of the Cumbrian Mountains. 65 



the effects of this action to be certain^ that it cannot be ever brought under 

 the mechanical laws of any constant force. To assume therefore, indepen- 

 dently of an incomparably wider induction from facts than has yet been made^ 

 that volcanic forces have acted on the earth during all geological periods 

 with equal intensiti/, is so far from being- sound philosophy, that it is a mere 

 gratuitous assumption — founded, it may be, upon a mistaken analogy be- 

 tween the calculated effects of constant forces, and the effects of other forces, 

 which are not only not constant, but which are unknown in their origin and 

 undefined in their power. 



Judging from facts alone, and independently of all hypothesis, I adopt some 

 of the leading principles of the Essay on the Epochs of Elevation by M. Elie 

 de Beaumont, to which I before referred. They are certainly open to nu- 

 merous exceptions ; especially in low geological regions like many of those 

 in our Island. But among mountain chains which are well defined and of 

 considerable elevation, they are, I believe, of very extensive application and 

 of unquestionable importance. The two systems of dislocations, described in 

 the second and third sections of this paper, mark two epochs of elevation which 

 have had a great effect in modifying the structure of this Island. 



Note. — I hope I may be excused in very briefly anticipating some of the 

 conclusions which seem to follow from a detailed examination of the Cumbrian 

 mountains. 



1. The crystalline structure of some of the slates appears to have been 

 superinduced after their deposit by an action from below. This seems to be 

 certain,, as far as regards a zone of indurated slates which surrounds the boss 

 of Shap granite ; and it is at least a probable explanation of the crystalline 

 structure of the lowest group of slate rocks in Skiddaw Forest. 



2. There are, in many large tracts of the Cumbrian system, slate beds with 

 a decided transverse cleavage, alternating with coarse flagstone and grey- 

 wacke, and not associated with any igneous or unstratified rocks. Similar 

 facts are repeated in North and South Wales on a more extended scale, where 

 the slates sometimes pass into true chlorite slate, alternating with other unal- 

 tered slaty masses. In such cases we have no right to assume that the crystal- 

 line structure has been produced by any modification of igneous action. Hence 

 it appears to be a rash generalization to attribute the crystalline structure of 

 all primary slate rocks to direct igneous action. This negative conclusion is 

 further confirmed when we consider that secondary deposits, by the mere 

 chemical action of their particles, unaided by any igneous cause, sometimes 

 assume a perfectly crystalline structure. For example ; many beds of moun- 



VOL. IV. — SECOND SERIES. K 



