250 The Rev. A. Sedgwick's Outline of 



any fixed law ; for cases are pointed out where cleavage planes are 

 much less inclined than the beds, and others in which these planes, 

 even in fine quarries, deviate one or two points from the strike of 

 the true beds. Five cases are exhibited in the sections where the 

 cleavage planes continue their strike and dip through all the con- 

 tortions of the beds ; and a few cases are given of a good second 

 cleavage plane. From all the facts the author concludes that clea- 

 vage planes are true crystalline phenomena, produced by the mutual 

 action of the elementary particles of the rock while passing into a 

 solid state. 



Jointed structure is also discussed at some length : and joints are 

 divided into four classes, called dip, strike, diagonal, and tabular joints. 

 The two former are most constant, often highly inclined, and divide 

 the slaty beds into great rhombohedral masses. They are supposed 

 to have been formed by mechanical tension while the rocks passed 

 into a solid state. Cleavage planes are not parallel to them ; and they 

 cannot arise from molecular or true crystalline action ; because, as 

 shown from examples in North Wales, they cut through the pebbles 

 of beds of conglomerate : sometimes however, among the accidents 

 of structure, true cleavage planes and joints become confounded. 



Bala limestone. — System of the Berwyns. — Fossiliferous beds on the 

 line of the Holyhead road, #c. 



This limestone ranges through Cader Dinmael to Glyn Diffwys on 

 the Holyhead road, a few miles east of Corwen. Thence it is pro- 

 longed towards the south ; but its continuity is broken, and for some 

 miles its range has not yet been made out. Exactly on the line of strike 

 (N.N.E. and S.S.W.) it breaks out again near Bala, and ranges 

 thence to the neighbourhood of Dinas Mawddwy, dipping steadily 

 to the E.S.E. 



From this limestone there is an ascending section to the very crest 

 of the Berwyn chain, south of the road from Bala to Llangynog. 

 In this ascending section are higher calcareous slates, which in one 

 or two places have been burnt for lime. But on the east flank of this 

 part of the chain there is a synclinal line, beyond which for several 

 miles the beds dip to the N.W. ; and a series of slate rocks, alternating 

 with a few bands of porphyry, are again brought up to the surface. 

 Some of these are only a repetition of a portion of the slates and 

 porphyries on the east side of the range of the Bala limestone. These 

 older rocks abut against, and, in consequence of enormous convul- 

 sions, in one or two places seem to overlie the Silurian rocks (among 

 the tributaries of the Severn) described by Mr. Murchison. 



The synclinal line above noticed appears to strike about N.N.E. ; 

 but the mean direction of the water-shed of the Berwyns is about 

 N.E. : hence the synclinal line, and the calcareous slates overlying 

 the Bala limestone, are, near the Llangynog road, brought to the west 

 side of the chain ; and the crest of the ridge, extending beyond Ca- 

 der Ferwyn, is composed of the older slates and porphyries dipping 

 towards the N.W. Still further north, either from a great flexure 

 or (more probably) from an enormous fault, we have for several 

 miles a great series of beds dipping to a point a few degrees east of 



