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VOLCANIC SERIES OF ST. DAVIDS. 261 



ing coast-line to the east. They may be regarded as definitely 

 connected with the volcanic series. Others, however, are of far more 

 recent origin and are fonnd cutting alike through Dimetiac, Pebidian, 

 and the overlying Cambrian sedimentary beds. Fine examples may 

 be seen in the Dimetian in the Porthlisky promontory, in the Pebidian 

 at St. Non's Bay, and in the Cambrian sedimentaries at Porth- 

 y-rhaw. 



The acidic dykes are the quartz-porphyries of Dr. Geikie and the 

 felsites of other authors. Some of these in the Church-school and 

 Board-school quarries, for example, and in St. Non's Bay, are 

 familiar to all who know the literature of St. David's geology. But 

 there are other examples in many places along the Pebidian coast- 

 section. One of these is mentioned by Dr. Geikie as occurring near 

 Pen-y-foel. Tt is readily seen as one stands on this headland and 

 looks westwards across the Ogof Mrs. Morgan inlet. Near the sea- 

 line there are two converging limbs which appear to meet above to 

 form a common mass. Between the limbs a triangular mass of tuff 

 (basic) is enclosed, and similar tuff is seen on either side. I have had 

 half a dozen slices cut from various parts of this dyke for micro- 

 scopic determination. Mr. Grenville Cole has kindly afforded me 

 much valuable assistance in this examination. 



The rock is granophyric in structure, with abundant spherulitic 

 aggregates in which the radial structure is well seen. Good 

 porphyritic felspars occur, which Mr. Cole is inclined to regard as 

 oligoclase. The banded structure along the edge of the dyke is 

 beautifully seen some 20 or 30 feet above the water's edge, and in 

 one part waving flow-lines are very noticeable on the weathered 

 surface. Vesicular and amj^gdaloidal structure is well marked in 

 parts. The vesicles are large, elongated, and oval, with their long 

 axes parallel. This led me to suspect that the dyke was rather 

 volcanic than plutonic in origin. For in plutonic dykes the vesicles 

 are generally rounded and small. In one of my slides the rock 

 shows much brecciatiou, which may, as Mr. Cole suggests, contain 

 fragments from the adjacent tuffs or may have resulted from dragging 

 and flow along the contact plane. 



In the notes with which Mr. Cole was good enough to furnish me 

 he describes the dyke in general as " a gOod granophyric rock like 

 the wall of Cadcr Idris and many other British examples. The 

 analysis ought to give some 70 ^o of silica, and soda as the dominant 

 alkali, — a quartz-andesite, or perhaps more nearly an oligoclase- 

 trachyte had it erupted as a lava. It might well be an offshoot of 

 a soda- granite or (juartz-diorite." 



I subjoin an analysis of this rock (I.), together with an analysis 

 of the acidic tuffs (II.) of Pen-y-foel hard by for comparison. They 

 are kindly furnished me by Mr. Cecil Gibbins, who carried out the 

 analyses in the chemical laboratory of the University College, 

 Bristol. I have added in the third column (III.) an analysis of 

 Dimetian, quoted from Dr. Persifor Frazer by Dr. Ilicks. 



