1853.] RAMSAY — LOWER PAL^OZOICS OF N. WALES. 165 



The succession in all the sections is the same, and the thicknesses 

 nearly approximate to each other. The slates of the Ffestiniog slate- 

 quarries succeed them. They lie in the lowest part of the Bala heds 

 in their regular order of succession. The rocks at Tai-hirion, on 

 the Bala road, near Arenig-bach, which contain Trilobites and Lin- 

 gulce (see Mr. Salter's note on the fossils, infra), are their equivalents. 

 These underlie a set of ashy heds that form Arenig-bach and part 

 of the country on the north that passes by Cerrig-y-Uadion to the 

 valley of Penmachno. An upper portion of this igneous series, 

 therefore, belongs to the base of the Bala beds. 



North-west of Moel-wyn, and in the country generally overlying 

 the Ffestiniog felspathic traps towards Llyn Gwynant, numerous 

 lines of greenstone occur. Like the same kind of intrusive rocks near 

 Dolgelli, they are apt to run in the lines of bedding, but they also 

 cut more or less across the strike. 



No truly interstratified or contemporaneous traps occur between 

 Moel-wyn and the Snowdon felspathic trap, which is 6000 feet by 

 measurement above those of Moel-wyn and the other igneous rocks 

 of the date of those of the Arans. All of these have heretofore been 

 considered as belonging to one igneous group. The above thick- 

 ness can be well measured on section No. 2. Notwithstanding the in- 

 truded lines of greenstones, the dip is steadily north-west as far as 

 Castell and Yr-Arddu, which hills are capped by outliers of the 

 Snowdon trap. The Castell trap lies in a synclinal of the Bala beds. 

 The fault on the west, which repeats this trap, gives about 4300 feet 

 of downthrow. The rock so repeated forms part of the great mass 

 of the Snowdon trap, which from thence undulates westward in a 

 great trough of about five miles in width*. It is about 1300 feet 

 thick on the east side of Snowdon, but westward it splits into three 

 thin bands of slate separating the masses. Above it lies about 

 1000 feet of calcareous, sandy, and felspathic ashes, largely inter- 

 mingled with slaty sediment. Sometimes one element predominates, 

 sometimes another : on the whole, the slaty and calcareous elements 

 prevail. When this is the case, the rock is frequently fossiliferous. 

 The fossils are those of the Bala limestone. Its position also proves 

 that it is the equivalent of these beds ; a conclusion long since ar- 

 rived at by the Officers of the Geological Survey, first on physical, 

 and afterwards on palseontological grounds. The traps and fossili- 

 ferous ashy beds of Snowdon and the true Bala limestone are each, 

 therefore, about 6000 feet above the lower igneous series of Moel- 

 wyn and the Arans f- The physical and palseontological evidence 

 are thus in perfect accordance. 



On the ridges of Crib-goch and Llewedd (parts of Snowdon), 

 on Glyder fawr, and on Moel Hebog, near Beddgelert, there are eight 

 small patches of another mass of columnar felspathic rock, that must 

 once have entirely overlaid the ash. It may have been as large as 

 the whole mass of the Snowdon trap that underlies it. 



* This trough has been noticed by Prof. Sedgwick (Quart. Journ. Geol. Soc. 

 vol. iii. p. 138). 



t Horizontal Sections Geol. Survey, sheets 28 & 29. 



