438 Mr. A. Aikin on the Geological Structure of Cader Idris. 



phyritic slate, and this latter on imperfect amygdaloid, which 

 itself rests on the columnar basaltiform trap. 



On descending from Geygraig to Dolgelle by the edge of 

 the valley of the Aren, first occur craggy hummocks of massive 

 trap more or less columnar, surrounded by slaty trap in man- 

 tle-shaped strata, and covered in different parts by beds of a 

 shining micaceous clay-slate. Near Dolgelle the river flows 

 over beds alternately slaty and massive ; the former are di- 

 stinctly steatitical, and will probably be considered as pot- 

 stone ; the latter differ from them in being harder and con- 

 sisting of a larger proportion of quartz. Sometimes they con- 

 tain carbonate of lime, and then effervesce briskly for a short 

 time with acids. 



On tracing up the Dolgelle river, the Ynnion, for a few miles 

 towards Bala, no rock occurs iii situ except massive crystalline 

 greenstone ; but on turning to the N. up a small lateral valley, 

 I found beds of the same kind of rock as appear at Dolgelle in 

 the bed of the Aren, some of which are sufficiently calcareous 

 to burn into a reddish-brown sandy lime, and which therefore 

 may perhaps be called limestone : it contains no organic re- 

 mains. Beds very similar to these, and in all probability a 

 continuation of them, appear at the foot of the hill at Llaneltid 

 bridge over the Mawddach, on the road from Dolgelle to Bar- 

 mouth, where they rise N.W. at an angle of about 20°, and 

 form the upper members of a series of the grauwacke forma- 

 tion, composed of common blue slate, of finely foliated gray 

 slate more or less calcareous, and of coarse green slate. These 

 slaty beds alternate with sandstone consisting chiefly of quartz, 

 in grains varying from a very minute size to that of hazle-nuts, 

 intermixed with calcareous spar, the whole cemented together 

 partly by quartz and partly by carbonate of lime. This sand- 

 stone, in the lower beds, is less calcareous, and mixed more 

 or less with scales of slate ; it also by exposure to weather 

 splits into masses somewhat like starch, which stand nearly at 

 right angles to their planes of stratification. 



From the facts above detailed, it will I think be evident, 

 that Cader Idris and the ground between that mountain and 

 the Mawddach, as well as the northern boundary of the valley, 

 consist of various well-known transition rocks rising in gene- 

 ral 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 appi'oaching to 

 vertical ; that they rest upon and are interrupted by trap-rocks 

 more or less columnar; that the trap-rocks themselves are sur- 

 rounded in many places by mantle-form strata, which in some 

 instances are obviously of the same materials as the trap, and 



differs 



