Structure qfCader Idris. 437 



columns laterally aggregated and intersected by oblique irre- 

 gular joints. The columnar structure is remarkably perfect 

 on the northern side of the mountain, where terraces of co- 

 lumns sloping towards the body of the mountain, (each ter- 

 race in succession retired behind the lower one,) present a 

 flight of enormous steps, and show this remai'kable character 

 of the trap-rocks in great perfection. The base of each group 

 generally consists of slaty greenstone and porphyritic slate 

 surrounding the group, and rising up to it on all sides, and 

 therefore evidently a member of this trap formation. On the 

 slaty greenstone rests a bed of highly indurated siliceous schist, 

 which like the other regular strata rises N. by E. or W. 



On the south side of the mountain may be seen the con- 

 nection of the gi-een calcareous rock with those just mentioned : 

 its lowest bed rests on a porphyritic grauwacke, and this latter 

 on a slaty greenstone. 



About half way between Dolgelle, and the Stony Mountain, 

 on the Towyn road, is a large quarry of syenite. It is com- 

 posed of green hornblende intimately mixed with compact 

 felspar, and is rendered porphyritic by concretions of lamellar 

 flesh-red felspar. It is traversed by numerous veins of quartz, 

 in which are imbedded fibres and curved crystals of pale wine- 

 yellow thallite. On the S.W. flank of this syenite are ap- 

 plied nearly vertical beds of schist running about N.W. and 

 S.E, Those beds that immediately rest on the syenite are 

 considerably indurated, the others are less so. Beds of pot- 

 stone are found very near the schist, and probably interstrati- 

 fied with it. The northern side of the syenite is covered by 

 a soft dark green steatitical rock obscurely slaty, on which 

 rest beds of a green slaty rock not very different from the 

 former, and these are covered by beds of soft blue slate : all 

 these beds rise at a high angle S.E., that is, towards the nucleus 

 of syenite ; and the green steatitical beds are penetrated by veins 

 of quartz and thallite, stained green by carbonate of copper. 



The descent northwards from Geygraig, the eastern extre- 

 mity of Cadi^r III lis, also shows in a very interesting manner 

 tlie connection of the trap with the stratified beds. The south- 

 western part of the flat surface of Geygraig is covered by peat, 

 so as lo conceal the subjacent rock ; but the north-eastern end 

 consists of a few beds of unaltered blue slate, to which suc- 

 ceed beds of schist disturbed in position, in some places in- 

 durated, in others cellular ; and in every part containing im- 

 bedded lumps of very hard porphyritic greenstone : sometimes 

 the schist contains numerous small glands of quartz and of 

 felspar, and is largely mixed with siliceous schistus of a dull 

 green colour. This rests on highly inclined beds of a por- 

 phyritic 



