310 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



North Wales in which the parallelism between the strike of the 

 bedding and cleavage appears in the most marked degree, is all that 

 part of Carnarvonshire which lies north of Tremadoc. Throughout 

 this area, on the south of the Holyhead road, the beds as well as the 

 cleavage planes strike north-east. On the west side of the Snowdon 

 anticlinal the beds are much tossed about, and dip at various angles 

 either north-west or south-east, and the cleavage planes are nearly 

 vertical. On the east side of the anticlinal the beds dip south-east, 

 and the cleavage dips north-west from 60° to 65°, On the eastern 

 side of the Carnarvonshire synclinal the beds dip north-west, as does 

 also the cleavage, but at an angle which gradually diminishes as you 

 recede from the Snowdon chain. Thus at the Rhiw Brefder quar- 

 ries the angle is 55° ; at the Diffwys quarries it is 45°, and at Manod- 

 mawr 35°. Towards the northern extremity of the county of Car- 

 narvon the strike of the beds changes from north-east to N.N.E., as 

 does also the strike of the cleavage. To the south of Tremadoc 

 the beds change in strike from north-east to east, and the cleavage 

 changes in strike from north-east to E.S.E. 



The parallelism in the strike of the planes of bedding and clea- 

 vage prevails also, in a marked degree, in the slaty district north of" 

 the Dee, and also in the North Berwyns. The common strike of the 

 two planes approaches to east ; but it is subject to many local varia- 

 tions ; and in such cases the two planes vary in strike together. The 

 cleavage has a northerly dip, at angles varying from 25° to 65°. 



In the Barmouth chain the strike of the cleavage is somewhat irre- 

 gular ; but its mean direction is north and south, and its dip is from 

 east 60° to west 60°. 



In the district intersected by the great porphyritic eruption of 

 Arenig, Arran Mowddy, Szc, the planes of cleavage have lost their 

 original bearings, and are subject to the greatest irregularity both in 

 respect of direction and dip ; and the same observation applies to 

 the district of Lower Silurian rocks extending along the Holyhead 

 road between Bryn-y-ddinas and Corwen. 



From the circumstance that the position of the planes of cleavage 

 depends, not on the varying position of the beds at each particular 

 spot, but on their main position, the author infers that slaty clea- 

 vage cannot have arisen from any power analogous to that of crystal- 

 lization ; and from the almost mathematical regularity with which 

 those planes are arranged, he concludes that they are not the effect 

 of mechanical force or pressure exerted at the moving or upheaving 

 of the rocks. 



The author further concludes from his observations, that in those 

 parts of North Wales where the strata are least disturbed, the planes 

 of bedding and cleavage meet at an angle of from 15° to 30° ; and 

 hence he infers, that in those cases where, at the time of cleavage, 

 the beds were horizontal, such was also the angle at which the 

 cleavage intersected the bedding (15° to 30°). The author further 

 observed, that in the quarries of North Wales which afford the 

 slate of the best quality, the bedding and cleavage rarely meet at 



