128 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 1, 



ries, it might then be conjectured that the hard vertical bands of 

 porphyry had uiterrupted the regularity of the pressure, and had 

 caused that want of order in the arrangement of the cleavage which 

 is so remarkable an exception in this district to its usual symmetry. 

 In either case the co-occurrence of faults and lines of perpendicular 

 cleavage points to a mechanical cause of the phsenomenon. 



It is a singular feature of the Lake district, that though the for- 

 mations strike about E.N.E., and the planes of cleavage usually 

 follow the same direction, the great bands of porphyritic rock run 

 nearly east. The physical geography of the district also presents a 

 remarkable feature connected with this subject, which is that most 

 of the valleys and great mountain ridges run nearly north, and very 

 rarely follow the strike of the strata, or cross it at right angles ; 

 their usual direction being at right angles to the bands of porphyry. 



Thus there seem to have been two great periods of elevation ; the 

 one when the porphyries were forced through long parallel chasms 

 running east, and those great rents in the surface were formed nearly 

 at right angles to them, which are now the lines of the great valleys 

 running north ; the other connected with the eruption of the sienites, 

 and probably also of the granites which have thrown up the slates as 

 we now find them with a strike of E.N.E., and have given the same 

 direction to the cleavage, but without effacing the leading physical 

 features of the district. I have not studied the country enough to 

 go more at length into the details of this difficult subject, but I 

 could not omit the mention of it, as it has an indirect bearing on my 

 present object, and has great interest in itself; and has, I believe, 

 been hitherto overlooked. 



Conclusio7i. — The following is a general summary of the conclu- 

 sions drawn from the observations detailed in this and the preceding 

 paper. 



The direction of the cleavage-planes is in direct relation to the 

 movements of elevation of the strata, being everywhere at right 

 angles to the direction of the elevating force ; and where the beds 

 have been raised with regularity over a single axis, the cleavage- 

 planes appear to be portions of curves, of which the width of the area 

 of elevation is the diameter. 



In slaty rocks there has been a considerable compression of the 

 mass of rock between the planes of cleavage ; that is, in the direction 

 corresponding to that of the elevating force ; this compression being 

 shown by the distortion of the included organic remains, and the flat- 

 tening of the component portions of the rock, and bearing a propor- 

 tion to the degree in which the cleavage is developed. 



The compression of the mass in a direction perpendicular to the 

 cleavage has been partially compensated by its expansion along the 

 dip of the cleavage, in which direction only its expansion was per- 

 mitted as the elevation of the beds enlarged the area occupied by 

 them. The difference between the amount of compression in one 

 direction and of expansion in another, is accounted for in the greater 

 density of the rock after compression. 



No connection has been detected between cleavage and crystalliza- 



