Uie Ordovician Lavas of Mynydd Prescelly. 311 



post-Valentian but pre-Old Red Sandstone age. It is hoped that the 

 detailed examination of the area, now in hand, will provide further 

 evidence bearing on this question. 



A preliminary visit to the district during the past summer served 

 to dispel much of the monotony from the rocks briefly noticed by 

 Parkinson, and as most of them differ somewhat from the other 

 occurrences in that part of Wales, it seems desirable to publish 

 a fuller description than apj^ears in his paper. The district is con- 

 tained in Sheet 210 (New Series) 1 in. Ordnance Survey, and Sheet 40 

 Old Series (geologically coloured), though the boundaries shown on 

 the latter are, in many cases, erroneous. 



Soda-trachytes, keratophyres, and allied types have ■ been 

 described from various parts of Pembrokeshire and Carmarthen- 

 shire — notably from the Abercastle district,^ Treffgarne,^ Skomer 

 and the mainland adjacent,^ and Coomb. ^ These are of Arenig 

 age, but in addition soda-rhyolites are known from several horizons 

 ranging from pre-Cambrian to Llanvirn. 



Unlike the Arenig occurrences mentioned above, the lavas fron^ 

 the Prescellys apparently occur in the Llanvirn series, but tbe 

 evidence is not complete. Owing to the strong cleavage by which 

 all the sedimentary rocks have been converted into slates in this 

 northern half of the county, fossils are both scarce and very ill- 

 preserved. The whole district abounds in slate quarries (many — 

 like the large ones at Rosebush — disused), and there are trial-holes 

 all over the hills round that village. Lately there has been a revival 

 in this local industry, and several quarries are at work near 

 Maenclochog and Llandilo. 



In addition to the cleavage the area has been subjected to a con- 

 siderable amount of faulting of the usual North Pembrokeshire 

 type, i.e. east-west strike-faults accompanied by numerous, but 

 not usually individually important, dip-faults. 



(a) The Soda-rhyolites and Felsites. 



(a) There is little to add to the description given by Parkinson 

 of the spherulitic and other varieties from Cam Alwand Foel Trigarn. 

 Dr. Elsden has already drawn attention to the similarity of many 

 of these rocks to the rhyolites of Llanrian near Abereiddy Bay.^ 



One interesting feature which has hitherto escaped notice is the 

 presence of the Foel Trigarn rocks of numerous relatively large 

 crystals of zircon — ranging up to -Ol in. in length. 



East of Rosebush, near the farm Eithbed-fach, there are exposures 

 of silicified rhyolites and rhyolitic ashes. Other ashes — usiially 

 porcellanous and quite featureless under the microscope — occur 



1 J. V. Elsden, Q.J.G.S., vol. Ixi, 1905, p. 594 ; A. H. Cox, ibid., vol. Ixxi, 

 1915, p. 307. 



" Mem. Geol. Surv., Sheet 228 (Haverfordwest), p. 21. 

 3 H. H. Thomas, Q.J.G.S., vol. Ixvii, 1911, p. 175. 

 " CantriU & Thomas, Q.J.O.S., vol. Ixii, 190(i, p. 243. 

 ^ Op. cit., p. 581 ; see also Cox, op. cit., p. 316. 



