VOLCANIC SERIES OF ST. DAVIDS. 



3. The Pebidian Succession. 



255 



a. The Results of previous Investigations. — In his earlier paper 

 (Q. J. G. S. xxiv. p. 15) Dr. Hicks divides the Pebidians into eleven 

 groups. Hubseqiiently (Q. J. G. S. xl. plate xxiv.) he maps five 

 groups : — 



c. Purple and green schistose breccias. 



d. Diabase breccias and sheets. 



e. Quartz-felsite breccias and sheets. 

 h. Felsite tuffs &c. 



a. Agglomerates, breccias, and porcellanites. 



These are mapped in parallel bands, little disturbed by faults 

 except where they adjoin the Dimetian and Arvouian axis. In 

 describing the area to the north of the axis, Dr. Hicks says: — 

 " Here, again, the evidence of a regularlj- ascending succession in the 

 Pebidian, but broken by a few faults, is perfectly clear, and there 

 is no indication of an isocline. The beds dij) regularly in one 

 direction at a high angle, and the alternations are well marked and 

 easily traced." (Q. J. G. S. xl. p. 544.) 



Dr. Geikie divides the series provisionally into four groups : — 



4. Pine tuffs and silky schists. 



3. Diabase sheets with intruded quartz- porphyry and hardened tuffs. 

 2. Compact green granular tuff, 



I . Thick purplish-red, green-flecked tuff, with abundant small lapilli of 

 felsite. 



These beds, according to Dr. Geikie, are isocliiially folded, the 

 axis of the isocline cutting the coast-line between Pen-maen-melyn 

 and Pen-y-foel. 



Professor Blake found only a glorious confusion, which enchanted 

 him. 



b. (jrtii'sral Results of the Authors /Purvey. — Although the time at 

 ray disposal did not permit of anything like an exhaustive resurvey 

 of the district, yet I was able to lay down on the six-inch survey 

 map the evidence afforded by all the important inland exposures 

 and that presented by the long coast-section. The main facts are 

 indicated in the accompanying sketch maps on the six-inch scale 

 (I'late X.), in which direct observations alone, and such inferences 

 as are inevitable therefrom, are included. A short description of 

 certain special features is also given in the pages which follow. Here 

 I will describe the main results wliich I have obtained. 



There can be no question that with the exception of some cinder- 

 beds to the west which appear to be subaerial, the whole series was 

 accumulated under water, many of the beds containing (juartzose 

 and other sedimentary material. The tuffs vary in colour from 

 dark green, or dark green and red, through lighter shades to light 

 green, light pink, and white. The former are decidedly basic, the 

 latter acidic in appearance. Sometimes, as may ))e well seen in the 

 Allan Valley, east of the cathedral, differing bands alternate some- 

 what rapidly. The lighter more acidic beds contain great numbers 

 of often large felsitic and felspathic fragments. In the more basic 



