376 ME. H. H. THOMAS AND PROF. 0. T. JONES ON THE [Sept. I912, 



exposures are mentioned in his notes ; he had, however, included 

 in the Cambrian a good many pyroclastic rocks of greater antiquity 

 (p. 379). He gave a description of the granite at several localities, 

 but erroneously considered, on macroscopic evidence, that it con- 

 tained hornblende. 



On the later edition of the Geological Survey Map (7) the 

 rhyolitic rocks of Rhindaston are indicated as intrusive porphyry, 

 but the mass at Pointz Castle is still coloured as granite. 



This region has not been revised since 1857 by the Geological 

 Survey. 



After an interval of twenty years, we enter upon a period which 

 was marked by a great advance in South Welsh stratigraphy, 

 namely that period over which were spread the researches of the late 

 Dr. Hicks and J. W. Salter ; for, with the recognition of a base to 

 the Cambrian System and the establishment of a faunal succession 

 within the Cambrian itself, modification of the pioneer work of 

 the Survey was not only necessary but expected. Hicks had 

 already suggested the names 'Dimetian' and 'Pebidian' for two 

 pre-Cambrian rock-groups at St. David's, and in 1878 (9) he re- 

 cognized their extension into the Hayscastle area. He remarked 

 that, in the latter district, the Dimetian rocks 



' form an axis to the newer rocks . . . similar in many respects to the one at 

 St. Davids; and they are flanked in part by metamorphic Pebidian, and 

 along both sides by unaltered Cambrian rocks.' 



In 1879 (10) he suggested the name ' Arvonian' for a group of 

 pre-Cambrian bedded felsitic rocks, and as such rightly claimed the 

 rocks of Pointz Castle and Rhindaston. The Arvonian, however, 

 included rocks of post-Cambrian age, and the remainder has proved 

 to be inseparable from the Pebidian. 



Sir Archibald Geikie in 1883 (12) questioned the pre-Cambrian 

 age of the older rocks at St. David's and incidentally those of 

 this region, so that Dr. Hicks (14) in the following year found it 

 necessary, with additional evidence, to reiterate his statement that 

 the igneous masses of Pointz Castle, Brawdy, and Hayscastle were 

 older than any of the Cambrian deposits. His last paper bearing 

 on this district was published in 1886 (15), and in an appendix 

 by T. Davies were given several descriptions of Dimetian and 

 Pebidian types. 



Since this date nothing had been written on this particular area, 

 until one of us gave a brief mention of the rock-types and a few of 

 the chief exposures in the Summary of Progress of the Geological 

 Survey for 1908 (16). In the summer of last year one of us con- 

 ducted certain Members of the Geologists' Association over a portion 

 of the ground that we had surveyed, and Mr. J. P. N. Green 

 published a report on this excursion in the ' Proceedings of the 

 Geologists' Association' (17), based on notes supplied by us. 



