﻿376 ME. J. F. X. GEEEX ON THE GEOLOGICAL [-^Ug. I908, 



member of the two lower series of the PebidiaD. At the eastern- 

 most point of the district which has been mapped in detail, near 

 Yachelich and Llandruidion, it is in contact with the lowest 

 division (Dl) of the Ilamsey-Sound Series ; and, from the breadth 

 of outcrop of the latter, it must be near the top. Tracing the 

 conglomerate westwards to the Caerbwdy Yalley, it is seen resting 

 upon the basal bands of the Eamsej-Sound Series, and so small is the 

 thickness of these beds here that the staining described on p. 377 

 has extended through them to the underlying Caerbwdy Series (C 5). 

 At the arch in St. iNon's Bay, which has been repeatedly described,^ 

 the conglomerate lies at some depth below the top of the Caerbwdy 

 Series, as, allowing for a neighbouring dyke of quartz-porphyry, 

 not more than 140 feet of its uppermost division (C 5) appears to 

 be present. Passing to the west of the Bay, the base of the 

 Cambrian is clearly transgressive, as at the Stacks of the bathing- 

 place, although it still reposes on C 5, there is now only 40 feet of 

 this division present. In the little Bay, south-south-west of St. N'on's 

 Chapel, only 15 feet of the division intervenes between the easily 

 recognized halleflinta (C 4) and the base of the Cambrian. After 

 passing this point, the junction is a faulted one and continues 

 so for some distance. 



It is thus seen that, between the Caerbwdy Yalley and the 

 western part of St. Xon's Bay, the Cambrian has transgressed over 

 fully 400 feet of the Pebidian. 



On crossing the granophyre to the Porthlisky area, a patch of 

 Cambrian hitherto unrecorded is seen to occur a littlfe inland from the 

 cliff; but, as its exact relations to the tuffs are not known, and the 

 Pebidian rocks are greatly deformed, it does not afford trustworthy 

 evidence of transgression. Xevertheless, if the view (also Sir 

 Archibald Geikie's) that these schistose rocks are identical with 

 those of Ramsey Sound be correct, the base of the Cambrian must 

 be at least 1000 feet higher in the Pebidian than at St. Xon's Bay. 



The Cambrian conglomerate is not seen again, until the cliffs of 

 Eamsey Sound are reached ; but in the southern outcrops no normal 

 junction occurs. As, however, it is now clear that the highest series 

 is undoubtedly represented here, the argument just put forward 

 holds good in this case, for we have at least 900 feet more Pebidian 

 present below the Cambrian than at the western end of St. Xon's Bay 

 without reaching the base of the Cambrian. A normal junction 

 does actually occur at the arch in Maen Bachau, the horizon in the 

 Pebidian being probably the upper part (D 3) of the Ramsey-Sound 

 Series ; but the outcrop is so narrow, and the rock so deeply stained, 

 that its identification is difficult. The only other normal junction 

 exposed is a mile and a half farther north, at T^'hitesand Bay, where 

 the conglomerate rests upon D 2, about the middle of the Ramsey- 

 Sound Series, an horizon that is certainly at least 600 feet above 

 that of St. Xon's Bay. 



Prom the evidence just given it may be fairly claimed that an 

 unconformity between the Cambrian and the Pebidian has been 

 established. 



^ See Quart. Joiirn. Geol. Soc. vol. xlvi (1890) pp. 2-i4 ct seqq. 



