The Ordovician Lavas of Mynydd Prescelly. 321 



wise been unfortunate at Rosebush, but a quarry in similar 

 slates on Banc-du yielded one specimen of a small " tuning- 

 fork " Didymograptus, which I am indebted to Dr. Gertrude 

 Elles for examining. The specimen is unfortunately rather 

 smeared by the cleavage, but Dr. Elles is inclined to regard it 

 as being of D. Murchisoni type rather than D. hifidus. These slates 

 immediately overlie the rhyolites of Greenway, and it is therefore 

 suggested that these occur either high up in the Bifidus (it must be 

 remembered that B. Murchisoni var. geminus extends well down 

 into this zone in South "Wales) or low in the Murchisoni slates, i.e. 

 about the horizon of the Llanrian rhyolites as originally suggested 

 oy Dr. Elsden. Until further evidence is available it would not 

 be wise to attemj)t any closer correlation of these rocks. 



(e) Chemical Composition. 



Unfortunately there are not many analyses of the soda-rich rocks 

 of South Wales published, and most authors have been content to 

 quote Dr. Elsden's original analysis of the Abercastle 

 " Keratophyre ". A few analyses were made in the Surv^ey 

 laboratory in connexion with the Skomer series, and are given in 

 Dr. Thomas's paper and in the Memoir on Sheet 227 (Milford). Only 

 two of these are applicable to the present case, and are given in 

 columns II and III below, together with Dr. Elsden's analysis 

 as quoted by Professor Cox (IV). 



Mr. Radley, who was responsible for the Skomer analyses, has 

 very kindly made a partial determination for me of the rock from 

 Cnwc-y-felin, west of Rosebush, and his figures are shown in column I. 



II. 



III. 



IV. 



I. Soda-rhyolite, Cnwc-y-felin, west of Rosebush. 



II. Soda-rhvolite, near the Table, Skomer, Pembrokeshire. (Anal. 345) 

 Mem. Geol. Survey, Sheet 227 (Milford), p. 31. 



III. Soda-trachvte, north cliff, Skomer, Pembrokeshire. (Anal. 347) Mem. 

 Geol. Survey, Sheet 227 (Jlilford), p. 33. 



IV. " Lime-bostonite ", Longhouse, Abercastle, Pembrokesliire. Elsden, 

 O.J.G.S., vol. Ixi, 1906, p. 596. 



VOL. Lix. — NO. vn. 21 



