SUCCESSION OF THE ANCIENT ROCKS OF ST. DAVID'S. 16' 



13. On the Succession of the Ancient Rocks in the vicinity of St. 

 David's, Pembrokeshire, with special reference to those of the 

 Arenig and Llandeilo Groups, and their Fossil Contents. By 

 Henry Hicks, Esq., E.G.S. (Read December 2, 1874,) 



[Plates YIXI.-XL] 



Introduction. 



In my last paper, communicated to the Geological Society in De- 

 cember 1872, the order of the succession of the strata in the vicinity 

 of St. David's was carefully tabulated to the top of the Tremadoc 

 group ; and it was there shown that the succession was a perfectly 

 continuous one from the base of the Cambrian series to the top of 

 the Tremadoc group, and that the only break or unconformity recog- 

 nized was at the base of the Cambrian, where it rested on the pre- 

 Cambrian ridge. In the accompanying map and sections (PI. VIII.) 

 the following order of the rocks is shown. Directly under the city of 

 St. David's, or, rather, under its eastern portion, are some massive 

 beds of quartzy conglomerates, very compact in structure, but alter- 

 nating with dark-green shales, with a strike from ~N.W. to S.E. 

 These have a thickness as we follow them directly eastward from 

 this spot of about 4000 feet ; and they run, with a varying thickness 

 not exceeding this, for about five miles in a IST.E. direction from St. 

 David's, but are then cut off by a fault. They also extend in a line 

 to the S.W., reaching the coast of St. Bride's Bay, on the east side 

 of Porth-lisky Harbour, but are considerably reduced in thickness 

 at this part by a fault running in a IST.E. direction. The ridge 

 formed by these rocks underlies the whole of the remaining strata of 

 the district, being the axis on which the Cambrian and other rocks 

 are supported. This ridge is marked in the Geological- Survey maps 

 as a mass of intrusive syenite ; and in some places, especially near 

 the centre, it may be said, from its very compact nature, to assume 

 almost that character ; but when the mass is more carefully ex- 

 amined, it is evident that the whole is composed of bedded rock, 

 partly metamorphosed. 



The difference in the thickness of the ridge in our map in places 

 from that shown on the Survey maps is accounted for by our 

 having excluded some strata along the edge which belong to the 

 overlying Cambrian series, and which rest unconformably on the 

 ridge. The bedding of the strata composing the ridge may be 

 distinctly seen in the hill to the N.E. of the Cathedral, also on the 

 west side of the valley beyond the mill between St. David's and 

 Porth-clais, and on the east side of Porth-lisky Harbour. The beds 

 forming the Harlech or Longmynd group, consisting of green shales, 

 conglomerates, and purple, red, and green sandstones and slates, are 

 seen on both sides of the ridge ; they rest unconformably on the 

 edges of the beds of the axis ; and there are indications that they once 



