Transitions of Bocks. 13 



transition in the direction of the thickness. But the occurrence 

 is often met with of transitions in the direction of the strike and 

 of the dip. When we are occupied with a bed of gneiss, and ad- 

 vance in the direction of its strike, we not unfrequently find 

 that the rock is gradually changed into mica-slate, into clay 

 or hornblende- slate, or even into a (sometimes red) sandstone. 

 Transitions of the last-mentioned kind are, however, more fre- 

 quent in the direction of the thickness. Since we know how 

 different are the features of repositories of useful minerals 

 in different mountain-masses (although this is certainly not 

 without exception), we can easily understand with what care 

 we should observe the transitions in the direction of the 

 strike and dip ; and how little confidence should be placed 

 in sifigle sections, which have been prepared to assist the in- 

 vestigation of a district, although they have been sketched with 

 all possible exactness, and which, taken by themselves, are 

 therefore quite accurate ; and hence, that it is indispensably 

 necessary to become possessed of as many of these sections as 

 possible. Thus, no lateral valley should remain imexamined 

 in reference to such transitions ; and if the mountain ridges 

 between them possess too considerable a breadth, they also 

 should be sufficiently investigated, so that all means may come 

 into operation which serve, or at least can give assistance, in 

 accomplishing a task of so uncertain an issue, with that de- 

 gree of precision which is necessary, if a decisive result is to 

 be expected. *•»***** 



Bocks subordinate to the Slate Series. — The slate series, 

 which we have hitherto been considering according to the 

 relations that are presented in reference to the discovery of 

 useful minerals, is, as it were, the general repository of 

 several other mountain-rocks, which appear imbedded in 

 it ; and a knowledge of which, as well as of the phenomena 

 of the arrangement of their beds, is just as indispensable 

 as that of the slate series itself. Some of these agree so 

 perfectly with the slate rocks, from which they are separated 

 by some observers, and are so directly connected with them, 

 that, in a purely geognostical point of view, they need 

 hardly be enumerated. These are the masses of talc-slate 

 and chlorite-slate, which generally occur in great variety in 



