Geological Society. 59 



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 coarser slate, with ochry spots and small cell ; of grauwacke ; 

 of porphyritic quartz and slaty pot-stone. About the middle 

 of the series is a single bed of brownish-gray 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, 

 is 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 Goafs 

 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 of a very irregular and thick slaty structure ; but when 

 visited in front or looking down upon them, they are evidently 

 clusters of columns, laterally aggregated, and intersected by 

 oblique irregular joints. 



The large quarry of sienite on the Tawyn Road is noticed, 

 as showing the connexion of the trap and of the stratified 

 rocks ; and this is also shown, in a very interesting manner, 

 on the descent northwards from Guy Graig, the eastern ex- 

 tremity 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, consists of various well-known transi- 

 tion rocks, rising in general N. by E. or W. : that the beds 

 both at the northern and southern extremities are at low an- 

 gles, 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 ma- 

 terials as the trap, and differ only in structure; but which, 



H 2 sometimes, 



