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



divided by vertical joints at right angles to the tabular structure, and thus 

 form tolerably regular quadrangular columns. Between every two steps of 

 porphyry lies abed or step of green calcareous rock disposed in rude irregular 

 columns ; 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 appearances very similar to those de- 

 scribed. The top is slate, resting on a very hard compact porphyritic 

 greenstone in thick curved tabular masses, which are rudely subdivided into 

 rhomboidal columns, 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, imbedding angular pieces of slate ; then nearly 

 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 little 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 involved masses of the calcareous rock, so cellular as to appear like 

 large sponges. 



A very remarkable mountain (the name of which 1 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 composed 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 pas- 

 ture and bog. The surface of the bosses is comparatively smooth, and gene- 

 rally reticulated by veins of quartz usually not more than half an inch thick; 

 some of them, however, are considerably 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 



