1/0 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [A.pr. 20, 



these points the slates of Trwyn-y-tal on the coast dip southerly. 

 They contain pisolitic iron-ore, and belong to that part of the Lmgula 

 flags in which the pisolitic iron-ore of Llyn Cwellyn (above the road 

 from Caernarvon to Beddgelert) occurs. Iron-ore, probably on the 

 same parallel, again crops out in the black shales between St. Tud- 

 wall's Road and Hell's Mouth. They dip northerly, and the rocks, 

 therefore, that occupy the midst of the promontory, lie in a trough, 

 and belong to part of the Bala series. The fossils of the district 

 confirm this idea. Wherever any jmiction of these beds and of the 

 metamorphic schists is exposed, they are always found to be brought 

 against each other by faults. Greenstones, felspathic porphyries, 

 and syenites are intruded in great bosses amid the above-mentioned 

 strata. 



Recapitulation. — Such is a brief outline of the disposition and 

 structure of the rocks of North Wales that lie beneath the Caradoc 

 Sandstone. I shall briefly recapitulate the sequence. The lowest 

 rocks are the Barmouth and Harlech grits, or their sandy and slaty 

 equivalents of Llanberis and Penrhyn, which are also the equivalents 

 of the main masses of metamorphic strata of Anglesea and the north 

 horn of Cardigan Bay. Their greatest known thickness is about 

 6500 feet. Then come the Lingula beds, about 7000 feet in thick- 

 ness. Both of these are pierced by greenstone dykes, and by bosses 

 of intrusive greenstone, syenites, and felspathic traps. Toward the 

 close of the Lingula flag depositions volcanic outbursts took place, in 

 consequence of which great ashy deposits were formed interstratified 

 with ordinary muddy sediment, and here and there associated with 

 thick beds of felspathic lava. It has been stated that these rocks 

 forming the Arans and the Arenigs are on diiferent horizons. This 

 has not been found to be the case. They are all on the same horizon 

 and merely repeated by faults. Numerous greenstones are associated 

 with them, especially among the ashes. Although these often run 

 in the lines of bedding for a space, yet they frequently break suddenly 

 across the strike, and are therefore intrusive. Much of the ash seems 

 to have been subaerial. Islands, like Graham Island, may have 

 sometimes raised their craters for various periods above the water, 

 and by the waste of such islands some of the ashy matter became 

 water-worn, — whence the ashy conglomerates. Viscous matter seems 

 also to have been shot into the air, as volcanic bombs, which fell 

 among the dust and broken crystals (that often form the ashes) before 

 perfect cooling and consolidation had taken place. The volcanic 

 activity ceased for a time, during which 6000 feet of the lower Bala 

 beds were accumulated. It then broke out afresh in a new area, 

 further north, and the volcanic rocks of Moel Hebog, Snowdon, 

 Carnedd Llewellyn, Conway, &c., were produced. Numerous lines 

 of intrusive greenstone occur in the slates that lie between these two 

 great igneous series. They often run in the line of strike, but they 

 also sometimes cut across it and branch into two. The rocks also 

 both below and above are altered, which is never the case with the 

 slates that rest on the contemporaneous felspathic lavas. The great 

 centre of tliis later volcanic outburst was in the Snowdonian range. 



