84 STRATIGRAPHICAL GEOLOGY 



in which. Conocoryplie viola was found (see p. 81), but nothing like 

 the black Menevian slates and mudstones appears to exist in that 

 area. Whether originally present and now cut out by faults, or 

 whether they were eroded before the deposition of the Lingula 

 flags, is a question for future investigation. Mr. Fearnsides is of 

 opinion that there are local unconformities at the base of the 

 Upper Cambrian, and that this absence of the Menevian in 

 Carnarvonshire is probably due to erosion. 



Olenian or Upper Cambrian. As already stated this 

 consists of the groups known as the Lingula flags and the 

 Tremadoc Beds, and as in the case of the underlying Middle and 

 Lower Cambrians their thickness is greatest in Merioneth, which 

 seems to have been near the centre of the Welsh basin of deposi- 

 tion in Cambrian time. 



In Pembrokeshire the total thickness of this division is not 

 half that which it attains in Merioneth, though some of the same 

 zones or subdivisions seem to be represented. The area has not 

 yet been resurveyed by the Geological Survey and Dr. Hicks' 

 account of them is still the best available, but Mr. J. F. N. Green 

 considers it probable that there is a break or non-sequence be- 

 tween the Lingula flags and the Tremadoc. 7 The succession is as 

 follows : 



. n 



13 & 1 1 



c^ &o I - 1 



Feet, 



Black flagstones and slates with Neseuretus Ramseyensis, 

 Niobc Homfrayi, and Lamellibranchs .... 1000 



f Upper Fine grey slates without fossils .... 300 

 Middle Hard grey micaceous flags with Lingulella Davisi 



SE 1 * n Sundance ......... 1000 



h3 \.Lower Grey flagstones and black slates without fossils . 700 



These beds appear to form a continuous band along the northern 

 side of the Cambrian area from Whitesand Bay to the country south 

 of Llanvirn and beyond Crugglas (see Fig. 1 6) ; east of St. David's 

 they are seen in Solva Harbour, and inland they spread eastward 

 round another tract of Lower and Middle Cambrian rocks of 

 which little is yet known. 



Another small tract of Lower Tremadoc Beds is found round 

 Tremanhire, a place about 4 miles east of St. David's ; they 

 are brought in above the Lingula flags by a synclinal flexure, 

 and have yielded many of the same fossils as in Eamsey Island. 



Tremadoc slates have also been found to the south of Caermarthen 

 by the Misses Crossfield and Skeat. and their presence there is 

 interesting as it was unsuspected. They yielded Orthoceras sericeum 

 and two new species of Trilobites, Peltura punctata and Ogygia 



