1846.] SEDGWICK ON THE FOSSIL SLATES OF N.WALES, ETC. 137 



discover a single fossil, not even so much as a fucoid; though I be- 

 lieve the rocks belong to a fossiliferons group. A similar group 

 of dark slates breaks out farther south, and is at length interrupted, 

 and broken up, by the great erupted porphyries of the Rivals. Asso- 

 ciated with these rocks we find some irregular beds or masses of 

 iron ore, sometimes highly magnetic, and with a remarkable piso- 

 litic structure. They are probably contemporaneous; and the beds 

 along which they run sometimes pass into the condition of a load- 

 stone, exhibiting a true polarity. I should not have noticed these 

 phasnomena had they not been exhibited in several other places 

 among beds which are, I believe, exactly on the same parallel. On 

 the east side of the great intrusive rib of porphyry, above mentioned, 

 and towards the base of the higher mountains of the Carnarvon 

 chain, are several other alternations of slates and contemporaneous 

 trappean rocks. They are, I believe, in symmetrical undulations, 

 which strike nearly with the beds ; but the country is low, and much 

 covered with the drift of the higher mountains, as well as with great 

 masses of a northern drift, which (as is well known) contains ma- 

 rine shells to the height of more than 1200 feet. 



Again, to the east of this low country is another and larger rib of 

 porphyry, extending from the country near Llanllyfni to the hills 

 north of the foot of Llanberis lake (see the Map)*. In position and 

 structure it very nearly resembles the more western rib of syenitic 

 porphyry. On the other hand, the beds of its upper surface are of 

 such remarkable structure, and offer such apparent passages into the 

 overlying slates, that I cannot, in all the quarries, separate one for- 

 mation from the other ; I am compelled therefore to consider the 

 date of this enormous rib of porphyry as doubtful. But, at any rate, 

 it leaves undisturbed the general symmetry of the great undulating 

 east and west section through the Carnarvon chain : for a section 

 from the Menai near Bangor through the Penrhyn quarries, and 

 thence to Glyder Fawr, presents the same undulations, and the 

 same succession of stratified masses, with another section from Car- 

 narvon across the two ribs of porphyry to the top of Snowdon ; or, 

 still farther south, to the top of Moel Hebog. 



Section I. 

 Menai Straits to Glyder Faivr on the Carnarvon Chain. 



Horizontal line 9 miles. E.S.E. 



Glyder Fa^vr. 



W.S.W. 

 Menai Straits, 



a. Black slates. d. Penrhyn slates. 



5. Porphyry with traces of bedding. e. Coarse greyvvacke. 



c. Porphyry and undulating slates". /. Slate and porphyry. 



To the east of the second great rib of porphyry commences the 

 great zone of the Carnarvon slates, ending with the quarries of Pen- 

 rhyn, Llanberis, Llanllyfni, &c. They alternate with many bands 



* The Map referred to is that which accompanies Professor Sedgwick's former 

 paper, and will be found in vol. i. p. 5 of the Journal. 



