216 PREVAILING THEORY CONCERNING [Ch. XL 



tain chains ; and in the prosecution of this interesting enquiry, 

 he has been led to conclude that all chains of the same date 

 are parallel to each other. This new branch of geological 

 speculations was, at first, almost generally received ; but, on 

 more mature consideration, many very formidable objections 

 have been suggested. We shall have occasion to discuss this 

 subject hereafter : it need only be now mentioned, that there 

 appears to be but one opinion, that these mountain chains 

 have been elevated at different periods, by the operation of an 

 internal igneous agency; but all are not agreed as to the 

 manner in which this has taken place. M. Elie de Beaumont 

 supposes these elevations to have been produced by sudden 

 and violent convulsions ; whilst, on the other hand, Conybeare, 

 Lyell, and others, attribute them to a gradual, gentle, and 

 protracted upheaving, continued without interuption during 

 the whole period of the formation of the elevated strata. 



According, therefore, to this modification of the theory, 

 the granite must, in some instances, have been protruded in 

 a solid state, subsequent to the alteration or metamorphosis 

 of the adjacent strata ; for it happens, that whilst the lowest 

 strata are gneiss, clay- slate, or rocks analogous thereto, the 

 upper, though equally inclined, and also adjacent to the 

 granite, have not undergone any analogous change. Thus, 

 Sedgwick and Murchison, in their observations on the isle of 

 Arran*, say, "that the great dislocations of the secondary 

 strata of this island were produced by the elevation of the 

 granite, and that the upheaving forces must have been in 

 action some time after the deposition and consolidation of the 

 new red sandstone : that the granite could not have been in 

 a fluid state, at the time of its elevation ; for, had that been 

 the case, it could never have risen into lofty mountains, and 

 mural precipices, overhanging the secondary strata, without 

 overflowing their broken edges, or penetrating their mass in 

 the form of dykes." The same experienced geologists are 

 also of opinion, that the Ord of Caithness f was likewise pro- 



* Geol. Trans, vol. iii. p. 35. New Series. f Idem, p. 354. 



