volcanic series of st. dayids. 243 



2. Tfe Eelation op Pebidian to Cambrian. 



a. General Considerations. — The Cambrian conglomerate is a 

 deposit of variable thickness, reaching in some places 100 feet. It 

 is composed of rolled fragments, consisting in- the main of quartz and 

 quartzite. These are well rounded and sometimes of large size, as 

 big as a man's head or larger. 



The underlying beds, where direct contact is observable, belong 

 to the Pebidian series. According to Dr. Hicks they formed shore- 

 lines and were very much in their present mineral condition before 

 any of the Cambrian sediments were deposited. The conglomerate 

 itself is, he says, very largely indeed made up of fragments and finer 

 materials derived from them by denudation, and constantly overlaps 

 different members of the volcanic series, from which it is separated 

 by a systematic unconformity. 



Dr. Geikie denies the systematic unconformity, admitting only 

 contemporaneous erosion ; he denies that the included pebbles are 

 " almost invariably for the most part " derived from Pebidian rocks, 

 and he denies that they constantly overlap different members of 

 the volcanic series. 



Let us, before examining the evidence in detail, consider what we 

 are entitled to expect on the one view and on the other. 



On Dr. Hicks's view we should expect to find a more or less well- 

 marked line of demarcation between the Cambrian conglomerate and 

 the upturned and eroded edges of the more ancient strata. We 

 should expect a varying discordance of strike between the uncon- 

 formable systems. We should expect the included fragments, or at 

 any rate a large proportion of them, to consist of well-rounded 

 pebbles of the volcanic breccias similar to those which are abundant 

 on the present shore-line where Pebidian strata are undergoing 

 denudation. 



On Dr. Geikie's view we should expect the conglomerate to lie 

 on a current-swept surface of the volcanic tuffs and breccias which 

 were then still soft. We might even expect a considerable thick- 

 ness of the volcanic beds to have been in some places swept away 

 by currents capable of rolling onwards such largo masses of 

 quartzite. Together with this local contemporaneous erosion we 

 should expect systematic conformity and accordance in the main 

 lines of strike. We should expect some shading of the conglomerate 

 into the underlying volcanic beds at the point of junction and some 

 inwashing of the volcanic beds into the conglomerate. Wc should 

 expect to find occasionally, among the materials of the conglomerate, 

 subangular fragments of the larger inclusions of the breccias and 

 tuffs over which the current passed. 



Such would seem to bo legitimate expectations. What are the 

 facts? 



There are four localities where the Pebidian-Cambrian junction 

 may be observed. Two (Cacrbwdy Valley and St. N'on's 13ay) arc 

 to the south of St, Davids, two (Ogof Golchfa and llamsey Sound) 

 to the north. We will consider the stratigraphical evidence in each 



