184-6.] SHARPE ON SLATY CLEAVAGE. 103 



present position to the beds of Carnarvonshire and Merionethshire 

 traversed by the section fig. 17, ceased to exert its influence at the 

 boundaries which limit the area of elevation of the cleavage ; the 

 lines of vertical cleavage serving to mark the boundaries both to the 

 cleavage elevation and to the elevation of the beds. Thus the ar- 

 rangement of the cleavage planes and the present disturbed position 

 of the beds, appear the consequences of one elevation ; which was 

 probably caused by fluid igneous matter forcing its way upwards 

 along a longitudinal rent below the crust of the earth. It is also 

 probable that the greenstones which rise along the centre and flanks 

 of the Snowdon chain are portions of that fluid mass. 



In accordance with the preceding reasoning, we find the lines of 

 vertical cleavage passing at the outside of the flank of the principal 

 mountains included within an area of elevation. Thus the line first 

 described runs just to the W. of the extreme flank of Snowdon : the 

 second line described passes between Plynlimmon and Cader Idris. 

 In the case of the Devonshire area this is less marked, still both the 

 vertical lines pass through ground lower than that on each side of it. 

 I have also some evidence to show that a line of vertical cleavage 

 runs close along the southern edge of the Lake Mountains, passing 

 near Coniston and Ambleside. 



Near Barnstaple there are two lines of vertical cleavage, the one 

 already described at Bickington, the other six or seven miles more 

 to the N. passing through Swinham Down ; and between them there 

 is some irregularity in the dip of the cleavage. So again on the S. 

 of Devonshire near the Start Point there seem to be two vertical 

 lines running parallel between one and two miles apart. It is pro- 

 bable that this will generally be the case. Each line must be con- 

 sidered as the boundary of its own area of elevation, and the narrow 

 band between as neutral ground, in which the dip will probably al- 

 ways be found irregular. 



I have not the means of judging whether Devonshire presents the 

 features of elevation here described ; as there are several parts of 

 that county where the dip of the beds has never been described, so 

 that no section across it yet published is really complete. But it may 

 be inferred from Sir H. T. De la Beche's account of the joints pre- 

 vailing in the granite of Devonshire and Cornwall that these counties 

 have been elevated since the solidification of the granite, by the up- 

 heaval of a mass below the surface whose axis extended from W.S.W. 

 to E.N.E. The prevailing divisional joints of the district are found 

 in two directions (with many exceptions which are stated) ; these di- 

 rections are about from N.N.W. to S.S.E. and at right angles thereto, 

 or from W.S.W. to E.N.E,* The latter of these lines is parallel to 

 what appears to be the axis of the elevation of the cleavage planes. 

 Mr. Hopkins has taught us to expect fissures both parallel to the axis 

 of elevation of an area and at right angles to it, so we may conclude 

 that the axis of elevation of these counties runs nearly from W.S.W. 

 to E.N.E., coincident with the axis of the elevation of the cleavage 



* Report on Cornwall, &c., p. 274. 



