274 Mr. A. Airin on the Geological Structure of Cader Idris. 



Cwm y Cae, the beds, without changing their direction^ bend up sharply so as 

 to increase their angle to about 50°, Induced by this circumstance, I took a 

 line of section from this part of the hill by the edge of Cwm y Cae, over 

 Craig y Cae, to the margin of the crater, and thence to the Goat's Pool. 

 Mynydd pen y Coed consists almost entirely of strata of soft bluish-grey clay- 

 slate. The beds which lie in succession below this, and occupy the ground to 

 the edge of the crater, are grauwacke, consisting of coarse blackish-blue slate 

 inclosing concretions of splintery quartz more or less earthy, and compact 

 splintery quartz with imbedded crystals of pyrites, in parts more or less 

 ochrey and cellular, probably from decomposition of the pyrites. 



To these succeed beds of quartz-rock differing from the preceding only in 

 being more vitreous, and these last rest upon a bluish-grey quartz-rock ren- 

 dered porphyritic by a few crystals of felspar. 



These beds all rise N.E. by N., but their angle of elevation is continually 

 increasing, and the last bed forms the summit of Craig y Cae. The space from 

 this to the margin of the crater is occupied by alternations, in nearly vertical 

 beds, of soft glossy clay-slate, of coarser slate with ochrey spots and small cells, 

 and of grauwacke with concretions of quartz, in some beds rendered porphy- 

 ritic by imbedded crystals of felspar as well as quartz. Porphyritic quartz- 

 rock also occurs, as well as slaty potstone, that is, slate intimately mixed with 

 steatite. There is also inserted about the middle of this series a single bed 

 of a brownish-grey rock, which effervesces strongly for a short time with acids, 

 and by exposure to weather becomes porous and of a darker brown. It ap- 

 pears to be ferruginous quartz, intimately mixed with carbonate of lime. 



The next bed (which forms part of the summit of Cader Idris) is composed 

 of globular concretions a foot or more in diameter, which separate into com- 

 pact angular pieces. This rock is excessively hard and strongly resists decom- 

 position ; in consequence of which it appears as a ridge or wall, a few feet 

 higher than the adjacent beds. It contains numerous specks of pyrites, and has 

 small roundish prominences on the weathered surface, somewhat resembling 

 the orifices of tubes. In very thin shivers it melts into a black glass, and is 

 probably a trap-rock. 



The perpendicular wall of the crater is between 900 and 1000 feet in 

 height, and presents the following series of beds from the top. 

 Three or four alternations of slate and grauwacke. 

 Fine-grained greenstone in irregular columns. 

 Slate. 



Fine-grained columnar greenstone similar to the preceding. 

 Slate inclosing a bed of calcareous iron-shot quartz. 



