Ch. XII.] NATURE OF STRATIFICATION. 255 



rocks to their peculiar concretionary structure, and not to 

 stratification. Indeed, occasions will frequently offer them- 

 selves, in the following pages, for this illustration : we shall, 

 therefore, conclude the chapter, with remarking, that this 

 view not only reconciles many of the incongruous opinions 

 which have been advanced on this subject by the most 

 eminent geologists ; but it offers us an explanation of those 

 parallel lines which often intersect formations, undoubtedly 

 produced at different times ; an occurrence that has caused 

 no little perplexity. And lastly, it points out, how a differ- 

 ence of opinion has frequently existed on a subject so appa- 

 rently simple, as whether a given rock is or is not stratified ; 

 for the term stratum has reference not only to the arrange- 

 ment of rocks in parallel layers by deposition, but also to the 

 tabular forms produced by joints or seams : and since this is 

 only a variety of structure, common in a greater or less de- 

 gree to all rocks, it clearly shows that it cannot be made a 

 ground of distinction, much less the fundamental basis for a 

 classification of rocks. The terms stratified and unstratified 

 are, in many respects, synonymous with the two kinds of in- 

 ternal structure, the massive or compact, and the schistose 

 or fissile : the former predominating in granitic, trappean, 

 and other rocks of this nature, commonly called igneous ; 

 the latter being more characteristic of the primary slates, and 

 of the secondary and tertiary deposits, which are of aqueous 

 origin. We have seen, that though each kind of internal 

 structure respectively prevails in certain series of rocks, yet 

 that neither is exclusively confined thereto : and thus it has 

 happened, that some granitic or unstratified rocks have been 

 pronounced to be stratified ; and some members of the strati- 

 fied have been called unstratified intrusive masses, though 

 intimately blended with, and perfectly enveloped in, the 

 former. We contend, therefore, that the various kinds of 

 granitic and primary slates have been unnecessarily separated 

 from each other ; that these slates are not stratified, in the 

 usual acceptation of the word, only differing from granite in 



