Structure ofCader Idris. 435 



It ascends obliquely on the west, to that part of the ridge of 

 the mountain called the Saddle, 2655 feet high, which is over- 

 spread with columns similar to those just mentioned, except 

 that they are mostly lying prostrate : it also forms the basin in 

 which the Goat's Pool at the bottom of the crater is contained. 



On the northern side of the pool is a kind of mound con- 

 sisting of a rock composed of chlorite, calcareous spar, quartz 

 and glassy felspar. The calcareous spar is white, or of a dull 

 purplish colour. There are several of these beds; of which 

 the upper is a kind of conglomerate imbedding roundish masses 

 of the same kind of rock, but harder, and with a smaller pro- 

 portion of calcareous spar, and large angular pieces of slate. 

 Below this is a bed harder and more compact than the former, 

 and arranged in irregular columns like starch. This rests upon 

 a more slaty rock of the same kind. 



All these beds decompose by exposure to weather through 

 the decay of the calcareous spar, showing the greater part of 

 the mass to consist of this ingredient : some of the decomposed 

 pieces are very light and spongy, and I presume are the sub- 

 stance which by some persons has been described as cellular 

 lava. The relation of these beds to the columnar porphyry 

 which precedes them I have not been able satisfactorily to 

 make out in this place ; but upon the whole I am inclined to 

 think that they rest upon the porphyry, as this latter rock makes 

 its appearance on the north as well as on the south of them. 



These calcareous beds being readily distinguishable from 

 the others among which they occur, I endeavoured to trace 

 their extent. To the east, as far as the ridge of the mountain 

 stretches, the valley in which they ought to be found is so co- 

 vered with peat bog and with large masses that have fallen 

 from the chffs above, that I could not examine it with the re- 

 quisite minuteness : I obtained, however, a very good section 

 of the descent northwards from Geygraig (the eastern extre- 

 mity of the mountain), without finding these beds ; and I con- 

 clude that they do not exist there. I then examined the foot 

 of the ridge to the west of the crater, and had the satisfaction 

 of finding what I sought below Twrr mawr, one of the points 

 of the mountain 2148 feet high. An inaccessible precipice of 

 irregular columnar shafts occupies the first 700 or 800 feet 

 from the top, which, lower down, breaking into groups or 

 steps, allows of examination. Here I found a columnar por- 

 phyry, evidently the same rock as is associated with the cal- 

 careous beds at the Goat's Pool. Three of these fa9ades or 

 steps consist of nearly vertical coarsely slaty tables, which are 

 again divided by vertical joints at right angles to the tabular 

 structure, and thus form tolerably regular (juadrangular co- 

 3 K 2 lumns. 



