230 H. HICKS ON TIIE PRE-CAMBRIAN HOCKS OF ST. DAYJd's. 



however, I have given a good deal of time to the further and special 

 examination of these rocks, and I have obtained much additional 

 and valuable information in regard to their distribution and mineral 

 composition. 



General Description. 



The ridge formed by these rocks attains its greatest breadth near 

 the city of St. David's, where it is rather over a mile wide. It ex- 

 tends in a direction from St. David's to the N.E. for about 4 miles, 

 with an average width of rather less than a mile, and is then cut off 

 by a fault running nearly east and west. To the south of St. David's 

 it is greatly reduced in width by faults on either side. The portion 

 immediately under St. David's is thrown down to some extent to the 

 S.E. by two faults ; and hence this part is not entirely in line with 

 the remainder of the ridge. Except in the immediate neighbour- 

 hood of the faults, which cross it, the ridge stands boldly out, with 

 a tolerably even rounded surface, the highest points being at least 

 300 feet above the level of the sea. Low marshy ground marks the 

 lines of the three chief faults. In my map of St. David's (Quart. 

 Journ. Geol. Soc. vol. xxxi. pi. viii.) the width in some places is 

 understated, as I find it necessary, after further examination, to 

 include as Pre-Cambrian all the beds below the Lower Cambrian con- 

 glomerates, and to which the latter are unconformable. 



I propose now to divide the Pre-Cambrian rocks into two distinct 

 series under the local names of Dimetian (Dimetia being the ancient 

 name for a kingdom which included this part of Wales) for the 

 lower, and Pebidian (Pebidiauc being the name of the division or 

 hundred in which these rocks are chiefly exposed) for the upper series. 

 Up to the present time no name has been given to rocks in England 

 or Wales which occupy an earlier position than the Cambrians ; and 

 as there can be no doubt that these two series of rocks are overlain 

 unconformably at St. David's by the lowest Cambrian rocks hitherto 

 known in Wales, I think I am justified in giving them local names 

 in preference to attempting at present to correlate them with rocks 

 in other countries which may appear to hold a somewhat similar 

 position, but which may nevertheless indicate separate periods in 

 the world's history. 



The rocks composing the ridge consist chiefly of compact quartzi- 

 ferous beds, altered shales, and limestones in the lower series, and 

 of altered conglomerates and shales in the upper. The two series 

 are everywhere unconformable to one another, the strike of the beds 

 in the lower being from N.W. to S.E., and in the upper from S.W. to 

 N.E. The Cambrian conglomerates overlap them irregularly, in 

 some places entirely covering over the upper series. The conglome- 

 rates of the Cambrian also are almost altogether made up of pebbles 

 derived from the underlying rocks. It is difficult to give a cor- 

 rect estimate of the thickness of either series, because of the faults 

 in the lower, and of the overlapping by the Cambrians of the upper ; 

 but it is certain that the lower (Dimetian) cannot be less than 15,000 



