﻿364 MR. J. F. Is^. GEEEX OX THE GEOLOGICAL [Aug. IQoS^ 



(2) Hicks. The tuffs (Pebidian) are unconformably overlain bv 

 Cambrian deposits, which are largely composed of their detritus. The 

 St. David's granitoid rock is both pre-Cambriau and pre-Pebidian, the 

 exposed junctions with the tuffs and Cambrian being all faulted. The 

 quartz-porphjries are not connected with the granitoid rock. 



The literature of the subject, up to 1883, has been fully sum- 

 marized by Sir Archibald Geikie in his paper in toI. xxxix of this 

 Journal (pp. 262-66). Of those T\'hich have appeared since, the 

 most important are Dr. Hicks's defence in vol. xl (1884) p. 507^ 

 and the paper with a map of rock-exposures by Prof. Lloyd 

 Morgan in vol. xlvi (1890) p. 241. 



A study of these papers led to the conclusion that the only hope 

 of definitely settling the problem lay in the preparation of a 

 detailed map of the district ; and the work was begun by laying 

 down the outcrops of the Cambrian rocks, as accurately as possible, 

 on the 6-inch maps. These outcrops were found to be much 

 faulted, and the faults thus indicated could be followed into the 

 adjacent tuffs, which latter, after allowing for the discontinuity 

 caused by these faults, were successfully resolved into a series of 

 marked constancy when traced from one part of the district to 

 the other. 



The faults are so numerous in and near the cliffs that the coast- 

 section fails to afford complete guidance to the complex structure 

 of the area, which could not have been unravelled if the inland 

 exposures were not very numerous. Some difficulties in mapping 

 are caused by the local presence of Boulder-Clay, but the only large 

 continuous mass of it occurs in a band, running east and west, to 

 the north of the city, and thus lies outside the critical area. 



Thanks to Dr. Hicks the Cambrian succession is well known, and 

 the following short account may be given, as providing the most 

 trustworthy basis and the datum-line for other rocks. 



II. The Cambeian. 



The Cambrian in the St. David's area is represented by (a) the 

 Caerfai or Olenellus-^eds ; (6) The Solva or Lower Paradoxidian ; 

 (c) The Menevian, or Lpper Paradoxidian ; and (d) The Lingula- 

 or OZem(s-Beds. 



(a) The Caerfai Beds. 



(1) Basal Conglomerate. — This has been described in detail 

 by several observers, particularly by Sir Archibald Geikie & Dr. 

 Peach ^ and by Dr. Hicks.^ It is variable in texture, but always 

 very coarse at or near the base, where, in addition to rolled quartz 

 and quartzite-pebbles, there are usually boulders of the neighbouring 

 Pebidian. The pebbles and grit are set in a sparse matrix, which 

 is highly micaceous. The conglomerate causes a marked feature 



1 Quart. Journ. Geol. Soc. vol. xxxix (1883) p. 288. 



2 Eep. Brit. Assoc. 1890 (Leeds) pp. 803-804. 



