Arenig \ 



Upper 

 Tremadoc" 



Lower 

 Tremadoc 



THE CAMBRIAN 8Y8TRM 87 



12. Black slates with iron stains. 

 11. Garth Hill grit. 



10. Hard bluish flags with many fossils, Awjeliiiti Sfd-j 

 llomfrayi, Ogygia. smtatrix, etc. 



9. Soft sandy shales with Shumardia. 

 8, 7. Alternating beds of hard bluish slate and black pencil 



slate. 

 6, 5. Thick beds of pencil slate, with some hard sandy layers, 



Asaptiellus Homfrayi. 

 4. Hard bluish slaty rock in massive beds. 

 3, 2. Dark iron-stained slates, with Psilocephalut innotatut, 



Niobe Homfrayi, Dictyograptut tocialis. 

 1. Lingula flags Black slates with Olenus. 



Mr. Fearnsides has recently described a similar succession near 

 Tremadoc on the east side of the Lleyn peninsula, where the 

 Upper Tremadoc Beds (440 feet thick) yield species of Ataphtllim, 

 Angelina Shunuirdia, Symphysurus, and Dikrllocephalut? 



The Tremadoc Beds do not form a continuous band round 

 the Lingula flags of Merioneth, for in certain places they seem 

 to disappear beneath the base of the Arenig Series (Ordovician) ; 

 it is now believed that there is an actual unconformity between the 

 two series and that the Arenig often completely oversteps the 

 whole of the Tremadoc Group so as to rest on Lingula flags. 



In Carnarvonshire the thickness of the Lingula flags is much 

 less, but near Llanberis and Nant Ffrancon they are apparently 

 from 2000 to 2500 feet thick. On each side of Llyn Peris the 

 nearly vertical Harlech grits are succeeded by dark slates which 

 represent the Haentwrog Beds, with a thickness of about 1000 feet, 

 and these are followed at the eastern end of the hike by hard 

 quartzose grits (? Ffestiniog beds), which are estimated by Ramsay 

 to be about 1300 feet thick. 10 These beds are traceable north- 

 ward through Elidyr-fawr, and a good section is exposed in the 

 southern cliffs of Marchlyn-mawr, where Ramsay found Olenut 

 micrurus and Lingulella Davisii in certain grey and brown grita. 

 It is noticeable that some of these grits are of coarse grain, and 

 occasionally conglomeratic, thus differing from the finer-grained 

 flagstones of Merioneth. The Upper Lingula flags or Dolgelly 

 Beds have not yet been identified in Carnarvon owing to the rarity 

 of fossils, but are probably represented by a part of the overlying 

 slates. 



On the western side of the Llanberis anticlnu- the thickness of 

 the Cambrian Series seems to be still further reduced, and when they 

 again emerge on the eastern flank of the Bangor-Carnanron ridge 

 there are only slates overlying the basal grita and conglomerate*. 



Professor Hughes n has traced these basal beds along the eastern 

 flank of the pre -Cambrian ridge all the way from Twt Hill near 



