SHARPE ON NORTH WALES. 299 



On the continuation of the line of section passing through Snow- 

 don, the author regards as Cambrian some of the schists which are 

 seen in the pass of Llanberris, and among these he ranks the bed 

 which is quarried for slate opposite Llanberris Church. Owing to 

 the extreme complication of this part of the section and the altered 

 condition of many of the beds when near to the greenstone dikes, he 

 draws the line of separation between the Silurian and Cambrian rocks 

 with great doubt of its correctness. 



A little to the west of the church, on the south side of the Llanber- 

 ris Pass, the Cambrian rocks are cut off by a great fault, which is re- 

 markable for severing in twain an arch of greenstone. In the meta- 

 morphic schists above the greenstone a copper-mine is worked. 

 Westward of the fault are metamorphic slates and slaty grits, which 

 probably belong to the Bala grits. These are followed by softer 

 schists and slates, which, when near the greenstone dikes, are altered 

 and disturbed. This series, as was the case on the Holyhead road, 

 is overlaid by the purple slate. Beyond this is a great mass of igne- 

 ous rocks, which are covered on the west by gravel. 



Purple Slate of the Western side of the Snowdon Chain The 



purple slate-bed of the quarries of Penhryn, Llanberris, &c. is, in an 

 economical point of view, the most important bed in North Wales, 

 supplying, as it does, more than half of all the slates which are raised 

 in the Principality. 



The slates that are worked on the west of Snowdon are regarded 

 by the author as all belonging to one bed, which, having been much 

 tossed about by the greenstone, is repeated many times in some lo- 

 calities. On the Holyhead road*, its first appearance towards the 

 west is in the Penhryn quarry. It there lies in a trough between the 

 metamorphic rocks of Bronllwyd on the east, and a ridge of green- 

 stone on the west. The bed being equally upheaved on both sides, 

 dips towards the middle of the quarry at an angle of about 45°. 



The effect produced by the doubling up of the bed is shown, at the 

 back of the quarry, by a mass of broken and curved slate which fills 

 the middle of the trough, and is there squeezed up together. The 

 axis of the trough strikes north-east, as do the planes of cleavage in 

 all parts of the quarry, including the crushed portion. These planes 

 dip south-east, from 80° to 85°, with great regularity, showing that 

 the elevating forces had ceased to act before the cleavage took place. 

 The peculiar position of the beds has been taken advantage of in 

 working the quarry, which is disposed in terraces that give it nearly 

 the form of a Roman theatre. 



The author estimates the thickness of the bed of good slate at 

 not more than 200 feet, though it appears greater, owing to the 

 filling-up of the trough with crushed portions of its upper part. 

 The slate is of a rich purple colour, with green spots and green lines 

 at the junction of the beds. 



Below the fine slate lies another bed of slate of the same colour, 

 but too hard for use ; and beneath that is a bed of hard, green slate- 

 rock. Then follow coarser schists, which rest on the igneous rocks, 

 * See Section 4. 



