434 Mr. A. Aikin's Notes om the Geological 



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 viti'eous, and these last rest upon 

 a bluish-grey quartz-rock rendered porphyritic by a few cry- 

 stals 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 cra- 

 ter 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 porphyritic 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 appears 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 compact angular pieces. This 

 rock is excessively hard and strongly resists decomposition ; 

 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 pro- 

 bably 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. 



A porphyritic rock, consisting of quartzy compact felspar 

 containing, imbedded, crystals of felspar, and a green matter 

 which decomposes by exposure to weather, leaving cells lined 

 with red oxide of iron. This bed rises about N.W. by N., 

 and is composed of irregular columns from 6 to 14 feet long. 



It 



