1846.] SHARPE ON SLATY CLEAVAGE. 91 



to trace a general plan of two series of planes dipping from a central 

 axis towards the boundaries of the area. 



On the western side of the area the dips of the cleavage follow 

 certain laws with a great regularity. I measured the dip of the 

 cleavage at every convenient opportunity along two sections. On 

 the Holyhead road proceeding eastward from the W. flank of Snow- 

 don where the cleavage is vertical, there is a gradual decrease in the 

 inclination at which the cleavage dips to the N.W. : at the Penrhyn 

 quarry its angle of dip is 85°, and in crossing the Snowdon chain it 

 diminishes 25°, being 60° at Capel Cerrig ; for about six miles it 

 continues at about 60°, and close to the Bryn-Ddinas dike, at the 

 forty-seventh mile-stone, it is 50° ; beyond this spot the strike of the 

 beds and of the cleavage changes to E., and we enter suddenly 

 another area of elevation. 



The section through the pass of Llanberris, and thence eastward 

 to Bala, gives a complete line across the whole area : in crossing 

 Snowdon the angle of inclination of the cleavage decreases gradually 

 about 25°, being vertical at the Glyn slate-quarries and dipping at 

 65° to the N.W. at Llyn Gwinant ; the regularity is broken by a 

 reversal of the dip near the village of Llanberris for about half a 

 mile, beyond which the series recovers its order. On the E. of Snow- 

 don the angle continues falling with the greatest regularity as we 

 proceed eastward, as may be seen in all the slate-quarries near Ffes- 

 tiniog, till it reaches 35° about two miles E. of that village : we 

 have then a mile or two of disturbed country at the great anticlinal 

 axis of the strata near Rhaiadr Cwm, between seventeen and eighteen 

 miles from each of the two lines of vertical cleavage which bound 

 the area : at this spot the direction of the cleavage planes is confused. 

 E. of the axis at Rhaiadr Cwm the dip of the cleavage planes suddenly 

 changes to E.S.E. with an inclination of 50°, and it continues to dip 

 either in that direction or to the E. or E.N.E. with a gradual in- 

 crease of inclination, till we reach the line of vertical cleavage beyond 

 Bala ; but there is less regularity here than on the Snowdon side of 

 the area. 



E. of the vertical line of Bala and Mallwyd the dip again changes 

 to N.W., and on that side of the line it has a westerly dip all the 

 way from the Dee to St. David's, the greatest variation being be- 

 tween W. and N.W. 



Omitting therefore minor irregularities and local disturbances, 

 these statements show a symmetrical arrangement of the cleavage 

 planes over an area thirty-five miles wide, with a dip in opposite di- 

 rections away from a central axis, their inclination increasing with 

 the distance from the axis, till it reaches the perpendicular at an 

 equal distance on each side. And on the only side which we have 

 the opportunity of examining, we find on passing the vertical bound- 

 ary the commencement of another similar arrangement. 



Mr. Darwin suggested in a passage already quoted, "that the 

 cleavage laminas, though to the eye appearing straight, may be parts 

 of large curves*." The arrangement just described bears out that 

 * Geological Observations on South America, p. 155. 



