92 PROCEEDINGS OF T«& GEOLOGICAL SOCIETY. [DeC. 2, 



suggestion, as the lines appear to correspond to portions of curves 

 having a common axis. But, as each of the planes runs in the same 

 position for a great distance, if the curves were completed they would 

 represent a series of semicylinders turned over a common axis*. 



The arrangement of the planes of cleavage on so symmetrical a 

 plan is in itself deserving of attention ; but when their direction is 

 compared with the position of the beds over the same area, we find 

 coincidences too remarkable to be due to accident, and which lead 

 to considerations of great importance connected with the elevation 

 and disturbance of portions of the crust of the globe. The remarks 

 which follow will be made intelligible by reference to my map of part 

 of North Walesf, and to the sections figs. 17 and 18, in which the 



"^ ' Figs. 17 and 18. 



1. Snow don. 2. Rhaiadr Cwm. 3. Bala. 



minor irregularities are omitted and only the main features preserved* 

 to prevent the attention from being distracted by minutiae all subor- 

 dinate to the larger phsenomena. Fig. 17 represents the position 

 of the beds along a section of the country from the W. side of Snow- 

 don to the slate bed E. of Bala, fig. 18 the dip of the cleavage planes 

 along the same line of section. 



At Rhaiadr Cwm, which is on the axis of the elevation of all the 

 planes of cleavage of the area, there is an anticlinal axis of the stra- 

 tification along which the oldest rocks of this part of North Wales 

 are brought to the surface in a low flat arch J, which forms but a slight 

 feature in the country ; this now proves of great significance, as it 

 must be considered the true axis of elevation of the whole area, both 

 of the cleavage planes and the stratification. We cannot trace this 

 axis for more than a few miles either to the N.E. or the S.W., as in 

 both directions it is entangled in great masses of erupted igneous 

 rocks. In Professor Sedgwick's map of North Wales §, the line laid 

 down as the "great Merioneth anticlinal" is parallel to the axis at 

 Rhaiadr Cwm, but is drawn five miles to the S.E. of it. 



On both sides of this central axis, there is a relation between the 

 inclination of the cleavage planes and the elevation of the strata : 

 where the dip of the cleavage is at the lowest angle, the elevation of 

 the beds is most moderate and they are but slightly disturbed ; where 

 the cleavage dips at a higher angle, the beds are more elevated and 

 disturbed and their dip occasionally reversed ; and near the boundaries 



'*• * Although in theory we ought to regard the cleavage as arranged in curves, I 

 have continued to use the term cleavage jy/anes to which we are accustomed, from 

 dislike to introducing a new term, and because to our senses they appear planes, and 

 not curves. 

 t Quarterly Journal of Geological Society, vol. ii. pi. 12. xltefs'i airiT * 



X Ibid. vol. ii. pp. 291 and 293. § Ibid. vol. i..pL 1..- .^.-^ 



