Ch. X.] ON THE PRIMARY ROCKS. 209 



allude : viz. the apparent fragmentary nature of some primary 

 rocks ; and the phenomena of veins. 



Concerning the rounded or conglomerated, and the angular 

 or brecciated appearance of these rocks, no repetition of 

 details need be here given, as a connected description of 

 them will be found in the sixth chapter : but it may be re- 

 marked, that both the conglomerates and breccias of this 

 kind occur in the granitic as well as the schistose groups. So 

 that, if these fragment-like rocks be derivative and detrital 

 when interstratified with the primary slates, as is generally 

 supposed ; they must be of the same nature, when connected 

 with the granitic rocks : and since their alternation with 

 these slates, has been received as a confirmation of the de- 

 rivative nature of the latter, it ought to apply with equal 

 force to the granitic rocks which we are taught to regard as 

 of igneous origin. If we are compelled to consider this as 

 inadmissible, because it appears more probable that, in the 

 case of granite and volcanic rocks, these fragment~like forms 

 are modifications of the concretionary structure ; then the 

 same solution ought to be extended to those primary slates 

 that are similarly circumstanced. We do not argue here for 

 either mode of origin ; our intention is only to. state that 

 analogous appearances occur both in the granitic and schistose 

 groups, which also in every other respect resemble each 

 other; and if this be a correct conclusion from the facts 

 recorded in the preceding chapters, the conglomerated and 

 brecciated structures of the rocks of either group would 

 seem to demand one and the same explanation. 



Nor need any recapitulation be offered concerning veins, 

 as these are separately described in the last chapter : but, it 

 may be observed, that the granite-veins and elvan-courses, 

 though treated of in other chapters, exhibit precisely the same 

 phenomena. All these kinds of veins, traverse both granite 

 and slate, or are contained in either ; they intersect many 

 layers of rocks, or are confined to one layer, or, indeed, to an 

 individual concretion, according to the size and position of 



p 



