Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/91

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81
Dr. Kidd on the Mineralogy of St. David's.

of the cliffs which they compose a bold but graceful curvature, very characteristic of a coast of this kind, and productive of scenery the most magnificent. A very striking illustration of this effect, though it is not clear whether it proceed from rocks of this class or from those more immediately belonging to the coal grits, occurs at Saunders' Foot, a small cove situated about four miles to the north east of Tenby.

It not unfrequently happens that the partial removal of the superincumbent laminæ of the schist, from the surface of those placed relatively beneath them, has given rise to that appearance of a succession of broad flat steps or stairs which suggested to the Swedish mineralogists the term Trapezius; a term applied by them to that class of rocks, in which from the action of the weather and other causes, there is a tendency to assume an appearance of this kind.

These slaty strata are occasionally traversed by beds of clay porphyry; and by veins of quartz affording very large and beautiful specimens of rock crystal.

It is worth noticing that in none of the stratified or unstratified rocks of this neighbourhood, did the extemporaneous test of an acid give any evidence of the presence of carbonate of lime: nor did there occur in them, with the exception of one equivocal instance, the smallest trace of any organic remains.

St. David's Head.

The rock which forms this promontory consists of a mixture of blackish green hornblende and white felspar; but the proportion of the hornblende often so far predominates, and its crystalline form is so regularly developed, that the felspar appears rather

Vol. II.L