177 



the illusti-ation of his opinions. "Has the advocate," he says, "who would 

 account for all such dismemberments by long continuance only of existing 

 causes, whether by the active erosion of breakers on a shore or by atmospheric 

 action, ever satisfactorily accounted for the complete and entire denudation of 

 our clean-swept valleys of elevation? Let him inspect that British model of 

 such phenomena, the Silurian valley of elevation at Woolhope, first described 

 by myself. "What agency, I ask, except that of very powerful currents of 

 water, could have removed every fragment of the debris that must have re- 

 sulted, whether at one or several periods of elevation, from the destruction of 

 all the once superposed arches of rock, and have scooped out all the detritus 

 arising from such destruction, from the circling depressions, the central dome, 

 flanking ridges, and former cover of tho-e Silurian strata? And if that water 

 had not been impelled with great force, caused by sudden uprises of these rocks 

 from beneath the Old Red Sandstone, what other agency will account for so 

 complete a denudation, the broken materials having only found issue by one 

 lateral gorge, which was, we see, opened out by a great transverse fi'acture 

 of the encircling ridges." — (p. 492). It is impossible to deny the cogency of 

 these arguments; satisfied as we may be that deposition and denudation are 

 correlative, and are measures of one another, anxious, too, as we may be to 

 maintain the identity of former and existing causes, still we cannot be blind to 

 the fact that on the deep sea-bed little or no change is effected by the supcrin- 

 ciunbent ocean. But in judging upon the comparative merits of these two 

 rival theories we must not forget the hydrostatical law of the diminution of 

 weight of bodies in water as facilitating their more speedy removal. We must 

 always pay attention in this particular case to the inclination of the axis of 

 elevation as afi'ording an easier field for submarine currents to work upon ; as 

 the plane A B, Fig. 1, moved up towai'ds the surface, it continually presented 

 some unprotected protuberance for the sea to exert its force upon, and held 

 out a challenge to it to level it to the horizontal plane A D. We must also 

 remember the mighty results of the glacial epoch, and the gradual removal of 

 the surface of the earth above the level of the glacial sea, while the inner 

 valleys were being eroded. There must have been a long period when the dome 

 and its two lines of circumvallation alone stood out of the water, and the pre- 

 sent basins of the Leadon and "VVye were united in one sea, when resistless 

 waves and currents poured through the Cockshoots, and through the fissures 

 in the Wenlock ridge, working everywhere as on coast-lines, dragging down 

 masses of dihris to the Mordiford gorge, and depositing them where they met 

 the ocean driving in. There must have been a time, too, when the glacial 

 epoch ended, and swollen rivers took the place of these sea-cuiTents, to follow 

 in the same course, and complete the work that had been begun, by pouiing 

 down their sediment-laden tribute to the Wye chain of lakes. 



The Mordiford gorge has lately had a reminiscence of these denuding 

 processes. The following accoiuit is from the Hercfmd Joanuil for May 29th, 



