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



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 mantle-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 distinctly steatitical, 

 and will probably be considered as potstone ; the latter differ from them in 

 being harder and consisting of a larger proportion of quartz. Sometimes they 

 contain 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 in 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 suffi- 

 ciently calcareous to burn into a reddish-brown sandy lime, and which there- 

 fore may perhaps be called limestone : it contains no organic remains. 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 Barmouth, where they rise N.W. at an angle of about 20°, and 

 form the upper members of a series of the grauwacke formation, composed of 

 common blue slate, of finely foliated grey 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 sandstone, 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 wefl-known transition 

 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 in- 

 termediate 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 themselves are surrounded in many places by mantle-form strata, which 

 in some instances are obviously of the same materials as the trap, and differ 

 only in structure, — but which sometimes bear a less obvious resemblance to 

 the trap; and, from exhibiting transitions from that rock to those which com- 

 pose the regular strata, are probably the latter, more or less changed by conti- 

 guity with the trap. 



