1846.] SEDGWICK ON THE FOSSIL SLATES OF N. WALES, ETC. 139 



appear here and there in the beds of this trough, first breaking out 

 on the higher part of the brow ascending from the Penrhyn quarries 

 to Llyn Ogvven. Along this line we have a Ibssiliferous zone about 

 half a mile wide, and it ranges through the higher crests of Snowdon 

 to Moel Hebog, as stated in former papers*. In many extensive 

 tracts it is true that fossils disappear, especially where contempora- 

 neous porphyries and trappean shales are most abundant ; but the 

 whole series of undulations, continued from the high crests here no- 

 ticed, over the Berwyns, and to the edge of Shropshire, belong to 

 one great physical group, the whole of which (where the conditions 

 are favourable to the development of animal life) is fossiliferous. 



Such are the sections from the Menai to the crests of the Car- 

 narvon chain ; and I cannot estimate the thickness of the beds, be- 

 fore we reach the fossiliferous trough above noticed, at less than six 

 or eight thousand feet. 



But there is one great imperfection in these sections — they have 

 no well-defined base ; and we have no evidence on which to estimate 

 the thickness of deposits which may have been interpolated between 

 the hypozoic group and the dark, earthy slates of Carnarvon and 

 Bangor. Moreover, it is almost impossible to form any correct esti- 

 mate of the thickness of the masses occupying the low country be- 

 tween the Menai and the western flank of the Carnarvon chain ; we 

 must therefore seek for better CN^idence in other sections 



Section III. 

 Near Tremadoc. 



Horizontal distance 8 mUes. . ■"' " 



S. by W. 





a. Lingulabeds. 



b. Black slates with Fucoids and Trilobites. 



c. Trilobites, &c. 



I have stated in my former papers that a great dislocated group of 

 slates and porphyries (not to be mineralogically distinguished from 

 the general mass of the Carnarvon chain) occupied the promontory 

 south of Tremadoc, and was continued northward till it abutted 

 against the south flanks of Moel Hebog and Moel Ddu, a few miles 

 south of Beddgelert ; but I had not seen this group since 1831. Its 

 beds first dip nearly north, and then bending round toward the 

 western side of the great estuary, called Traeth Mawr, the beds dip 

 about N.N.E. Before rising to the flanks of Moel Hebog and Moel 

 Ddu, they reach a great elevation, and become in some places almost 



* See Proceedings of the Geological Society, vol. iii. p. 548. 



