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
In my last paper, communicated to the Geological Society in December 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 recognized was at the base of the Cambrian, where it rested on the pre-Cambrian ridge. In the accompanying map and sections (P1. 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 alternating 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 N.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 N.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