1846.] SIIARPE ON Si/^fllJ CLEAVAGE. 93 



of the area where the cleavage is vertical or nearly so, the beds have 

 been broken up and thrown into great confusion '^. Along this sec- 

 tion the cleavage planes usually dip 20° or 30° more than the bedding; 

 but this is not general elsewhere, for in the middle of Devonshire 

 and Cornwall they are less inclined than the bedding. 



A remarkable point of contrast in the two sections 17 and 18 is, 

 that in this wide area we have only one axis of the cleavage, but 

 there are several anticlinal and synclinal axes of the stratification ; 

 these (with the exception of the central one at Rhaiadr Cwm) have had 

 no effect on the cleavage, which follows its own direction indifferently 

 through beds dipping in opposite directions. Still there is so much 

 relation between the direction of the cleavage planes and the posi- 

 tion of the beds, that we might infer from this section alone that the 

 cause which produced the cleavage of the rocks had helped to de- 

 termine the elevation of the beds. 



The area of elevation of Carnarvonshire and Merionethshire is 

 bounded on the N.E. by an irregular line reaching from the mouth 

 of the Conway to the junction of the Dee and the Calettwr about 

 three miles E. of Bala, with a general bearing of N.N.W. to S.S.E., 

 which is nearly at right angles to the direction of the axis of eleva- 

 tion. Commencing at the mouth of the Conway, it follows a great 

 fault which for about fifteen miles is nearly on the line of that river; 

 it then runs along or close to the Bryn-y-Ddinas dike : the rest of its 

 course has still to be traced a little S. of the Holyhead road through 

 a district not yet examined. To the N.E. and E. of the district 

 described, the cleavage planes mostly strike E. and W. in conformity 

 with the prevailing strike of the beds, which must be referred to 

 another area of elevation. The south-western boundary of our area 

 is lost in the sea, except from Clynog to Tremaddoc, where a line of 

 beds striking E. and W. forms its limit. 



Direction of the cleavage planes in Anglesea,- ,<? }»«!« 



It will be desirable to ascertain the direction of the cleavage planes 

 through the districts lying on both sides of the area already described. 

 For Anglesea I can only refer to the remarks scattered through 

 Professor Henslow's description of that island in the first volume 

 of the Transactions of the Cambridge Philosophical Society, from 

 which I gather that the strike of the cleavage is almost always to the 

 N.E., and that the most usual direction of its dip is to the N.W. 

 at high angles. But between Dulas and Llanerchymmed the laminae 

 are vertical, thence to the Menai they are mentioned at some places 

 as dipping to the N.W. In the chloritic slate at the S.W. point of 

 Carnarvonshire the cleavage dips at 50° to the S.E. These slates, 

 lying beyond the Snowdon line of vertical cleavage, must be referred 

 to the area of elevation of Anglesea. Thus there are indications of 

 another arch of the cleavage commencing on the E. side of the 

 Menai and including Anglesea. 



* This relation between the inclination of the cleavage and the position of the 

 beds appears to be very general, and has been observed in many other districts. 



