1846 LETTER TO LYELL 89 



tion, but simply refer to the phenomena exhibited in a 

 given district which came during the progress of the 

 Survey under my especial notice. 



First, With regard to the disturbance and upheaval 

 of rocks. You will observe that in different parts of 

 this essay I recognise two distinct species of pheno 

 mena, one that of violent and extensive disturbance 

 on a gigantic scale, and another that of slow upheaval 

 and depression, such as we now have experience of on 

 the shores of the Baltic and elsewhere. With regard 

 to the latter, I grant that any possible height may be 

 attained by an unlimited succession of comparatively 

 small elevating disturbances. But in reference to the 

 former I cannot conceive that the early forces that 

 affected the Palaeozoic strata in the district treated of, 

 and which I believe then elevated these strata, were at 

 all of the same intensity as those that in later periods 

 occurred in the same tracts. You may raise to a 

 given height a certain bar of iron with your finger, but 

 no succession of the same forces, however numerous, 

 could crush that bar laterally like a plaited frill. A 

 more sudden and powerful effort is requisite. Such I 

 believe to have been the case with the Mendip Hills, 

 viewing them solely as part of a much wider area. 

 But even granting this to be the case, it does not 

 follow that I therefore restrict the period required for 

 denudation. In the case referred to (the Mendips) 

 you will observe that I state, p. 320, that a mass of 

 limestone, etc., once existing above part of the Mendip 

 Hills, to an extent of at least 6000 feet high, had been 

 removed by the denuding agency of the New Red sea, 

 or possibly by that sea and the earliest Liassic waters, 

 since we find the lower beds of the Lias resting 

 horizontally on the upturned water-worn edges of the 



