

SHARPE ON SLATY CLEAVAGE. 



101 



disturbing agent, and the connection between it and the effect it 

 has produced are obvious. 



A more difficult case is that of an elevated area which has not 

 been broken through in the middle by any great igneous eruption ; 

 where we can only infer what was the nature of the disturbing cause 

 from observing the effects it has produced. This is the case of 

 the Wealden elevation which Mr. Hopkins has illustrated. This is 

 also the case of the elevations of Carnarvonshire and Merionethshire, 

 and of Devonshire and Cornwall ; for though in all these counties 

 abundant eruptions of igneous matter have taken place, some probably 

 connected with the elevation we have to consider, these are secondary 

 pheenomena resulting from a wider-spreading cause ; and in Devon- 

 shire and Cornwall the great granitic eruptions do not form part of 

 the case under consideration, for they were evidently hardened and 

 solidified on the surface before the elevation took place which was 

 connected with the cleavage and gave it that symmetrical arrange- 

 ment we now find. 



The hypothetical case put by Mr. Hopkins, of a fluid mass below 

 the surface forced upwards along a rent bounded laterally by two 

 parallel walls, and covered over by the beds forming the crust of the 

 earth, is in itself probable, and will be found sufficient to explain 

 most of the phaenomena observed. The effects produced on the 

 mass above will depend on the width of the rent relatively to the 

 fluidity of the moving mass, supposing always, for simplicity of ar- 

 gument, that the fluid matter finds no means of escape between the 

 beds, but continues always to press upwards. If the rent is narrow 

 compared to the matter forced into it, the surface must give way in 

 a long crack through which the fluid will escape, as in the Malvern 

 Hills. If the rent is broader the fluid must force up the beds above 

 it in a curve, for the surfaces of the fluid mass must rise more in the 

 middle than at the sides. 



Fig. 22. 



Figure 22 represents a section of the supposed case; the lines 

 A B are the vertical walls of the fissure bounding a fluid mass which 

 can only escape upwards by displacing the mass above, of which the 

 part first opposed to it is shown in its original position by the line 

 C. The beds above will be successively raised into each of the 

 curves a c a: since the pressure is greatest in the middle of a fluid 

 column, and the resistance of the mass above is greatest at the sides 



