Remarks on Dr. Boast's Primary Geology. t 551 



intended to assert that they are all of the same nature, and referrible 

 to precisely the same geological epoch : on the contrary, it is wished 

 for the present to avoid all conjectures concerning the nature of 

 their origin, and only to express that these slates do, as in Cornwall, 

 bear a certain relation to the granite with which they are associated : 

 and consequently all slates, both foreign and Cornish, which have 

 the same relative connection and position with the granite, may be 

 regarded as parallel or equivalent rocks." 



The 6th Chapter contains an investigation of the varieties 

 of structure, both external and internal, which is found in the 

 various primary rocks, both granitic and schistose* 



In the 7th Chapter, 



" The relative position, and the nature of the granitic and schis- 

 tose rocks at their junction," 



is considered, and after shewing that an unconformable posi- 

 tion of the schists incumbent upon granite is common, and 

 they are known in many cases to abut against the granite with 

 the seams of the lamination : it is proved that at the junc- 

 tion not only is the structure of the schists and their mineral 

 composition much modified, but that the granite also is 

 changed in a similar manner \ and the author remarks, 



" In short, the granite and slate of Cornwall, at their junction, 

 are frequently so similar, both in composition and concretionary 

 structure, that the detached blocks, as they lie side by side, cannot 

 at a little distance be distinguished from each other ; the darker 

 colour of the slate, and its tendency, often slight, to break into 

 laminae, are sometimes the only differences to be detected on a closer 

 inspection." 



And the same remarks are shewn to be applicable to the 

 granitic and schistose rocks of other countries also. 

 Of the 8th Chapter, the purpose is thus stated : 



" Having detailed in the preceding chapter the nature and position 

 of the schistose rocks at their point of contact with granite, it is 



