352 DE. H. HICKS ON THE PRE-CAMBKIAlf AGE 



a grain of the Cambrian rocks had been deposited. In the specimens 

 of the Cambrian conglomerate which were collected by me at 

 different points on the former occasion and submitted to Mr. T. 

 Davies for microscopical examination* abundant evidence was 

 obtained to show that a very large amount of the material had been 

 derived from Granitoid rocks. This w^as particularly the case with 

 the specimens collected at and near Chanter's Seat, on the coast S. W* 

 of Nun's chapel. During my late visit I again obtained specimens 

 of the grits and conglomerates at Chanter's Seat, and I found them, 

 in places, to be almost wholly made up of fragments of characteristic 

 varieties of the Granitoid rocks found in the Dimetian ridge near 

 by ; therefore the facts prove most conclusively that the Granitoid 

 rocks must have been present in the area in Pre-Cambrian times. 

 Moreover certain peculiar structural appearances now observable in 

 these rocks in situ are also equally evident in the fragments in the 

 conglomerate (see notes on slides 1-5 by Prof. Bonney). The facts 

 given with regard to the contents of the Cambrian conglomerates 

 will, I think, be deemed an amply sufficient reply to some of the 

 assertions made by Dr. Geikie in his paper, such as in the following 

 passage, which occurs in his paper at p. 288. — " Dr. Hicks has stated 

 more than once that the Cambrian conglomerates are largely made up 

 of the underlying 'Pre-Carabrian' rocks. As the result of a most 

 careful examination of the conglomerate belt along both sides of 

 the fold, I feel myself warranted in stating confidently that it 

 contains not a single pebble of the characteristic granite of the St* 

 David's ridge." 



The distance between the point where the Dimetian fragments 

 occur in greatest profusion in the Conglomerate, and the Dimetian 

 in situ, is about 800 feet ; but fragments are found also in the inter- 

 vening series. When the Chanter's-Seat beds which belong to the 

 Lower Cambrian series were deposited, it is clear that Granitoid rocks 

 were being freely denuded ; for on tracing the beds at this horizon 

 for some miles to the east and also as far west as Ogof-llesugn, I 

 found that they were very largely composed of the debris of such 

 rocks. In the very lowest beds, however, there is a greater pro- 

 portion of fragments from the Pebidians, and it seems tolerably clear 

 that the Dimetian rocks were but little exposed when those beds 

 were deposited, either from their being covered by Pebidian rocks 

 or because of the presence of a great amount of loose material on 

 the Pre-Cambrian land. The great thickness of Cambrian sediments 

 found in almost all areas shows clearly that there must have been 

 much loose material ready at hand to be washed away as each 

 portion of the Pre-Cambrian land became submerged. This material 

 must have accumulated during great and, possibly, peculiar atmo- 

 spheric changes, as the rocks show clear indications of having been at 

 the time subjected to powerful processes of disintegration. Almost 

 all the rocks we claim to be of Pre-Cambrian age in the St. David's 

 area are freely represented in the Cambrian conglomerates ; and it 

 is important to note that the fragments found of the Granitoid rocks, 

 the felsitic rocks, the halleflintas, the porcellanites, and the various 

 * See Notes 43-71. Quart. Journ. Geol. Soc. vol xl. p. 553. 



