284 Reports and Proceedings — Geological Society of London. 



4.— April 15tli, 1908.— Dr. J. J. Harris Teall, M.A., F.R.S., 



Vice-President, in the Chair. 

 The following communications were read : — 



1. "The Geological Strnctiire of the St. David's Area (Pembroke- 

 shire)." By J. Frederick N. Green, B.A., F.G.8. 



With a view to the elucidation of controverted points in the geology 

 of the St. David's area, the author has mapped the district in consider- 

 able detail on the 6 inch scale. The Cambrian Rocks, the succession of 

 which is well known, were first traced and were found to be greatly 

 faulted. The faults have been followed into the underlying volcanic 

 tuifs (Pebidian), and the succession within the intervening blocks of 

 ■country determined and pieced together. In this way the Pebidian 

 has been subdivided into fourteen horizons, with a total visible thick- 

 ness of over 3,000 feet. The tuffs are described in some detail, and 

 the subdivisions classified into four series — the lower two of which are 

 composed chiefly of trachytic pebbles in a chloritic matrix, and are 

 usually separated by a peculiar schistose quartz-felspar-porphyry sill. 

 The third series is composed of fragments of rhyolite and hiilleflinta in 

 a silicified matrix, and the topmost now principally consists of highly 

 sheared schistose beds. The volcanic fragments throughout the Pebidian 

 are rolled, and have often undergone changes before deposition ; thus 

 the tuffs appear to be mainly, if not wholly, detrital. 



By the aid of this succession, an unconformity between the Pebidian 

 and the Cambrian is demonstrated, the position of the basal Cambrian 

 conglomerate on the volcanic series varying by at least 1,000 feet. 

 The red coloration of the beds immediately underlying the con- 

 glomerate is due to staining. 



The schistose sill has been traced into the porphyritic margin of 

 the St. David's granophyre (Dimetian), to which it is allied petro- 

 graphicalh' ; and it is inferred that the granophyre is a laccolitic 

 intrusion in the Pebidian. The boundaries between the granophyre 

 and the Cambrian are prolongations of faults proved in the latter; 

 except at one point in the well-known Porthclais district, which, on 

 account of its importance, has been mapped on the scale of 25 inches 

 to the mile. A trench specially opened at this point exposed basal 

 Cambrian rocks resting upon a denuded surface of the granophyre, 

 which is therefore of pre-Cambrian but of post-Pebidian age. 



The relationships of the basic igneous rocks west of St. David's, 

 which have hitherto been held to be Pebidian lavas, are discussed, 

 and the conclusion is reached that they are all post-Cambrian 

 intrusions. Finally, it is suggested that the word Pebidian should be 

 revived as a general term. 



2. "Notes on the Geology of Burma." By Leonard V. Dalton, 

 B.Sc, F.R.G.S. (Communicated by Dr. A. Smith "Woodward, 

 F.E.S., F.L.S., V.P.G.S.) 



The object of this paper is to present the results of geological 

 expeditions in the Irawadi Valley, carried out by the author and 

 Mr. W. H. Dalton between 1904 and 1906, and "to correlate these 

 observations with those made by previous writers, thus summarizing 

 present knowledge of the geology of Burma in general, and of the 



