Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/528

 of Pembrokeshire by Mr. Hicks induced other geologists to seek for similar fossils among the equivalents of these rocks in connexion with the Lower Lingula-flags, which repose conformably upon the upper portion of the purple rocks forming the Merionethshire anticlinal.

In reference to the rocks which rest upon the purple strata on the east side of this anticlinal, Mr. Plant communicated a memoir entitled " Notes relating to the Discovery of Primordial Fossils in the Lingula-flags in the neighbourhood of Tyddyngwladi's Silver-lead Mine." An abstract of this was published in the Quart. Journal of the Geol. Soc. in 1866 ; and the memoir appeared in extenso in the Trans, of the Geol. Soc. of Manchester. A list of fossils obtained from the Lower Lingula-flags of this portion of Merionethshire was given by Mr. Plant ; and this list exhibits a series having a very intimate relation with that containing the fossils obtained by Mr. Hicks from Porth-y-Rhaw, near St. David's.

The Lower Lingula-flags of the valley of the Mawddach, Merionethshire, are seen occurring between hard dark-grey shale-beds, which afford Oleni, and the highest member of the purple rocks of the anticlinal ; and the Lower Lingula-flags here are conformable to the deposits above and below them. In their mineral nature they have great affinity to their equivalents near St. David's, consisting of what Mr. Salter termed " sandstones probably accumulated in deep water."

The strata at Porth-y-Rhaw, which are rich in Trilobites, contain, no red or purple rocks associated with them, but are grey in colour at their base, being banded by light and dark shades, and black in their upper portion ; and it is in the latter that fossils are most abundant.

Beneath the grey beds, and having the same inclination as the Lower Lingula-flags (Menevians) there is, in the St. David's promontory, a great development of rocks, which exhibit red, purple, green, and greenish-grey colours. These rest upon a conglomerate composed of quartz pebbles, of various sizes, cemented together by a reddish or purple sandy matrix.

The beds upon which the conglomerates rest are greenish in colour, and these are supported by rocks of rather peculiar characters.

In many spots they have an aspect which so nearly resembles syenite that it is, at first sight, very difficult to make out their true nature ; for they appear to be made up of crystals imbedded in a base of quartz. When, however, these apparent crystals are carefully examined, they are found to be, for the most part, angular fragments of quartz, not possessing the proper crystalline form which this mineral assumes. Some of the fragments have a subangular outline ; and a few even manifest a distinctly rounded surface. The matrix in which these fragments are imbedded does not exhibit a crystalline arrangement, and contains a very large proportion of silica as a constituent. The chemical composition of a specimen of these rocks has been kindly determined by Dr. Blyth, of Queen's College, Cork, and is as follows : —