68 UPPER CAMBRIAN (ORDOVICIAN). 



The Upper beds are developed at Arenig, and in Caernarvonshire, 

 Shropshire, and at St. Davids. The Lower beds are exhibited at 

 Portmadoc, St. Davids and Ramsey Island. 



Among the fossils, Graptolites are abundant ; in the lower 

 portion about 22 species have been found, including Didymograpius 

 and Denchograptus, and in the upper, Teiragraptiis and Diplograptus 

 occur. Dr. Hicks observes that the deep-sea conditions pre- 

 valent at the time were no doubt favourable to the growth and 

 development of Graptolites.^ In the upper beds, the Grit of Garth 

 Hill and Ty Obry, east of Tremadoc, in North Wales, has yielded 

 no fossils, but other beds have yielded Ogygia hiiUina, O. peltata, 

 Irinucltus Gibbsii, y^glhia grandis, Ampyx Sa//en' and Calymene. 



The lower beds have yielded Asaphus Hom/rayi, Ogygia scutatrix, 

 Trinuckiis Stdgwickii, Conularia Homfrayi, Theca, Obolella plicata, 

 Lingida, Orihis rtmo/a, etc.* 



Between Shrewsbury and Montgomery are the Shelve and 

 Corndon areas of older rocks, to the east of which are the Stiper 

 Stones. The quartzose rocks called Stiper Stones in Shropshire be- 

 long to the Arenig series : they extend for ten miles, from Pontes- 

 bury, near Shrewsbury, to Snead, near Bishop's Castle, and were by 

 Murchison originally considered as the base of the Silurian 

 System (1833-4). They form a celebrated ridge of rocks which 

 jut out upon a lofty moorland, at heights varying from 1500 to 

 1600 feet above the sea. They consist of outstanding masses of 

 a thick band of siliceous sandstones, in parts veined, altered and 

 fractured, and occasionally passing into crystalline quartz-rock. 

 Lingula and also Annelide-burrows {Scoliihus linearis) have been 

 found in the Stiper Stones.^ 



Dr. Hicks observes that in Shropshire the order of the beds is 

 almost similar to that observed in North Wales, and the Stiper 

 Stones are doubtless, as first suggested by IVIr. Salter, the equiva- 

 lents of the Arenig grit beds in Caernarvonshire. Hitherto the 

 black beds under the Stiper Stones have proved almost barren of 

 organic remains, and they cannot be correlated with other series 

 except by position ; but in the beds immediately upon the Stiper 

 Stones the fauna is exceedingly like that in the Upper Arenig 

 group at St. Davids, and contains, like it, the genera IllcEnus and 

 JlliFnopsis, in addition to most of the genera in the beds at Ty 

 Obry, in Caernarvonshire.* 



Slates are worked in the Arenig series, as at the Crown Slate 

 quarry near Dolgelly, Maengwynedd quarry near Cader Berwyn, 

 and on the north side of Whitesand Bay. In the Shelve district 

 lead-mines were worked shortly after the time of the Roman 

 invasion. 



^ See also J. Hopkinson, G. Mag. 1872, p. 467. 



'^ See Hicks, P. Geol. Assoc, iii. 99; Q.J. xxix. 42, xxxi. 175, 192. 



^ See J. W. Salter, Q. J. xiii. 200 ; Murchison, Siluria, edit. 5, 1872, p. 37. 



* Q. J. xxxi. 175. 



