80 Reviews — Dr. John Ball — 



IV. Conclusion. 



The different formations at all the above localities on the St. Annes- 

 Llanllyfni ridge lie in the same relative position to one another as 

 the purple slates, grits, conglomerate, and rhyolitic series at Craig-y- 

 Dinas, and the grits, conglomerate, and rhyolitic series at Cil-y-Coed. 

 Further, the different formations are each to each lithologically 

 similar. This resemblance is well brought out in Plate II, showing 

 microphotographs of the conglomerate: Fig. 1, from Cil-y-Coed; 

 Fig. 2, from Mynydd Cilgwyn ; the fine grit : Fig. 3, from Cil-y-Coed ; 

 Fig. 4, from Bwlch-y-llyn ; the coarse grit: Fig. 5, from Cil-y-Coed; 

 Fig. 6, from Bwlch-y-llyn. 



The evidence seems, therefore, conclusive that the series at 

 Cil-y-Coed and Craig-y-Dinas are of the same age as those of the 

 St. Annes-Llanllyfni ridge. Accepting the view, usually held, that 

 the porphyry on this ridge is pre-Carnbrian and that the conglomerate 

 is basal Cambrian, we have — 



1. At Cil-y-Coed — Lower Cambrian grits and conglomerate lying on Pre- 



Cambrian rbyolites and tuffs. 



2. At Craig-y-Dinas — Lower Cambrian green and purple slates, grits, and 



basal conglomerate lying on Pre-Cambrian rbyolites. 



3. At Bryn-mawr — Pre-Cambrian rhyolites and tuffs. 



Pre-Cambrian Rhyolites and Tuffs. — To sum up, the solid rocks 

 exposed in the area between Cil-y-Coed and the village of Pen-y-groes 

 comprise a volcanic series of rhyolitic type, probably Pre-Cambrian, 

 overlain by a series of conglomerates, grits, and slates of Lower 

 Cambrian age. This sequence represents substantially the succession 

 which obtains in the main portion of the St. Annes-Llanllyfni ridge 

 to the north-east ; and it is clear that the solid rocks of the St. Annes- 

 Llanllyfni ridge extend at least 3 miles further south-west than is 

 represented in the Geological Survey Map. 



In conclusion, I desire to express my indebtedness to Mr. C. J. 

 Edwards for suggesting the woi'k on Cil-y-Coed and for several 

 photographs, to Dr. Co>x for reading through the manuscripts and for 

 various suggestions during the course of the investigation, and to 

 Br. Gordon for his help in connexion with the microphotographs. 



EEVIEWS. 



I. — The Geography and Geology of West-Central Sinai. By 

 John Ball, Ph.D., D.Sc, F.G.S., A.R.S.M., Mem. Inst. C.E. 

 pp. ix, 219, with two geological maps printed in colour, 

 22 plates, and 54 text-figures. Cairo: Government Press, 1916. 

 Price 30 P.T. 



T01HIS important work owes its origin to the discovery of ores of 



T 



Mr. Thomas Barron during his reconnaissance survey of a vast area 

 in Western Siuai in the years 1898-9. Prospecting followed, the 

 Sinai Mining Co. was formed to exploit the deposits, and Dr. Ball 

 was deputed by the Director of the Geological Survey of Egypt to 



