286 a. geiexe on the supposed 



4. Relation oe " Pebidian " to Cambeian koce;s. 



The unconform ability between the top of the " Pebidian " rocks 

 and the conglomerate which Dr. Hicks assumes to be the base of 

 the Cambrian system likewise disappears on examination. As before, 

 he rests upon presumed discordance in the dip and strike of the 

 rocks, and on the alleged presence of fragments of the older rocks 

 in the younger. But both these tests fail him. 



In the first place it can be conclusively shown that in dip and 

 strike the volcanic group and the overlying conglomerate, sandstone, 

 and shales are perfectly conformable throughout. Mr. Peach and I 

 proved this by numerous measurements all over the district. We 

 observed that, at the locality on Ramsey Sound, near Castell, men- 

 tioned by Dr. Hicks as showing the unconformability of the con- 

 glomerate, there has been a slight local disturbance of the strata. 

 The beds below the cod glomerate have been bent up, and the con- 

 glomerate itself has been pushed over them. Seen from the top of the 

 cliff the conglomerate lies in part on their edges. But when examined 

 on the spot the unconformability disappears, and the strata below the 

 conglomerate are found at a little distance from the disturbance to 

 be perfectly conformable with it. The same complete conformability 

 is well exposed on the face of the next projecting cliff southward. 



At another locality, on the coast south of Caer-fai, also referred 

 to by Dr. Hicks, there is an apparent discordance between the con- 

 glomerate and the beds below it. But here again the seeming un- 

 conformability at once disappears on examination. It is an instance 

 of the familiar phenomenon of what has been called " contempo- 

 raneous erosion." Every field-geologist knows that this structure 

 constantly occurs among pebbly strata, from the most recent valley- 

 gravels to the most ancient sedimentary rocks yet known. Were it 

 to be used as indicative of serious unconformability, we might have 

 half a dozen discordant formations in a single gravel-pit. But the 

 section at Caer-fai, as shown in fig. 7, might as well be taken to 

 prove unconformability above the conglomerate as below it. In 

 reality these are merely common local accidents of sedimentation, 

 and, but for their relation to the question now under discussion, 

 would not be worthy of special notice. 



Dr. Hicks, in one of his papers, remarks that " the lino of strike 

 of the Cambrian rocks appears at first sight to be nearly identical 

 with that of the underlyiDg Pebidian beds ; but when examined 

 carefully it will be seen that in no case is it truly so, but that the 

 conglomerates overlap the beds irregularly and at different points in 

 the succession"*. In another paper he states that the Cambrian 

 conglomerate overlaps the " Pebidian " group altogether, so as to rest 

 upon the " Dimetian" rocksf. 



I was not surprised by these statements when I found that he 

 had wholly missed the structure of the ground between Eamsey 

 Sound and the granite ridge. At first an observer traversing 



* Quart. Journ. G-eol. Soc. vol. xxxiv. p. 159 (1878). 

 t Ibid. vol. xxxiii. p. 230 (1877). 



