90 PROCEEDINGS OF THE GEOLOGICAL SOCIETY- [DeC. 2, 



one system of cleavage, and may be called an area of elevation of 

 the cleavage. These general views will be confirmed by the ex- 

 amination of particular districts, where we must first ascertain the 

 lines along which the cleavage planes are vertical. 



Area of elevation of Carnarvojzshire and Merionethshire. 



A line of vertical cleavage runs along the slate beds which lie on 

 the western flank of the Snowdon chain, from the valley of the Lyfni 

 to the coast near Aber on the N.E. of Bangor, a distance of more 

 than twenty miles : this line runs N.E. It cannot be followed farther 

 to the south, as the direction of the beds changes suddenly near 

 Clynog. On the W. of the vertical line there is a long band of 

 greenstone, and then a broad bed of gravel which completely conceal 

 the stratification and cleavage. On the E. side of the vertical line, 

 over an area of about seventeen miles wide, the cleavage dips with 

 few exceptions to the N.W., at various angles which will be stated 

 shortly ; farther eastward for a similar distance of seventeen miles, 

 the cleavage planes dip at various angles to some point between S.E. 

 and E ; the nearest line at which a vertical cleavage is met with, 

 runs through the great slate quarries between Dinas Mowddy and 

 Mallwyd striking N.N.E. Thus thirty-five miles from the line of 

 vertical cleavage at the W. flank of Snowdon, runs another vertical 

 line nearly parallel to it. Not having followed up this second vertical 

 line, I can only lay down its course by drawing it between those 

 points where the cleavage dips at very high angles towards a common 

 axis : from such help I infer that the cleavage will be found vertical 

 a little to the W. of the pass of Bwlch-y-groes, and thence to the 

 N.N.E. at about two miles E. of the Bala Lake as far as the Dee. 

 From some observations made by Mr. J. E. Davis, I conclude that 

 the line runs from Mallwyd to the coast in a direction from N.E. to 

 S.W., passing about five miles N. of Machynlleth through a moun- 

 tainous district of some elevation, and reaching the coast a little to 

 the N. of Aber Dovey. We might here give up our line as lost 

 beneath the sea, but Sir R. I. Murchison has recorded that on the 

 N. side of Whitesand Bay near St. David's, vertical cleavage planes, 

 coinciding exactly with the bedding, strike E.N.E. subject to some 

 undulations. The direction of the cleavage planes at this spot and 

 its bearing from the points before stated, show this to be the con- 

 tinuation of the same line of vertical cleavage. The distance from 

 the Dee near Bala to Whitesand Bay is 110 miles. 



We have thus established two nearly parallel lines of vertical 

 cleavage in North Wales, the western perfectly straight, the other 

 slightly bent, which enclose an area thirty-five miles wide, within 

 which the cleavage planes usually strike nearly Is.E. ; except in all 

 that large district between the coast and the porphyritic chains of 

 Cader Idris, the Arenigs, Arran Mowddy, &c., where the cleavage 

 usually strikes N., but is subject to great irregularities both of dip 

 and strike ; these porphyritic eruptions appearing to have disturbed 

 the uniformity of arrangement of the cleavage : still we are able 



