14d Proceedings of Philosophical Societies, [Aug* 



crystals of felspar. These beds all rise NE by N, but their 

 angle of elevation is continually increasing, and the last forms 

 the summit of Craig y Cae. 



From hence to the margin of the crater the space is occu- 

 pied by alternations in nearly vertical beds of soft glossy slate, 

 of coarse slate with ochry spots and small cells, of grey wakke, 

 of porphyritic quartz, and slaty potstone. About tlie middle 

 of the series is a single bed of brownish grey rock, appearing 

 to be ferruginous quartz intimately mixed with carbonate of 

 lime. 



The next bed, forming part of the summit of Cader Idris, 

 composed of globular concretions, very hard, containing specks 

 of pyrites, and melting in very thin shivers into a black glass, 

 k supposed to be a trap-rock. 



After minutely detailing the other beds of Cader Idris, their 

 position and angles, the author proceeds to a mountain (forming 

 the northern boundary of the little valley wherein the Goat's 

 Pool and another small lake are situated), extending for about 

 two miles parallel with Cader Idris. This he calls " the Stony 

 mountain." It is composed of rounded tubercular crags and 

 hemispherical bosses of trap, like enormous ovens, rising group 

 above group. Their surfaces are comparatively smooth, and 

 generally reticulated with veins of quartz, which sometimes 

 occurs in areas four or five yards across, several inches thick, of 

 an obscurely slaty structure, and adhering to the surface of the 

 trap. Many of the groups when seen in profile appear to be o^ 

 a very irreeular and thick slaty structure, but, wnen visited in 

 front and looking down upon them, are evidently clusters of 

 columns laterally aggregated, and intersected by oblique irre- 

 gular joints. 



'the large quarry of sienite on the Tawyn road is noticed as 

 showing the connection of the trap and of the stratified rocks, 

 and this is also shown in a very interesting manner on the de- 

 scent northwards from Grey Graig, the eastern extremity of 

 Cader Idris. 



From these and other facts detailed in his paper the author 

 considers it evident that Cader Idris, and the ground between 

 that mountain and the Mawddoch, as well as the northern 

 boundary of the valley, consist of various well known tran- 

 sition rocks, rising in general N. by E. or W. — that the beds 

 both at the northern and southern extremities are at low angles, 

 not greater than 20°, — that the intermediate beds are at high 

 angles, approaching to vertical, — that they rest upon and are 

 interrupted by trap-rocks more or less columnar, — that the 

 trap-rocks are surrounded in many places by mantle-form 

 strata, which in some instances are obviously of the same mate- 

 rials as the trap, and differ only in structure, but which some- 



