﻿S66 ME. J. E. N. GREEX ON THE GEOLOGICAL [Aug. I908, 



faults, the position of which can be determined with great accuracy 

 owing to the marked lithological differences among the Cambrian 

 rocks, cut up the volcanic group into a series of blocks, or areas 

 within which faulting does not occur, unless the possibility of pre- 

 Cambrian faulting be taken into consideration. Xo evidence has, 

 however, been found that any fault exists which does not cut the 

 Cambrian rocks when traced up to their boundary. 



The order of sequence of the beds in each block has been noted, 

 and the portions of the succession thus obtained have been pieced 

 together into a series that has yielded consistent results over the 

 whole district. 



The entire succession, so far as known, has a visible thickness of 

 over 3000 feet on the west side of the St. David's granite, where 

 the whole of the sequence is seen. On the east the upper half only 

 occurs, but seems thicker than the corresponding portion on the west. 

 The real base is nowhere seen. 



III. The Pebidian. 



Prom the highest to the lowest known beds, the Pebidian series 

 consists essentially of submarine rhyolitic and trachytic tuffs. A 

 certain amount of intermediate material (augite-andesite) occurs 

 in the lower half, which is distinctly more basic, and, speaking 

 broadly, coarser than the upper half. In the eastern area, the later 

 rhyolitic phase is ushered in by a great acid conglomerate. The 

 fragments in this conglomerate and elsewhere are, except when very 

 small, well rolled, and are associated (except in the bands of finest 

 grain) with a variable amount of quartz-grains which seem in all 

 cases to have been derived from igneous rocks, and occasionally, 

 though rarely, show unabraded crystal-forms. Broken felspars, 

 both orthoclase and acid plagioclases, are found plentifully through- 

 out the series. Lenticles of pink or green shale occur at several 

 horizons. Contemporaneous lavas appear to be totally absent. The 

 rocks that have been described as lavas are, as will be shown later, 

 all post-Cambrian intrusions. 



The Pebidian rocks fall readily into fourteen or fifteen constant 

 subdivisions, which group themselves naturally into four series 

 (A, B, C, D), to which it seems convenient to give local names from 

 the locality where each is typically developed and can be best 

 studied. Their sequence is tabulated in the index accompanying 

 the map (PI. XLIV), with the letters which I have attached to 

 the various bands. Assistance in mapping is also afforded by a 

 quartz-felspar-porphyry sill which maintains a constant horizon. 



(A) The Penrhiw Series. 



The lowest or Penrhiw Series is named from the fine exposures 

 in the neighbourhood of Penrhiw Vicarage, north of St. David's. 

 It consists of alternations of red and green tuffs, becoming pro- 

 gressively more felspathic and gritty towards the top. 



