121 THE GEOLOGY OF THE DISTRICT AROUND ST. DAVID'S, PEMBROKESHIRE. By J. FREDERICK N. GREEN, B.A., F.G.S. [Read March 3rd, 191f.1 Advance copies of this Paper and of the Paper on Ramsey Island were printed and issued to Members in connection with the Easter Excursion of IgII. The two Papers are now reprinted with a few additions and alterations. INTRODUCTION. ---HE Council of the Geologists' Association have done me 1 the honour of appointing me Director of an excursion to St. David's.'*' My experience of the district is not extended, and during my stay there I have devoted myself wholly to the struc• tural problems, leaving on one side the zonal paheontology and surface geology. Most of what follows must therefore be looked upon as a compilation from the papers mentioned in the Biblio• graphy (p. 136). Further, the area has been very insufficiently worked, especially in respect of its remarkable glacial geology, so that in these notes there must be many imperfections, the elimination of which may, I hope, be advanced by our excursion. The structure of this region, properly named" Dewisland," has occasioned one of the most severe disputes in the history of British geology. This controversy attained grave proportions owing to its bearing on other areas and on the generalisations drawn therefrom. I should not have ventured to assume in these notes the correctness of the interpretation given by myself in 19°8, which substantially upheld the main contentions of the late Henry Hicks, who made this corner of Wales a classical district for the study of our oldest rocks, had I not been authorised by the kindness of Mr. H. H. Thomas and Professor O. T. Jones to say that, in a forthcoming paper on country some miles farther east, they would describe a closely similar succes• sion, viz., a series of tuffs penetrated by ancient crystalline intrusions and overlain unconformably by the basal Cambrian. The stratigraphical interpretations of those who previously studied the district were very various in detail, but settled down to certain main points of difference: (I) The existence or non-existence of an unconformity above the ancient tuffs (Pebidian) which lie at the base of the stratified series. (2) The Pre-Cambrian or Post-Cambrian age of the St. David's granitoid rock (Dimetian). (3) The relationship of the quartz-porphyries; whether Pre-Cambrian and distinct (Arvonian) from the Dimetian or not. • This refer. to the Easter Excursion of '9". PROC. GEOL. AssoC., VOL. XXII, PART 3, IgIL] II 122 ]. FREDERICK N. GREEN ON These structural questions are, however, not now of much interest compared to those presented by the two great groups of fossiliferous and igneous rocks. Although St. David's itself is difficult of access, once there almost all the main points of interest are within easy reach. A circle of three miles radius with the Cathedral as a centre con• tains nearly all the best exposures, including fourteen miles of magnificent coast-line and the fine sections along the Alan, Caerbwdy and Porthyrhaw Valleys. The Solva Valley lies close along the circumference. The roads and tracks are good. GENERAL STRUCTURE. A fold of Cambrian and Ordovician rocks, with axis about east-north-east and west-south-west, exposes a core of Pre• Cambrian tuffs and intrusions, usually about a mile in diameter. St. David's itself lies on this core one-third of a mile from the southern boundary of the Cambrian. The fold is overturned south-west of the city, flattens eastward to a more symmetrical arch, but is overfolded again and strongly thrust when seen in the Upper Solva Valley, three miles east-north-east. South-east of the St. David's axis other overfolds and thrusts occur, one of the latter bringing the Cambrians over the Millstone Grit and Coal Measures at Newgale Sands, six miles east of the city. Newgale is situated near the seaward termination of another core of Arch~an rocks, parallel to the St. David's core, the intervening strip being, so far as yet ascertained, wholly occupied by lower Pal~ozoic beds with various intrusions. This general structure is, however, complicated by a multitude of faults, mostly small, but sometimes with a throw of two to four thousand feet. Many of these faults are reversed. In the neigh• bourhood of the city they have usually been flooded with basic material which has accumulated in considerable masses west and north of St. David's. When in Pliocene times the sea wiped out the traces of earlier denudation by planing down the country to a platform, now standing at a level of about 200 ft. above O.D., these larger intrusions with some other bits of hard rock were left standing out as rugged" stacks," which now constitute the most characteristic feature of the local scenery. It is now proposed to give a general account of the rocks and '3tructures of the district in their chronological order. PRE-CAMBRIAN. Volcanic ROCRS.-The oldest rocks of the district are the Pre-Cambrian Volcanics or Pebidian, which, in the St. David's axis, consist entirely of tuffs, for the most part detrital. THE GEOLOGY OF THE DISTRICT AROUND ST. DAVID'S. 123 They are probably of the same age as the Bangor Volcanic series, the Uriconian, and perhaps the Charnwood Rocks. A thickness of over 3,000 ft. is exposed. Excellent sections me seen close to the hotel, but they are best studied to the south-west, especially about Porth-henllys and Treginnis-isaf. Table I (p. 134) gives a conspectus of the succession, but for a more detailed description I must refer to my paper of 1908.* The compositicn is very varied, including pebbles ranging from acid rhyolite, through trachyte and hornblende• andesite to augite-andesite, with acid and basic glasses, bits of halleflintas and other tuffs, and broken crystals of quartz, felspar, augite, apatite and magnetite. Among the most interesting points are boulders of red fluidal rhyolite scattered through the Treginnis series, andesite with ferruginous pseudomorphs after hornblende in the peculiar quartz-felspar tuff at the top of the Penrhiw series, bedded halleflinta quarried in the Caerbwdy Valley, and the sheared tuffs of Porthlisky. At Pointz Castle rhyolite flows are intercalated. Intrusive Rocks.-Special attention must be given to the quartz-felspar-porphyry sill, mimicking an augen-gneiss, which, except in the extreme west, comes in above the Penrhiw series. Its schistosity appears to be due to pressure while still hot and viscous; and it can be traced from Treginnis-uchaf into the porphyritic margin of the "Dimetian" near Rhoscribed. The sill, being hard, is often exposed, but its junctions are rarely seen. The roof can, however, be observed at Porth-henllys. The Dimetian granophyre, the true nature of which was first detected by Sir A. Geikie, is itself of interest owing to its Pre-Cambrian age and to the alteration which it has undergone. It covers an area immediately south-west of the city two miles in length and about a mile in greatest breadth. The rock is much jointed, often lesembling a rudely-bedded rock, especially when the joints are filled by slicken-sided calcite or ferruginous material. It is traversed by bands of brecciation, formerly mistaken for tuffs. The ferro-magnesian elements (biotite and perhaps horn• blende) are replaced by chlorite. The margin is delicately granophyric, with some felspar and many well-iormed bipyramidal quartz phenocrysts. It is best seen near Rock House, where it is cut by an elvan; but no exposure of the actual junction with the tuffs is as yet known. The quartz-porphyries associated with the granophyre are best developed in and north of the city, but are also found at St. Non's Bay, Pen Pedol, Rhodiad, Newgale Sands, LlanhowelI, etc. Beautiful spherulitic structures are developed in the finer grained varieties, and have been frequently described. More usually the base is micropcecilitic. The Cathedral and neigh• bouring buildings are founded upon a coarser rock with micro- " Quart. Journ. Geol. Soc., vol. lxiv, p. 363. 1 2 4 ]. FREDERICK N. GREEN ON gramtlc matrix. Sometimes the quartzes are spherical and weather out like pebbles. PALlEOZorC. Lower Palaozoic.-The actual junction of the Cambrian and the Pebidian is exposed on the south coast on both sides of St. Non's Bay, and on the west coast at Maen Bachau and Ogof Go1chfa (Whitesand Bay). The basal conglomerate contains a variety of rocks, including jasper, porphyry, many quartzites and boulders of tuff. A point of interest is the red staining which affects rock immediately underlying (in a stratigraphical sense) the conglomerate. The stained tuffs show a replacement of epidote and chlorite by hematite, and of felspar by a mixture of ferruginous and transparent products. This alteration, which seems similar to that which occurs under the Torridonian in the Highlands, usually extends about 15 feet from the junction and is clearly older than the folding. It has not affected the Granophyre. The relation is well shown at the Natural Arch in St. Non's Bay (PI. XX), where stained Pebidian tuffs with Porcellanite bands, belonging to the highest part of the Caerbwdy series, are thrust over the basal Cambrian conglomerate, which is seen in the lower part of the cliff resting unconformably on the tuffs. The conglomerate is again thrust against the (stratigraphically) overlying green flaggy sandstone, and the tunnel has been excavated along the band of shattered rock. This section has been described and sketched in the Quarterly Joumal ofthe Geological Society by Geikie (vol. xxxix, p. 287), Hicks (vol. xl, p.