1846.] SEDGWICK ON THE FOSSIL SLATES OF N. WALES, ETC M? 



I indicated the promontory to my companion John Ruthven ; and 

 directing him the day following to certain quarries near Ty Gwyn, 

 he found the same Fucoids and Lingulce which characterize the 

 Festiniog and Tremadoc flags. There is indeed no difficulty in finding 

 these fossils, as the neighbouring slate rocks abound with them. 

 Some one had before informed me that fossils were found in the 

 chain of Cader Idris, but I neither knew the species nor the locality. 

 The facts above stated are decisive as to the true geological age of 

 the range of Cader Idris. It is exactly where I had placed it before, 

 on mere physical evidence; and to this we may now add very 

 satisfactory zoological evidence, and a connected series of sections 

 referable to one common base-line. A line drawn from a point on 

 the south side of the Barmouth estuary over the top of Cader Idris, 

 and thence over the ridges of Arran y Gesail to the valley above 

 Machynlledd, will be nearly transverse to the general strike of the 

 country, and will give the following sequence : — 



(1.) A series of slates and contemporaneous porphyries, &c. de- 

 scending towards the parallel of the Merioneth anticlinal. These 

 beds are far better exposed on the north side of the estuary, between 

 the anticlinal (which is there well-defined) and Llanelltyd; and they 

 form a very thick group*. 



(2.) The Lingula beds, alternating with enormous ribs of por- 

 phyry, trap, shale, &c. &c. I have not traced these Lingula beds on 

 the exact line of section ; but, from the strike of the Ty Gwyn beds, 

 they must pass through the brows above Capel Arthog, near which 

 the line of section passes. 



(3.) Higher up the hills are more earthy slates (still alternating 

 with numerous beds of porphyry, &c.) containing the pisolitic iron 

 ore^ exactly like that of Tremadoc. 



(4.) Higher up still is an enormous mass of contemporaneous 

 porphyry, which forms a great feature on the north flank of Cader 

 Idris ; and higher still are numerous alternations of slate and por- 

 phyry to the very top of the chain. Any vsection I can draw must 

 fail to convey a true and adequate notion of the magnificence of 

 this sequence. It is now clear that the rocks composing this series 

 must be (as indeed I had placed them before) very nearly on the 

 parallel of the great slaty and porphyritic crests which range from 

 Carnedd Llewelyn to the top of Moel Hebog. It is true that the 

 upper part of the Cader Idris range is, so far as I know, without 

 fossils. But this may be easily accounted for by considering the 

 local conditions which may have been unfavourable to animal life 

 during the period of deposit. And nowhere in these groups are the 

 fossil bands strictly continuous; but only found here and there along 

 given lines of the several sections. 



(5.) On the south-east flank of Cader Idris the contemporaneous 

 porphyries begin to disappear ; and some soft and rather earthy dark 

 slates descend towards Tal-y-llyn. There is a partial contortion 

 which I have attempted to represent ; but a prevailing south-east dip is 

 constant. 



* The lowest beds here alluded to do not appear in the accompanying section 

 No. 6, as it commences on the south side of the Barmouth estuary. 



