Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/95

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85
Dr. Kidd on the Mineralogy of St. David's.

spersed with low rocky cliffs, which rise higher and higher in advancing to the southern extremity; at which point the rocks are particularly interesting from their variety; passing, sometimes abruptly, from the coarsest grained conglomerate, as from its appearance it might be called, to the finest schist.

The prevailing colours of these rocks are green and brownish purple; those colours alternating occasionally as in striped jasper. The cement of those parts which resemble a conglomerate appears to be in a great measure of chemical origin; containing minute crystals of semitransparent felspar, with small particles of glassy quartz. It seems worthy of remark, that in those parts of the rock which resemble a conglomerate, the pebble-shaped nodules of quartz are very frequently of the same purple colour as the schist. The sand of this beach when viewed through a microscope is seen to be a mixture of fragments of shells with small particles of variously coloured quartz and slate. A portion of it weighing 200 grains, which had been collected in July 1811, and had been kept in a room without a fire till March 1812, only lost one grain of its weight by exposure to a heat of 212°: after which, having been boiled in diluted muriatic acid, and then filtered, washed, and dried by the same heat, it weighed 157 grains; having lost or rather more than of its weight, which may be considered as very nearly the proportion of calcareous carbonate contained in this sand. The sand is extensively used as a manure.

Porthclais.

This is a small fishing harbour situated to the south or south east of St. David's, and is the termination of a narrow shallow valley, which extends two or three miles inland, and is longitudinally