"Upper 

 Gallery. 



Wall-case 4. 



186 



IGNEOUS ROCKS, NORTH WALES, AND 



embedded along with felspathic crystals, clearly prove its 



mechanical origin. 



38. — Felspathic ash. 



Craig 



Gwel, Mo 



8 miles south-east of Bala. Map 74, 

 grey felspathic sediment, with black slat; 

 tides. Weathers greyish-white. 



W 



Greenish- 



39. 



Porphyritic felspathic grit. — Llechrydau, 5 

 miles south of Llangollen, Montgomeryshire. Map 74, S.E. 

 Yellowish-brown sediment, with fragments and crystals of 

 white and red felspar, some very much decomposed ; gives 

 out an earthy odour. 



This is the equivalent of the lower ashy beds that under- 

 lies the Bala limestone near Bala, and they are probably 

 continuous underground in the synclinal curve that lies 

 between the Bala country and the Berwyn hills. 



The specimens from Nos. 27 to 39 are all from one set 

 of rocks, viz., the ashy rocks that lie on the felspathic traps 

 (lava beds) of Snowdon, as a centre, Moel Hebog, near 

 Beddgelert, and in the valley above Cwm Idwal, between 

 Y Glyder-fawr and Y Garn. As a whole, these rocks are 

 sometimes so purely felspathic and porphyritic, that it is 

 difficult (except for the beddir 

 felspathic porphyries that have been ejected as lava streams ; 

 but the greater mass on and around Snowdon is rough and 



scoriaceous-looking 



(near Llyn 



), or sometimes 



sandy, slaty, and calcareous, according as the volcanic 

 matter is variously intermingled with ordinary sediment. 

 Their uppermost part, on Snowdon, and in the outlier of 

 Dolwyddelan, is probably the equivalent of the Bala 

 limestone, which, even near Bala, is sometimes ashy in 

 its structure. The ashy beds on Snowdon, &c., contain 

 Bala fossils in places, They are about 1,000 feet thick on 

 Snowdon, (see section above, No. 4 ;) but ranging east to 

 Dollwyddelan, and from thence by Cerrig-y-Druidion to the 

 neighbourhood of Bala, and on the north and west flanks of 

 the Berwyn hills, they gradually thin out, and, with the 

 :est of the Snowdon igneous rocks, they finally disappear a 



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