 a considerable number of degrees, mutually neutralise one 

 another, as they are just as much on the one side as on the 

 other. This remarkable phenomenon sometimes even exists 

 on a larger scale. It is frequently observed, that in neigh- 

 bouring mountain-groups, that is, such as are separated from 

 one another by less elevated tracts (plains in the sense men- 

 tioned above), the general direction is identical or similar ; 

 and most mountain-groups are thus united into a whole, or 

 as it were into a unity, which is the largest that is reached 

 by geognosy in considering the constitution of mountain-masses. 

 But we must beware that we do not extend this process of 

 generalization beyond its limits, at least that we do not extend 

 it over the whole earth, that is by assuming that this direction 

 is the same round the whole earth. 



Classes of Bocks. — The slate-rocks consist of a considerable 

 number of mountain-masses, which are regarded by geognosts 

 as belonging to different formations, and separated by them into 

 two classes, of which the one is termed Primitive and the other 

 Transition. The mere name of the latter class, which presup- 

 poses a third, and which, as derived from the nature of the moun- 

 tain-rocks and of the mountain-masses, is quite correct as a 

 mere name, leaves it to be understood how it stands in respect 

 to the division of classes. When we have two unities (two 

 species or genera in the mineral or vegetable kingdom, or two 

 cImms in the geognostical systems), which are combined by 



