4S6 Mr. A. Aikin's Notes on the Geological 



lumns. Between every two steps of porphyry lies a bed or 

 step of green calcareous rock disposed in rude irregular co- 

 lumns; and, still lower down, is the same calcareous rock in 

 coarsely slaty beds, resting on hummocks of soft and not very 

 crystalline greenstone, and interstratified with common and 

 indurated slate. The beds rise N. W. at an angle of about 85°. 



The point or projection of the mountain next to the west of 

 Twrr mawr is called Mynydd Coeswyn, and presents appear- 

 ances very similar to those described. The top is slate, rest- 

 ing on a very hard compact porphyritic greenstone in thick 

 curved tabular masses, which are rudely subdivided into rhora- 

 boidal colunnis, the convex side of the curve being outwards. 

 At the foot of the porphyry are beds of common blue slate 

 rising W. by N. ; then comes the green calcareous rock, im- 

 bedding angular pieces of slate ; then neai'ly vertical beds of 

 a heavy black slate; then imperfectly slaty grey schist; then 

 beds of the green calcareous rock in which the general slaty 

 structure may be seen at a litde distance, but also passing into 

 the large lenticular and obscurely columnar structure. These 

 latter beds rest immediately on columnar greenstone ; which 

 at the plane of junction, but no where else, incloses or has in- 

 volved masses of the calcareous rock, so cellular as to appear 

 like large sponges. 



A very remarkable mountain (the name of which I could 

 not discover, and which I therefore for distinction's sake call the 

 Stony Mountain) extends for about two miles parallel to Cader 

 Idris, forming the northern boundary of the little valley in 

 which the Goat's Pool and another small lake are situated. 

 The height of it is about 1700 feet, and it is distinguished from 

 most others in its neighbourhood by several characters. On 

 looking towards Cader Idris from Dolgelle, the white roundish 

 crags of this mountain are very conspicuous ; and their height 

 is such, that only one peak of Cader Idris appears beyond and 

 above them. On a nearer examination it is found to be com- 

 posed of rounded tubercular crags and hemispherical bosses 

 of trap, like enormous ovens, rising group above group, the 

 intervals between which are filled up with coarse pasture and 

 bog. The surface of the bosses is comparatively smooth, 

 and generally reticulated by veins of quartz usually not more 

 than half an inch thick ; some of them, however, are consider- 

 ably wider ; and areas occur occasionally, four or five yards 

 across, of white massive quartz several inches thick, with an 

 obscurely slaty structure 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 viewed in 

 front or looking down upon them, are evidently clusters of 



columns 



