PKE-CAALBKIAN ROCKS OF ST. DAVID'S. 325 



with the development of porphyritic-felspar crystals and veins and 

 threads of crystalline quartz. 



It is suggested for future inquiry whether the granite and accom- 

 panying porphyries can possibly represent the roots of any of the 

 palaeozoic volcanoes which played so important a part in the geologi- 

 cal history of Pembrokeshire, as well as in the rest of Wales, during 

 the Lower Silurian period. 



The last episode is that of the diabase dykes, which represent the 

 third and final outbreak of eruptive rocks in the St. David's district. 

 These dykes rise particularly through the central core of granite, as 

 through an old area of weakness. Whether they were connected 

 with any outpouring of lava at the surface cannot be decided ; nor 

 does there appear to be any evidence that might lead even to an 

 approximation to their relative geological date. 



EXPLANATION OF PLATES YIH.-X. 



Plate VIII. 

 Geological Sketch map of the St. David's district (p. 268). 



Plate IX.* 



Fig. 1. Microscopic section of volcanic tuff, near Pen-y-foel, St. David's, 

 X 25 diameters. (See p. 298.) 



2. Ditto of fragment of lava in tuff, Pen-y-foel, X 100 cliam. The 



large crystal shows the perfect form occasionally assumed by the 

 augite in the lapilli of the basic tuffs. The finely cellular struc- 

 ture is also represented. (See p. 298.) 



3. Ditto of band of tuff from red shales, with Lingulella primceva, 



Castell, Ramsey Sound, viewed by polarized light, with crossed 

 nicols. The half marked A , X 50 diam. ; that marked B X 100 

 diam. The portions here drawn were selected to show the way 

 in which scattered felspars occur among the other lapilli. (See 

 p. 299.) 



4. Ditto of olivine diabase . from near Pen-y-foel, X 100 diam., 



showing the remarkably well developed forms presented by the 

 olivine. (See p. 304.) 



5. Ditto of schist, roadside north of St. David's, X 100 diameters. The 



granular-shaded parts are the abundant green chloritic consti- 

 tuent, separated into elongated nests, between which the foliation 

 is well marked. Here and there ortboclase, magnetite, or other 

 mineral has crystallized out. (See p. 311.) 



6. Ditto of schist, Ogfeydd-duon, X 100 diam. Among the consti- 



tuents occasional rounded grains of quartz appear, as on the 

 right-hand margin of the slide. (See p. 311.) 



Plate X. 



Fig. 7. Microscopic section of schist, with crumpled folia, from roadside 

 section north of St. David's, X 100 diam. (See p. 312.) 

 8. Ditto of schist, with refoliation, from Porth-clais, X 25 diam. 



* The drawings from wbich these plates of microscopic structure have been 

 prepared were made for me by my colleague Mr. F. W. Eudler, to whom my 

 best thanks are due. 



Q.J.G.S. No. 155. 2 b 



