THE OLDER ROCKS AT ST. DAVIDS. 121 



abundant of fibrous character, and pseudomorphing a bi silicate 

 probably, the polarising characters have not entirely disappeared : it 

 is usually, however, without prismatic outline, and scattered 

 through the ground, constitutes the chief part of the rock. 

 Altogether the rock shows less decomposition and calcification 

 than the vertical dyke at Forth Clais (/. c, p. 23^ note.) 



Pehidian. This series consists of a great variety of rocks, of 

 which many are enumerated by Dr. Hicks {I. c, p. 235) ; the 

 principal point to be noted about them is their relation to the 

 Dimetian, their strike is the direction of dip of the latter series — a 

 total unconformity. This result of Dr. Hicks' researches presents 

 to us, thus, two distinct series separated by wide intervals of time. 



We have spoken of the Dimetian as metamorphic, i.e., highly 

 crystallized rocks throughout, and over a portion of the area, at 

 any rate, showing not evident bedding enough to prevent our 

 classing it as a massive crystalline rock. The Pebidian series, on 

 the contrary, is not a crystalline but a sedimentary series, the 

 bedding being perfectly plain throughout. The beds seen 

 adjoining the Dimetian axis on the N. of the city are shales 

 and agglomerates, ashes mixed with mud, and seemingly the 

 product of submarine volcanos mixed with ordinary sediments. 

 The bedding is most unmistakeable, there is no metamorphosis 

 here, and the strata are in strong contrast to the crystalline rocks 

 of the lower series. It does not enter into our purpose to describe 

 the almost endless variety of this series ; sometimes coarse 

 agglomerates, at other times fine shales. At Trevithan, Sec, are 

 green and basic igneous rocks between bedded shales, probably 

 contemporaneous flows. At other times the shales are made up 

 of highly felspathic materials, pale in colour as if acidic out- 

 bursts were furnishing the products spread out over the sea-floor. 



One remarkable rock it is necessary to notice, viz., those beds 

 described {ihid. p. 236) as "the lower of the series resting 

 immediately on the Dimetian axis (S. side), hard, compact, 

 conglomerates, the distinctive outline of the pebbles for the most 



