1846.] SEDGWICK ON THE FOSSIL SLATES OF N. WALES, ETC. I^l 



7. Again, in ascending order, a series like No. 5, in which fossils 

 are rare. In it W6 found a Trilobite {Homalonotus bisulcatus, also 

 found at Wittingslaw). 



8. Then succeeds a fourth great terrace of greenstone ; and here 

 we do meet with a great vertical mass of greenstone (apparently re- 

 presenting the focus of eruption), above which the greenstone 

 spreads out into a tabular mass that is parallel to the beds above it 

 and below it. 



9. Higher still we have alternations almost innumerable, of flags, 

 slates, trappean shales, and igneous products — some of which may 

 have been recomposed ; and some truly erupted, and unchanged by 

 any subsequent aqueous action. 



10. At length we reach the steep brows ascending towards the 

 summits of Moel Ddu and Moel Hebog*. Slaty cleavage becomes 

 better defined; fossils are more numerousf. The beds are now 

 nearly horizontal, and almost identical in structure with the slates of 

 the neighbouring chain. Large irregular quartz veins mark the 

 passage of a break in the continuity of the strata, beyond which 

 the beds immediately recover the usual strike and dip of the neigh- 

 bouring ridges of the Carnarvon chain |. 



In conclusion I have to remark, that the base of this section (if 

 I have interpreted it correctly) is not only lower than the base 

 of the sections from the Menai above noticed, but has the great 

 advantage of exhibiting organic remains. I identify (at least pro- 

 visionally) the dark slates containing the pisolitic iron ore with the 

 dark slates on the shores of the Menai. The higher parts of the 

 Tremadoc section present details very analogous to those of the as- 

 cending sections from the Menai to the Snowdonian crest ; and all 

 the sections indicate a very great thickness before we reach the limit 

 of the fossiliferous slates which undulate through the higher crests 

 of the Carnarvon chain§. 



The beds I have called greenstone are contemporaneous, because 

 they are associated with trappean shales, and other recomposed 

 Jgneous rocks, which alternate with and pass into the slates. More- 

 over they cannot have produced the anomalous position of the Tre- 

 madoc groups, because they also themselves partake of that anoma- 

 lous position. Lastly, though this section throws light on the struc- 

 ture of Carnarvonshire, yet, as it ends among contorted and faulted 

 beds, there is some doubt as to its exact upper limit. On this point 

 I have only to remark, that I know not how to separate its upper 

 beds, either by mineral structure or by fossils, from the slates which 



* The section might be carried to the top of Moel Hebog, but it is here drawn, 

 a little farther east, towards the top of Moel Ddu, 



t Among the fossils are Trinucleus Caractaci, MurcMsonia scalaris, Leptcena 

 sericea, Orthis opercularis, &c. 



X It deserves remark, that the strike of the cleavage-planes is nearly the same 

 on both sides of the fault here noticed, and that this strike nearly coincides with 

 the mean strike of the Carnarvon chain, of which Moel Hebog and Moel Ddu 

 form a portion. 



§ The fossihferous slates hei'e alluded to are those, above noticed, which range 

 through the top of the Llyn Ogwen pass, by Cwra Idwal, the top of Snowdon, 

 and Moel Hebog, &c. 



