12 E. Wynne Hughes — Geology of 



from an underlying platform of older rocks. The specimen from 

 New Guinea was found with a series of corals and -Foraminifera which 

 indicate that it is Eocene and probably Middle Eocene (Gregory 

 and Trench, 1916, p. 53'2). The type-specimen from Java is probably 

 of the same age. 



References. 



Bernard (H. M.). 1896. Catalogue of the Madreporarian Corals in the British 



Museum. Vol. ii : The genus Turbinaria. The genus Astrceopora. 



iv, 106 pp., 33 pis. 

 Duncan (P. M.). 1880. " A Monograph of the Fossil Corals and Alcyonaria 



of Sind, collected by the Geological Survey of India": Pal. Indica, 



ser. xiv, iv, 110 pp., 28 pis. 

 Gregory (J. W.). 1900. " Polytremacis and the Ancestry of Helioporidse " : 



Proc. Eoy. Soc, vol. lxvi, pp. 291-305, pi. ii. 

 Gregory (J. W.) & Trench (J. B.). 1916. "Eocene Corals from the Fly 



River, Central New Guinea " : Geol. Mag., Dec. VI, Vol. Ill, pp. 481-8, 



529-36, Pis. XIX-XXII. 

 Reuss (A. E.). 1866. "tjber fossile Korallen von der Insel Java" : Reise 



Osterr. Fregatte Novara in 1857-9, Geologischer Theil, vol. ii, pt. ii, 



pp. 163-85, 3 pis. 



IV. — On the Geology of the District from Cil-y-Coed to the 



St. Annes-Llanllyfni Ridge (Carnarvonshire). 



By E. Wynne Hughes, M.Sc, F.G.S. 



(PLATES I and II and two Text Maps.) 



Contents. 

 I. Introduction. 

 II. Previous Literature. 



1. General. 2. Particular Area. 

 III. Exposures. 



1. Cil-y-Coed. A. Surface Features. B. Pre-Cambrian, theRhyolitic 



Series, (i) Field Relations, (ii) Petrology. C.Cambrian, (i) Basal 

 Conglomerate, (ii) The Grits. D. Summary of the Succession. 

 E. Relation to the Surrounding Country, (i) North and West. 

 (ii) South-East and East, (iii) South and South- West, (iv) North- 

 East. 



2. Pentwr, Craig- y-Dinas, and Bryn-mawr. A. Cambrian, (i) Basal 



Conglomerate, (ii) The Grits, (iii) The Purple Slates, (iv) The 

 Green Slates. B. Pre-Cambrian, the Rhyolitic Series, (i) Craig- 

 y-Dinas. (ii) Eithinog Wen. (iii) Bryn-mawr. C. Summary of 

 the Succession. D. Relation to the Surrounding Country. 



3. St. Annes-Llanllyfni Ridge (South-West End). A. Moel 



Tryfan. Conglomerate, Grits, Purple Slate. B. Bwlch-y-llyn and 

 Mynydd Cilgwyn. Conglomerate, Grits, Purple Slate, and Rhyolitic 

 Series. C. Clogwyn Melyn and Caer Engan. Rhyolitic Series, 

 Conglomerate. D. Summary of the Succession. 

 IV. Conclusion. 



I. Introduction. (See Map I in text.) 

 f HUE existence of two ridges of Pre-Cambrian rocks in South-West 

 JL Carnarvonshire is well known. The more northerly of these 

 ridges extends from near Bangor to Carnarvon, a distance of just over 

 8 miles. It runs in a south-westerly direction, close to and parallel 

 with the Menai Straits, and terminates at the hill called Twthill — 

 a prominent feature in the town of Carnarvon. The more southerly 



