Vol. 52.] LINGTTLA-FLAGS AND IGNEOUS ROCKS OF DOLGELLT. 515 



It may be remarked here that Belt attaches much importance 

 to the black streak characteristic of his ' Upper Dolgelly Group/ 

 Generally speaking, the slates above the andesitic lava, and no 

 others, possess this peculiarity ; but we have not been able to draw 

 a definite line between them and the somewhat paler-coloured 

 beds above. 



East of Nant Ceunant. — We commenced our description with the 

 Nant Ceunant because here the general succession is most clearly 

 shown; and we have described the character of the rocks west 

 of that stream. It is now necessary to give some account of those 

 which lie farther east. 



The lava-band may be followed from Nant Cennant with very 

 little interruption until it reaches the diabase of Mynydd Gader ; 

 and for the greater part of this distance it is well exposed, forming 

 the crest of a spur which juts out from the main ridge of Mynydd 

 Gader. The black slates below the lava are also very clearly seen ; 

 and in the streams which run down from Mynydd Gader, east of 

 Nant Ceunant, they are underlain by slaty and gritty beds which 

 in many places yield Lingulella Davisii in abundance, and clearly 

 belong to Belt's ' Ffestiniog Group.' These beds become more 

 gritty lower down. 



Similar beds occupy the greater part of the space between the 

 black slates and the Nant Ceunant fault, and are visible at 

 intervals over the whole of this area. They are particularly well 

 exposed, and Lingulella is particularly abundant, in the hill which 

 lies west of Brynmawr. They also occur in the wood between 

 Tan-y-fedw and Bryn-rhug. 



Another point in connexion with the Mynydd Gader slopes 

 requires notice. The band of lava which lies above the Dictyo- 

 graptus-beds in the west follows the base of the diabase as far as 

 the head of the Bryn-rhug stream ; and a mass of similar rock 

 occurs even as far east as Nant Ceunant. It is clear, however, that 

 in its eastern part this lava does not strike in the same direction as 

 the beds below, but lies across their edges. "We believe that it is 

 not here in its natural position, but has been dragged into its 

 present situation by the diabase. 



{b) West of the Dolgelly Fault. 



Bryn-y-gwin Woods. — A sequence similar to that in Nant Ceunant, 

 but by no means so complete, is developed along the northern 

 border of the great mass of diabase west of the Dolgelly fault. The 

 stream which flows past Pandy'r-odyn runs nearly along the line 

 of the fault, and on its left bank the same beds are exposed as in 

 the woods farther west. The right bank, on the other hand, is 

 formed, in part at least, by the ashy rocks which occur in the hill 

 on the east. 



Leaving the latter out of consideration for the present, we find 

 dark bluish slates forming the bed of the stream at Pandy r r-odvn. 

 These pass up gradually into black shaly slates which extend 



