ON THE GEOLOGY OF THE SNOWDON MOUNTAINS. 537 



other slate any where found. It splits readily and easily into the thinnest 

 laminae or plates, and may be obtained of any size ; and almost any kind of 

 architectural article can be made of it, as roofing-slate, flooring-flags^ window 

 and door jambs, chimney-pieces, tombstones, and various other articles. 



" This great vein of slate is penetrated about the middle, from south to north, 

 by a hard quartzy vein, having occasional traces of sulphuret of lead. The 

 slate in immediate contact with this dike or vein of quartz is much inferior in 

 quality, as it splits less readily. 



" The slate of this quarry cannot be raised without the assistance of gun- 

 powder. Though great quantities are thus separated and brought down, 

 much is shattered and rendered useless by the explosion. Being removed 

 from the quarry, and prepared for architectural purposes, it is then, by means 

 of an iron-railway, upwards of six miles in length, transported to Port Penryn, 

 within a mile of Bangor, where it is shipped for all quarters of the globe. 



"At this quarry a useful piece of machinery, put in motion by water, is 

 employed for sawing and cutting slate into tombstones, jambs, and such like. 

 Near Port Penryn is a writing-slate manufactory. Of the refuse slabs of slate 

 very good fences are made ; some that are long enough are fastened together 

 with iron-nails, like post and rails, and in that form look well, and answer a 

 good purpose. 



" The returns from this quarry in the course of a year are immense, many 

 thousand tons being sold and exported. Nearly a thousand men are daily to 

 be seen at work ; and from the interesting appearance of the various workings 

 of the Dolawen-slate quarry, and the great extent of the operations, few places 

 can afford a higher treat to the practical mineralogist. 



" Were instances wanting to exemplify the vast advantages iron-railways 

 confer in prosecuting great works, at less expense, and with greater expedition, 

 over common roads, repaired and engineered on the best principles, the iron- 

 railway at Dolawen slate-quarry may be adduced. It is said, that, before the 

 present one was laid down, nearly 400 horses were required to convey the 

 slates from the quarry to Port Penryn ; but with the assistance of the railway, 

 twenty horses can do all the carriage. 



" The next slate-quarry worthy of examination is that of Alt Dhu, belonging 

 to Mr. A. Smith, situate on the eastern bank of the lower lake of Llanberris, 

 on the steep declivity of a mountain, rising not much less than 2000 feet 

 above the level of the sea. This slate-vein, which is evidently the continuation 

 to the south-west of Dolawen vein, on the Ogwen, first described, passes along 

 the Alt Dhu mountain. In thickness it is 300 yards, and in height not much 

 less. It consists, like that of Dolawen, of three kinds, the blue, the red, and 

 the green. The blue, being most preferred, is the most valuable to the pro- 

 prietor, though the other two are equally durable. These veins are included 

 between hard dark-coloured greywacke rocks, and are penetrated through, 

 from the north to the south, by a sparry vein containing pieces of galena, and 

 now and then sulphuret of copper. This dike deranges the structure of the 

 slate near it, and renders it unfit for splitting. 



