538 ON THE GEOLOGY OF THE SNOWDON MOUNTAINS. 



" This quarry surpasses all others in convenience of removal of rubbish. 

 Placed on the high steep face of a hill, the refuse is easily got rid of, and all 

 obstruction in the working of it is prevented. On a railway of iron, seven 

 miles long, extending to the Menai Strait, the slate is conveyed to the 

 shipping. 



** In this quarry of Alt Dhu 800 workmen are generally employed. 



" The situation of these quarrymen is often exceedingly perilous. In the 

 course of boring and blasting the slate-rock, they have to descend by ropes 

 perpendicularly 100 feet or more ; and, in this position, suspended in the air, 

 they fearlessly proceed in their operations. 



" Two ropes are employed by these men ; the one they coil round their body 

 and one of their thighs ; and the other rope they hold in their hands, and with 

 wonderful dexterity, raise or lower themselves at pleasure. 



" Good workable slate is raised also in considerable quantities "at Clogwyn, 

 on the western bank of the lake of Llanberris, nearly opposite to that of Alt 

 Dhu, and on the north side of Nantle and Llanlyfne lakes. From the latter 

 place an iron-railway has just been completed, nine miles in length, to connect 

 these quarries with the port of Carnarvon. The colour of these slates is 

 reddish, and they are not equal in quality to those of Dolawen and Alt Dhu. 

 The veins of slate being entered from the surface, and sunk into a great depth, 

 considerable more expense in machinery is necessarily incurred in draining 

 and freeing the workings of rubbish in these quarries, than in the two first 

 described. 



" Before passing to the consideration of the other minerals of this side of the 

 Menai basin, it may not be uninteresting to remark, that no less a quantity 

 than 200,000 tons of slate is annually shipped from the above described quarries. 

 And when to this we add their value, not much less than 400,000/. a year being 

 paid for them, thus enriching the proprietor, and affording, at the same time, a 

 comfortable livelihood to a population of 20,000 people, including wives and 

 children, some idea may be formed of the immense importance of slate to the 

 county of Carnarvon. 



" The other transition-rocks, which are found in this portion of the Menai 

 basin, are more curious than valuable. Around the eastern and northern faces 

 of Snowdon, as seen from the pass of Llanberris, hornblende, porphyry, basalt, 

 greywacke, quartz, and micaceous rocks, are observed, as well as on the sides 

 of some others of the higher mountains of this range. The rocks in these 

 places have less of the slatey vertical structure, which is so predominant else- 

 where. They often assume, when the basalt prevails, a columnar form. Near 

 the summit of Snowdon, in the hard greywacke, impressions of shells are found, 

 proving, in the clearest manner, that all the rocks of this chain belong to the 

 transition-formation. 



" In cutting a new road along the western side of the lower Llanberris lake, 

 fine sections of the hard rocks are exposed to view. We can there trace veins 

 of asbestus, from an inch to three or four in thickness, passing through 



