XVII. Notes on the Geological Structure of Cader Idris, 

 By Arthur AIKIN, Esq. F.G.S., &c. 



[Read June 16th, 1826.] 



CvADER 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 ot 

 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 somewhat depressed 

 at each end, but on the whole considerably higher than the neighbouring 

 mountains. Its western extremity 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 trigonometrically by Lieut.-Col. Colby and his assistants. 



The northern face of the mountain is so steep as to be in few places acces- 

 sible, and, in many, approaches to perpendicular. The southern face is a long 

 slope with a varied undulating surface, consisting chiefly of bog and sheep- 

 walk, and concealing from view the subjacent rock. The summit overlooks 

 two remarkable hollows. One, of a semi-circular 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 pei-pendicular : 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 vegetation ; and wherever the rock can be seen it evi- 

 dently consists 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 



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