H. HICKS ON THE PRE-CAMJBRIAN ROCKS OF ST. DAVID'S. 231 



feet in vertical thickness, as shown by the sections near St. David's ; 

 and the upper (Pebidian) have a thickness of over 3000 feet exposed 

 at several points, and undoubtedly are of very much greater thick- 

 ness, only that they are everywhere more or less hidden by the 

 Cambrians. 



Dimetian. 



The portion of the ridge formed by these rocks gives an average 

 width of from 2000 to 3000 feet from St. David's northward to the 

 position where they are cut off by the great E. and W. fault at Tregle- 

 mais and Carnymyl. To the south of St. David's, and especially about 

 midway between St. David's and the coast, the width is greatly reduced 

 by faults on either side. Here only the more central portion of the 

 ridge seems to have been left ; and the faults which have so reduced 

 the thickness have in addition dropped the immediately overlying 

 Pebidian beds, as well as some of the lower Cambrians. At the 

 bend in Porthclais Valley the conglomerates of the Cambrian rest 

 immediately on the Dimetian rocks, and a little further east in the 

 harbour still higher beds of the Cambrians. On the coast to the 

 south the highest rocks of the Harlech group strike up against 

 the ridge. With these exceptions, and that of another small area 

 at the north-east end of the ridge near Carnymyl, which is again 

 covered immediately by Cambrian conglomerates, both sides are 

 flanked throughout by the Pebidian rocks. The rocks which com- 

 pose the Dimetian series are chiefly compact quartz schists, chloride 

 schists, and indurated shales. Sometimes these shale-bands, as under 

 St. David's, alternate frequently with the quartzose beds ; but usually 

 the latter predominate to a very great extent. Jn section I. (PL X.) 

 apparently, there are but few bands of shale ; but as there are only a 

 few exposures of the rocks in this section, I am unable to speak with 

 confidence on this point. In the direction of section II. I have had 

 frequent opportunities of examining the rocks in deep wells, in ad- 

 dition to the numerous quarries open ; and I have almost everywhere 

 found bands of shale associated w T ith the quartzose beds along this 

 line. The most highly crystalline beds which I have yet seen occur 

 to the north of St. David's. In these the quartz seems to be nearly 

 perfectly crystalline. Other beds between this point and St. David's 

 are also very much altered ; but as we go further south the rock 

 seems to be less highly metamorphosed. In section III. there are 

 several bands of shale at the base ; but in the exposures along the 

 side of the valley further south, and until we reach near the coast at 

 Porthlisky, the quartzose schists again are very greatly in excess. The 

 portion of this section which extends along the east side of Porth- 

 lisky harbour is the most important part, geologically, of the whole 

 ridge, as it not only shows the bedding very distinctly, but the 

 quartzose beds are less altered*, and a change also takes place in 



* Mr. T. Davies, F.G.S., of the British Museum, has kindly given me the 

 following report of a microscopical examination of a section of this rock: — 



