558 J. Pringle — Lower Tremadoc Rocks, St. David's. 



the question of the horizon of the group, and considered with the 

 evidence yielded by the fossils collected at localities XI, 2, and 3 

 (see Sketch-map), it became certain that the beds are of Lower 

 Arenig age. Of the fossils collected the following forms have 

 been identified : — 1 



Orthis calligramma, Dalm., var. carausi, Salter. 



O. mena/pice, Hicks. 



Calymene tristani, Brongn. [Neseuretus ramseyensis, Hicks] . 



Ogygia selwyni (Salter). 



The succession of the Arenig rocks at the north end of Ramsey 

 Island is briefly as follows. The lowest beds of the group are a series 

 of thin grey sandstones and sandy shales, which pass upwards into 

 dark-grey iron-stained mudstones which are abundantly fossiliferous. 

 They are faulted against the Lingula Flags at Ogof Velvet, and 

 are not conformable as has been supposed hitherto. A fault on 

 the west side of the fossiliferous mudstones brings the latter against 

 the thin-bedded chocolate-stained shales of the D. extensus zone, so 

 that a considerable thickness of shales belonging to the lower part 

 of the zone seen on the mainland at Porth Lleuog, Whitesand Bay, 

 is cut out. The non-occurrence of these shales on Ramsey Island 

 has been previously noted. 2 On the west side of the island the 

 D. extensus shales are brought by another dislocation against the 

 Lingula Flags, which form the headland of Trwyn Shon Owen. 



The above succession is similar to that observed in the Arenig 

 rocks in the Carmarthen district, 3 where grits, gritty shales, and 

 conglomerates, associated with a similar fauna, are strongly developed 

 at the base of the shales belonging to the D. extensus zone. 



Whitesand Bat. 



Hicks thought that the lower part of the 'Tremadoc' group was 

 to be seen resting conformably on the Lingula Flags at the north 

 end of Whitesand Bay. He believed the upper members to be cut 

 out by a fault which runs in a north-easterly direction, and brings 

 clown Arenig shales on the north side of Trwyn Hwrddyn. 



An examination of the section in the light of the results obtained 

 on Ramsey Island convinces me that these rocks do not belong to 

 the group to which they have been assigned. Lithologically, they 

 are inseparable from the Lingula Flags, and the beds which form 

 the promontory of Trwyn Hwrddyn are similar to the flags on the 

 east side of Ogof Velvet. No fossils were obtained except Lingulella 

 davisi, which occurs in great abundance. 



Further north, at Llanveran, the group is to be seen in its entire 

 thickness, but time did not permit me to visit the localities. Hicks, 

 however, records from the beds the same fauna as he obtained from 

 the ' Tremadoc ' of Ramsey Island, so that their Arenig age may 

 be safely assumed. 



1 The Trilobites were kindly identified by Mr. Philip Lake. 



2 Gertrude L. Biles, "Some Graptolite Zones in Arenig Eocks ": Geol. 

 Mag., 1904, p. 209. 



3 H. H. Tbomas, in the Geology of the South Wales Coal-field, pt. x 

 (Mem. Geol. Surv.), 1909, p. 10. 



