part 4] UPPER ORDOVICIAX OF THE BERWVX HILLS. 489 



II. Previous Literature. 



The existing literature on the subject is scanty. "Sedgwick 

 crossed this part of the Berwyn country, and noted the inversion 

 of the strata l ; but the most complete description of the area is 

 given bv J. B. Jukes in Ramsay's Geological Survey Memoir on 

 North Wales (2nd ed. 1881) ; here Jukes remarks (p. 297) that 



' the interest to be derived from the study of a locality in which so many and 

 such various physical features are so admirably shown, is heightened by the 

 fact of the limestone and parts of the ash-beds and all the beds about them 

 being crowded with a vast variety of fossils, containing, I believe, all the 

 characteristic species of the formation, and generally in a well-preserved 

 condition.' 



This contention is undoubtedly well founded, for the beds are 

 particularly well exposed and remarkably fossiliferous at nearly 

 every horizon. 



Following Jukes, there is a paper by D. C. Davies on the 

 phosphorite-deposits of the Berwyn Hills, in which he gives a 

 detailed section of the phosphate-mine at Cwm-gwynen-uchaf 3 ; 

 while of late years notes have appeared on the succession and 

 horizon of the beds near Pen-y-garnedd, at the north-eastern end 

 of the area, in the Summary of Progress of the Geological Survey. 3 

 Much work, however, has been done on the surrounding districts, 

 which enables close correlations to be made. The ground on the 

 east, and even the north-eastern part of the area under discussion, 

 has been surveyed in recent years by the officers of the Geological 

 Survey, largely by Mr. B. Smith and myself. Similarly, rocks of 

 the same age on the northern Hank of the Berwyn Hills have been 

 described by Mr. P. Lake & Dr. T. T. Groom, 4 and more recently 

 remapped by Dr. L. J. Wills, while on the Geological Survey, and 

 described by him and Mr. B. Smith in a recent paper read before 

 the Geological Society. 5 On the south-east lies the Welshpool 

 area, worked out by Dr. A. Wade in 1911. 6 On the west is 

 the classical Bala country, of which Miss G. L. Elles has given an 

 excellent account, 7 together with palajontological subdivisions for 

 the shelly facies of the Upper Ordovician rocks, wdiich are proving 

 of wide application and much value. 



1 A. Sedgwick, Q. J. G. S. vol. i (1845) p. 5. 



2 Q. J. G. 8. vol. xxxi (1875) pp. 357-67 ; also preliminary note in Geol. 

 Mag. vol. iv (1S67) pp. 251-53. On the 6-inch Ordnance Survey map, the 

 place-name is spelt ' Cwm-gwnen.' 



:t ' Summary of Progress for 1919 ' Mem. Geol. Surv. 1920, pp. 4-5. 

 1 Q. J. G. S. vol. lxiv (1908) pp. 546-95. 

 ' Ibid. vol. lxxviii (1922) pp. 176-226. 

 " Ibid. vol. lxvii (1911) pp. 415-59. 



7 Ibid. vol. lxxviii (1922) pp. 132-73; also Geol. Mag. vol. lix (1922) 

 pp. 109-14: 



