VOLCAXIC SERIES OF ST. DAVIDS. 251 



from one spot, of the perfect independence of the conglomerate and 

 the volcanic series, and in particular the silvery schists." I 

 approached this locality, therefore, in the full hope of finding satis- 

 factory evidence in favour of one theory or the other. 



The first (north-easterly) exposure of the conglomerate shows 

 this bed resting upon the surface of a lava. Mr. Grenville Cole 

 kindly examined my sections * of rocks from this locality, and the 

 words placed within inverted commas in the following descriptions 

 are from the notes he kindly sent me. Of this rock he says that 

 it is an " Andesite lava, with veins of chloritic decomposition. The 

 flow may result from squeeze, but was probably original when the 

 rock was brown and glassy." There are certainly no marked signs 

 of independence here. The appearance is as if the conglomerate 

 were deposited in the somewhat concave surface of the lava. 



This is, however, not the only exposure of the conglomerate. 

 Further to the south-west there is the clearest and most unmistakable 

 evidence of the interstratification of the conglomerate with Pebidian 

 volcanic beds. In the most westerly exposure there is fair, but not 

 perhaps convincing evidence that the conglomerate is succeeded by 

 volcanic beds four or five feet thick, and these, again, by conglom- 

 erate overlain by fine tuff. The details here are, however, difficult 

 of mastery and of description. But a little to the east of this 

 the relations are more readily decipherable. Beneath the conglom- 

 erate is tuff or broken-up lava. In the midst of the con- 

 glomerate is a thin band of ashy tuif which, under the influence of 

 squeeze, has a somewhat fluidal appearance. Above the conglom- 

 erate are ashy tuffs and perhaps lavas. Pebbles of quartzite are 

 found in the ashy tuft's adjoining the conglomerate. The two so 

 shade into one another that it is ofttimes diflicult to tell where one 

 begins and the other ends. The tonguing-in of one series with the 

 other is complete. One seems to be in presence of an island or 

 shoal in the sea in which the Pebidian volcanic series were accumu- 

 lating. In the hollows on the surface of the lavas and tuft's tongues 

 of conglomerate were formed by the currents that bore onwards 

 the quartzose materials. Then followed other volcanic outpourings 

 by which the conglomerate tongues were buried. 



Above and inland of these exposures is a highly vesicular amyg- 

 daloidal rock (in which a quarry has been opened). This Mr. Cole 

 describes as a " soda-trachyte or highly silicated andesite, with 

 tendency to spherulitic and granophyric structure." It may be a 

 lava, but is more probably a dyke — perhaps a volcanic dyke of con- 

 temporaneous origin. 



I carefully considered on the spot two possibilities — (1) that the 

 lavas and supposed ashy beds were dyke-tnaterial of long subsequent 

 date which had enveloped the conglomerate. (2) That the ashy 

 beds and supposed lavas were remade materials consequent on the 

 denudation of neighbouring Pebidian beds on Dr. Hicks's hypothesis. 

 The field-evidence and the microscopic evidence negative both 



* Prepared by Afr. Thomas Rylay. 



