VOLCANIC SEKIES OF ST. DAVIDS. 



257 



evidence for this on the ^vestern coast is not satisfactory, but its 

 existence is unquestionable. It would seem to flatten in the neigh- 

 bourhood of Ivhoson and Trefeithan. 



South-oast of the central syncline runs another anticlinal fold. 

 Below Treginnis Mill the evidence of the normal anticline is satis- 

 factorj^, the seaward dip of about 50° being readily observable even 

 from a distance. Tracing the beds eastwards 

 along the coast, the dip becomes higher, up 

 to 85°, seawards, and then passes over to a 

 landward (reversed) dip of 75°-G5°. The 

 normal anticline passes into an isocline. The 

 northerly dips in Porthlisky Bay are reversed 

 dips. 



The diagrammatic section (fig. 5) shows 

 the structure of the western portion of the 

 St. David's promontory as I read it. 



The thickness of the Pebidian volcanic 

 series exposed in the district, I estimate at 

 about from 1200 to 1500 feet — a modest 

 figure compared with the 8000 feet claimed 

 by Dr. Hicks. 



c. Farther Details. — There are one or two 

 points on which further details may be desi- 

 rable. 



(a) The Fcmlt at Ogof G6ch.—K well- 

 marked fault here brings basic beds against 

 the more acidic schistose strata of Ogofau- 

 Dduon (the Ogfeydd-duon of previous writers. 

 I have adopted the spelling of the G-iuch 

 survey map). Dr. Hicks is perfectly right 

 in contending that these Ogofau-Dduon beds 

 do not, as suggested by Dr. Geikie, imme- 

 diately overlie the more basic beds ; but by 

 failing to take note of the Ogof-Goch fault 

 he erroneously made the conglomerate rest 

 immediately and unconformably upon these 

 beds. The fault is not only, as previously 

 noted, perfectly obvious on the coast-line, 

 but further inland it divides the more acidic 

 crags of Carn Gwil and Carn Geli Fach 

 from a triplet of basic crags of which Carn 

 Coch is the most prominent. 



(/3) The Porphyry Gra(js of Tre(/innis. 

 — The strongly marked rock-exposures of 

 which Carn Gwil Geli is the most easterly 



perhaps faulted syncline. It may be, however, that the syncline lias flattened 

 out ; that, as Dr. Geikie suggests, the Clegyr Foia sheets lie on the southern side of 

 an isocline ; and that the northerly di[)3 there observable are reversed. 1 could 

 not determine this point with certainty. But 1 lean to the view that the I'olding 

 here is synclinal. 



