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

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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 ${\displaystyle \scriptstyle {\frac {42}{199}}}$ or rather more than ${\displaystyle \scriptstyle {\frac {1}{5}}}$ 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