Pebidian Volcanic Series of St. Davids. 283 



change emphasized by the conglomerate was one that followed 

 volcanic conditions after no great lapse of time. 



Hence the relation of the Pebidian to the Cambrian is that of a 

 volcanic series, for the most part submarine, to succeeding sedi- 

 mentary strata — these strata being introduced by a conglomerate 

 formed in the main of foreign pebbles borne onward by a current 

 which swept the surface of, and eroded channels in, the volcanic 

 tuffs and other deposits. He was disposed to retain the name 

 Pebidian as a volcanic series in the base of the Cambrian system. 



The Pebidian Succession. — With the exception of some cinder-beds, 

 which appear to be subaerial, the whole series was accumulated 

 under water. There is no justification for making separate sub- 

 divisions ; the series consists of alternating beds of tuff of varying 

 colour and basicity, the prevailing tints being dark green, red-grey, 

 and light sea-green. In the upper beds there is an increasing 

 amount of sedimentary material, and more rounded pebbles are found. 

 Basic lava-flows occur, for the most part, in the upper beds. Detailed 

 work, laid down on the 6-inch Ordnance map, appears to establish a 

 series of three folds — a northern anticline, a central syncline, and a 

 southern anticline — folded over to form an isocline, with reversed 

 dips to the S.E. The axis of folding is roughly parallel to the axis 

 of St. David's promontory. The total thickness is from 1200 to 

 1500 ft. 



The author devoted a considerable number of pages to further 

 details concerning this series of deposits. He failed to find the 

 alleged Cambrian overlap. " The probabilities are that it is by 

 step-faults between Rhoson and Porth Sele, and not by overlap, 

 that the displacement of the conglomerate has there been effected." 

 Also at Ogof Goch it does not rest upon the quartz-felsite breccia 

 and sheets (group C, of Dr. Hicks), but is faulted against them. A 

 section was devoted to the felsitic dykes, and it was suggested that 

 they may be volcanic dykes of Cambrian age. 



The Relation of the Pebidian to the Dimetian. — The author has 

 not been able to satisfy himself of the existence of the Arvonian as 

 a separate and distinct system. He notes the junction of Pebidian 

 and Dimetian in Porthlisky Bay and the Allen Yalley at Porth 

 Clais, at neither of which places are there satisfactory evidences of 

 intrusion. At Ogof Llesugu the intrusive character of the Dimetian 

 was strongly impressed upon him. He criticised the mapping 

 of Dr. Hicks, and pointed out the difficulties which present them- 

 selves in the way of mapping the Dimetian ridge as Pre-Cambrian. 

 He pointed out that not a single pebble of Dimetian rock, such 

 as those now lying on the beach in Porthlisky Bay, is to be found 

 in the conglomerate. He concluded that the Dimetian is intrusive 

 in the southern limb of the isocline, and that there are no Archaean 

 rocks in situ. 



