Mr. A. AiKiN on the Geological Structure of Cader Idris. 275 



A porphyritic rock^ consisting of quartzy compact felspar containing, im- 

 beddedj crystals of felspar,, and a green matter which decomposes by exposure 

 to weather, leaving cells lined with red oxyd of iron. This bed rises about 

 N.W by N., and is composed of irregular columns from 6 to 14 feet long. It 

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

 called the Saddle, 2655 feet high, which is overspread 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 consisting of a rock 

 composed of chlorite, calcareous spar, quartz and glassy felspar. The calca- 

 reous 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 proportion 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 ingre- 

 dient : some of the decomposed pieces are very light and spongy, and I pre- 

 sume are the substance 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 covered with peat bog and with large masses that have fallen from 

 the cliffs above, that I could not examine it with the requisite minuteness : 1 

 obtained, however, a very good section of the descent northwards from Gey- 

 graig(the eastern extremity of the mountain), without finding these beds ; and 

 I conclude 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 in- 

 accessible 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 porphyry, evidently the same rock 

 as is associated with the calcareous beds at the Goat's Pool. Three of these 

 fa9ades or steps consist of nearly vertical coarsely slaty tables, which are ag-ain 



