Mr. A. AiKiN on the Geological Structure of Cader Idris. 277 



evidently clusters of columns laterally aggregated and intersected by oblique 

 in"egular joints. The columnar structure is remarkably perfect on the north- 

 ern side of the mountain, where terraces of columns sloping towards the body 

 of the mountain, (each terrace in succession retired behind the lower one,) 

 present a flight of enormous steps, and show this remarkable 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 connection of the green 

 calcareous rock with those just mentioned : its lowest bed rests on a porphy- 

 ritic 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 composed of green hornblende inti- 

 mately 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 thaUite. 

 On the S.W. flank of this syenite are applied nearly vertical beds of schist 

 running about N.W. and S.E. Those beds that immediately rest on the sy- 

 enite are considerably indurated, the others are less so. Beds of potstone are 

 found very near the schist, and probably interstratified 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 diff'erent 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 extremity of Cader 

 Idris, also shows in a very interesting manner the connection of the trap with 

 the stratified beds. The south-western part of the flat surface of Geygraig 

 is covered by peat, so as to conceal the subjacent rock ; but the north-eastern 

 end consists of a few beds of unaltered blue slate, to which succeed beds of 

 schist disturbed in position, in some places indurated, in others cellular ; and 

 in every part containing imbedded 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 porphyritic slate, and this latter on imper- 

 fect amygdaloid, which itself rests on the columnar basaltiform trap. 



