10 MR. PHILIP LAKE ON THE DENBIGHSHIRE [Feb. 1 895, 



quarried at Glyn Ceiriog and elsewhere. He correlates the lower 

 part of the series with the Wenlock, and the upper part with the 

 Ludlow of Murchison, and considers the rocks of Dinas Bran to 

 represent the Upper Ludlow. In this he is probably more nearly 

 correct than is generally admitted. 



Sedgwick, in his various writings, refers several times to the 

 Denbighshire series of the Llangollen area, and in his description of 

 a section drawn from Llansaintffraid Glyn Ceiriog to Cyrn-y-brain * 

 he describes the succession in some detail. In the ' Upper Silurian ' 

 he recognizes a lower zone of roofing-slate with Graptolites ludensis, 

 and above this a great thickness of Denbighshire Flagstone. The 

 latter he divides into lower and middle divisions, with Oreseis 

 \_Orthoceras~] and Graptolites ludensis, and an upper division — the 

 rocks of Dinas Bran — with Terebratula \_Dayia] navicula, etc. Except 

 for the fact that several species of graptolites are confounded under 

 the same name, this description, so far as it goes, gives a good idea 

 of the succession. Sedgwick makes no attempt to correlate these 

 subdivisions with those of the Silurian elsewhere, but dissents from 

 the views expressed by Bowman on this subject. 



A few years later an account of the geology of North Wales was 

 published by Daniel Sharpe. 2 In the part of this paper which 

 treats of the Llangollen district, Sharpe agrees in general with 

 Bowman, and places a part of the Denbighshire series in the Wen- 

 lock and a part in the Ludlow. There is a certain amount of con- 

 fusion in this paper as to the base of the Wenlock ; but the author 

 seems to be inclined to draw the boundary between his Wenlock 

 and ' Lower Silurian ' below the flags [Nantglyn Flags] and above 

 the zone of roofing-slate [Pen-y-glog Slates]. 



The account of the Denbighshire series which is given in the 

 Geological Survey Memoir on North Wales 3 is very short, and no 

 serious attempt is made to describe the succession. The whole is 

 placed provisionally in the Wenlock, but it is pointed out that the 

 upper part may possibly belong to the Ludlow. In the palseonto- 

 logical appendix the beds of Castell Dinas Bran and the Llansannan 

 shales 4 are separated from the rest as a higher subdivision. 



Beyond a few casual references, nothing further seems to have 

 been written concerning this area until 1880, when a paper appeared 

 by Mr. J. E. Marr 3 on the geology of the Dee Valley. In this 

 Mr. Marr describes the Pen-y-glog Slates, at the base of the series ; 

 the Pen-y-glog Grits above them ; and the Dinas Bran Beds, which 

 he places at the top of the Silurian of this area. He correlates the 

 whole series with the Brathay Flags and Coldwell Beds of the Lake 

 District and with the Wenlock of Murchison. 



In an announcement of the discovery of remains of plants at the 



1 Proc. Geol. Soc. vol. iv. (1843) p. L ; 63. 



2 Quart. Journ. Geol. Soc. vol. ii. (1846) p. 283. 



3 Mem. Geol. Surv. vol. iii. ed. 2 (1881), pp. 284, 300. 



4 Prof. Hughes suspects that the Llansannan shales lie at a much lower 

 horizon than the top of the Denbigh grits, Proc. Chester Soc. Nat. Sci. & Lit. 

 pt. iv. (1893) p. 150. 



5 Quart. Journ. Geol. Soc. vol. xxxvi. (1880) p. 277. 



