94 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. % 



Direction of the cleavage planes through Pembrokeshire^ Cardigan' 

 shire and Radnorshire. 



Mr. Davis has favoured me with a few observations made in cross- 

 ing from Cader Idris to Rhaiadr in Radnorshire, showing the cleavage 

 vertical about a mile S. of the head of Tal-y-Llyn with a strike to 

 the E. (which is a local anomal}'^) ; the dip then continues towards 

 the N.W., the angle diminishing in six miles from 80° to 70° ; near 

 Rhaiadr he again observed it dipping in the same direction 65°, and 

 farther on 45°. The observations are unfortunately few, but they 

 make it probable that from the Mallwyd line of vertical cleavage to 

 Rhaiadr, a distance of about thirty miles, we have a dip to the N.W. 

 gradually diminishing to an angle of 45°, and that a central axis of 

 this area will be found a little to the S.E. of Rhaiadr. In the sections 

 across Pembrokeshire and Cardiganshire of the Geological Survey 

 of Great Britain, the cleavage planes are represented dipping every- 

 where to the N.W. at the uniform angle of 80°. Along the line of 

 strike the cleavage planes usually dip in one direction at the same 

 angle, but in crossing that line (as in these sections), all other ob- 

 servers have found the angle of dip to vary. As it is hardly proba- 

 ble that this one district should form an exception to so general a 

 rule, I conclude that the lines in the sections of the Survey are to be 

 taken as a conventional manner of expressing slaty rocks, and are 

 not intended to show the angle of dip of the cleavage. 



Area of elevation of Devonshire and Cornwall. 



The prevailing strike of the cleavage planes over a large part of 

 Devonshire and Cornwall is from W.S.W. to E.N.E., and the usual 

 dip is consequently either to the N.N.W. or the S.S.E. : to this there 

 are many exceptions, but they are trifling compared with the cases 

 which agree with these directions. In the western half of Devon- 

 shire and eastern end of Cornwall, to which the following remarks 

 apply, the most usual strike of the beds is from E. to W. or from 

 E.S.E. to W.N.W. ; so that it is rare to find the direction of the 

 cleavage coinciding exactly with that of the bedding. This has been 

 noticed by several observers whose evidence I am glad to quote, as 

 the case is not common*. 



Following the plan already pointed out, our first search must be 

 for the lines in which the cleavage is vertical. Sir H. T. De la 

 Beche's Report helps us to the southern vertical line, which passes 

 a little to the N. of the mica and chlorite slates of Start Point and 

 Bolt Point. There appear to be two vertical lines running at the 

 distance of about three miles apart, the more northerly near Stoke 

 Fleming, the other near Stokenham, with some confused lamination 

 between them : to the N. of the Stoke Fleming line, the dip of the 



* Viz. at Baggy Point and Croyde, by Professor Sedgwick and Sir R. I. Mur- 

 chison ; Geological Transactions, 2nd Series, vol. v. p. 648. At Brixham, by Sir 

 H. T. De la Beche ; Report on Cornwall, &c., p. 45. At Fremington, North Devon, 

 and in the limestone of South Devon, by Mr. Austin ; Geological Transactions, 

 2nd Series, vol. vi. p. 482. 



