Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/529

 Silica 78

Alumina 165

Lime, soda, and iron 5.5 100

The proportion of silica afforded by the foregoing analysis much exceeds that which is obtained from rocks having a syenitic nature.

These quartziferous rocks form an E.N.E. and W.S.W. course ; and near the centre of this ridge is the city of St. David's.

The arrangement of these rocks, which seem to be quartziferous breccias, is rather indistinct. In the immediate neighbourhood of St. David's, and also near Clegyr Bridge, about a mile E. from St. David's, they have associated with them irregular bands of hard greenish -coloured ashy-looking shales, considerably altered in character, but in many instances possessing distinct traces of foliation. Bands of this shale have also been met with in well-sinkings in St. David's*.

Differences prevail in the characters of the rocks which repose upon the central ridge, those on the S.S.E. side presenting an aspect somewhat different from those on the opposite side. This difference seems to have resulted from faults which have brought various rocks of the purple and green series into contact with the quartziferous breccias.

The quartziferous breccias on the S.S.E. have, for the most part, resting upon them quartz rocks of a greenish-grey colour and compact nature. These possess many of the features of hornstone, and they are intersected by closely approximated joints. They have, however, a distinct S.S.E. dip at a high angle, usually about 80° ; and in this position they are well seen in the neighbourhood of Clegyr Bridge, where their strike is parallel to the axis of the quartziferous breccias.

On following the line of strike of the hornstones for a short distance towards the "W.S.W., these rocks are found to disappear. At the ruins of Nun's Chapel, about a mile "W.S.W. from Clegyr Bridge, the compact quartz rocks are seen ; but immediately beyond this all traces of them are lost, and a greenish flaggy sandstone appears at Porth Clais Harbour resting against the quartziferous breccias.

The connexion between the quartziferous breccias and the greenish flaggy sandstones at Porth Clais Harbour results from a fault, having a throw-down on the E.S.E. side, by means of which rocks newer than the compact quartz rocks, or hornstones, are brought into con- tact with the quartziferous breccias. At Porthlisky, a little to the

spots in the ridge where the bedding is distinctly shown, and where the shales are seen alternating with the more massive quartziferous beds. The strike of the beds is from N.W. to S.E., and hence quite discordant to the overlying Cambrian series, in which the strike is from N.E. to S.W. This ridge, therefore, must represent a more ancient group of rocks than the Cambrian, occupying a position equivalent to that of the Laurentian group in Canada. — H. Hicks.
 * Since this paper was read, I have been fortunate enough to find two or three

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