54 



•CAMBRIAN. 



Here and there bands of grit are met with, while conglomerates 

 occur at the base. 



The large slate quarry of Penrhyn is probably familiar to all 

 visitors to North Wales. The summit of the quarry is about 500 

 feet above the base, and the slates are worked out in terraces, each 

 about 40 to 45 feet in height. The beds of good slate are about 

 200 feet in thickness, and are of a rich purple colour with green 

 spots. ^ The outstanding mass in the woodcut represents a dyke of 

 unprofitable rock. (See Fig. 7, p. 47.) 



The beds have yielded no fossils save some burrows of marine 

 worms, and very doubtful traces of Fucoids, termed Chondrites? 



The bluish varieties of slate owe their colour to protoxide of 

 iron ; the red and purple varieties to peroxide of iron. The green 

 banding of the slates, and the production of the large uniform 

 masses of green, are considered by Mr. G. Maw to be not only due 

 to independent causes, but probably to have occurred at different 

 times. The ordinary form of variegation of the slates consists 

 of nuclei chemically or mechanically formed and environed by pale 

 green slate, the bleaching of which has been due to the abstraction 

 of the greater part of the colouring oxides of iron. This banding 

 and blotching of the slate was formed probably before the slate 

 was cleaved, and some of the purple slate has been changed 

 into green at the junction of the greenstone dykes. An analysis 

 of the slate at the Glyn Quarries, Llanberis, showed 60 to 66 per 

 cent, of silica, 13 to 21 per cent, of alumina, with peroxide and 

 protoxide of iron, etc.^ Dr. Sorby has attributed the pale blotches 

 of slate to concretions lying in the planes of bedding.* Slate has 

 been worked also at Cilgwyn, and NantUe.^ 



Bangor. — Near Bangor, according to Prof. Hughes, the Cambrian 



Fig. 8. — Section near Bangor. 

 (Prof. T. McK. Hughes.) 



Menai 

 Straits. 



Friddodd. 



Bangor 

 Station. 



Bryniau. 

 Shafts to 

 Tunnel. 



Cambrian. 



a. Carboniferous Limestone. 

 lb. Black slate. 

 \c. Banded slate. 



{d. Greenish grit and subordinate 

 I purple and green slate. 



\^. Grit and conglomerate. 



Bangor 



Beds 



(Archaean). 



f. Greenish slates, 

 hornstones, 

 breccias, 8cc. 



g. Schists. 



^ D. Sharpe, Q. J. ii. 283. 



2 Salter, Q. J. xii. 246 ; Ramsay, Geol. N. Wales, ed. 2, p. 336. 



3 Q.J. xxiv. 381 ; Geol. Mag. 1868, p. 123. 

 * Edin. Phil. Journ. Jidy, 1853, p. 3. 



^ D. C. Davies, A Treatise on Slate and Slate Quarrying, 1878. 



