Ch. XIV.] OF THE STRATIFIED ROCKS. 323 



to produce fusion, be applied to the ends of the greywacke 

 beds, the various substances of which they consist would have 

 a tendency to resume their original state, at least that state 

 in which they existed in the crystalline rocks whence they 

 have been derived ; and, consequently, we should have com- 

 pounds resembling various crystalline stratified rocks." * 



Now, before this explanation can be admitted, it must be 

 proved that the granite has been thus protruded through the 

 strata over an extent of about twenty miles in diameter, and 

 that crystalline slates, which once existed, have disappeared, 

 after having provided materials for the structure of these 

 metamorphic and unaltered masses of greywacke. It would 

 be more consonant with reason to conclude that the fragments 

 in the greywacke have been derived from the existing crys- 

 talline strata which they resemble, than to have recourse to 

 rocks of which we have no knowledge, except through the 

 medium of the Plutonic hypothesis : for such a proceeding is 

 neither according to the general rules of induction, nor ac- 

 cording to the acknowledged principles of geology, by which 

 the relative ages of rocks are determined. Thus it has been 

 observed by Lyell, that, " in investigating a district composed 

 of two distinct formations, it is sometimes difficult to ascer- 

 tain their respective ages, from want of sections exhibiting 

 the order of their superposition. In such cases, another kind 

 of evidence, of a character no less conclusive s can sometimes 

 be obtained. One group of strata has frequently been de- 

 rived from the degradation of another in the immediate 

 neighbourhood, and may be observed to include within it 

 fragments of such rocks; from which we may confidently 

 infer that the group from which the fragments have been 

 derived, is the oldest of the two formations."f Thus, " there 

 can be no doubt that some granites are more ancient than 

 any of our regular series which we identify by organic re- 

 mains, because there are rounded pebbles of granite, as well 



* Geological Manual, p. 479. f Principles of Geology, vol. iii. p. 36. 



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