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LXXIV. Notes o?i the Geological Structure of Coder Idris. 

 By Arthur Aikin, Esq. F.G.S. 4'c* 



CADER IDRIS is a mountain ridge running almost E. 

 and W. to the south of, and nearly parallel with, the 

 course of the river that flows from the town of Dolgelle to the 

 sea at Barmouth. Its length is about eight miles and a half 

 in a straight line. It consists of a winding summit ridge some- 

 what depressed at each end, but on the whole considerably 

 higher than the neighbouring mountains. Its western extre- 

 mity terminates on the sea-shore in a steep slope ending in a 

 cliff, the entire height of which is about 1500 feet: its eastern 

 extremity called Geygraig, is 2267 feet high, the summit is 

 2914 feet, and the other points are of intermediate altitudes 

 between those just mentioned, and were ascertained trigono- 

 metrically by LieuL-Col. Colby and his assistants. 



The northern face of the mountain is so steep as to be in 

 few places accessible, and, in many, approaches to perpendi- 

 cular. The southern face is a long slope with a varied undu- 

 lating surfiace, consisting chiefly of bog and sheep-walk, and 

 concealing from view the subjacent rock. The summit over- 

 looks two remarkable hollows. One, of a semicircular figure, 

 commonly called the crater, forms part of the northern face : 

 the two extremities of the semicircle are respectively 2894 and 

 2655 feet high. The descent to the Goat's Pool (Llyn y Gafer), 

 which lies in the bottom of the crater, is above 1000 feet, and 

 almost perpendicular : the pool itself is 1835 feet above the 

 level of the sea. 



The other hollow, called Cwm y Cae, is on the S.E. of the 

 summit : it is of an irregular elliptical figure, with a break or 

 opening on the S.E. into the valley of Tal y Llyn : at the 

 bottom of this hollow is also a pool about 1660 feet above the 

 level of the sea. 



The hill called Mynydd pen y Coed is the highest of those 

 which stand out on the southern slope of the mountain, being 

 2504 feet. The upper part is but little obstructed by vege- 

 tation ; and wherever the rock can be seen it evidently con- 

 sists of beds of slate, very regular, and rising N.E. by N. at 

 an angle of about 35°: on the N.E. end, however, of the hill, 

 near the edge of Cwm y Cae, the beds, without changing 

 their direction, bend up sharply so as to increase their angle 

 to about 50°. Induced by this circumstance, I took a line of 

 section from this jjart of the hill by the edge of Cwm y Cae, 

 over Craig y Cae, to tlie margin of the crater, and thence to 

 the Goal's Pool. Mynydd pen y Coed consists almost entirely 



* From the Geological Transactions, Second Series, vol. ii. p. 273. 

 New Series. Vol.2. No. 12. Dec. 1827. 3 K of 



