Ch. XII.] NATURE OF STRATIFICATION. 24?7 



then are the primary slates of the same nature : that is, 

 that the primary rocks are not divisible into stratified and 

 unstratified ; and that the separation which has been made 

 on theoretical views, is not justified by the works of nature. 



Let us, then, enquire whence this difference of opinion has 

 originated. In the first place, we think that it will not be 

 difficult to show that much of this perplexity is to be attributed 

 to the structure of rocks; that is, to those lines or joints 

 which are more or less common to all, when perfectly consoli- 

 dated ; but which are not always visible in the older rocks, 

 until developed by a partial decomposition, and on that 

 account are generally most conspicuous in quarries, cliffs, and 

 other superficial sections. That an undue importance is not 

 attached to this cause, may be inferred from the lengthened 

 consideration given to it by Greenough, in his excellent essay 

 on stratification. 



In the chapter on the structure of the primary rocks, we 

 have endeavoured to show that if the fissile texture of the 

 slates be overlooked, that then these rocks would not be 

 found to differ in structure from the compact greenstones, 

 serpentines, porphyries, and similar unstratified or ignigenous 

 rocks with which they are associated: and that the layers of 

 these rocks, both schistose and compact, cannot in like man- 

 ner be distinguished from those of true granite, since the 

 principal joints of these rocks at their junction are commonly 

 inclined at similar angles to the horizon. We will not reca- 

 pitulate these details, but having referred to them, we will 

 rather proceed to enquire whether the fossiliferous rocks have 

 not a similar structure, and whether this is not sometimes per- 

 fectly independent of the operation of those causes by which 

 the deposition of these rocks was produced. 



In the transition or intermediate rocks, (such as occur on 

 the northern and southern adjacent boundaries of Devon and 

 Cornwall,) these lines of structure, crossing the beds in 

 various directions, are as distinctly marked as in the slate in 



