260 PROF. A. H. COX AND MR. A. K. WELLS ON THE [vol. lxxvi, 



Prof. FearnsLdes. 1 The colour of the slates gradually changes, 

 from grey in the lower part of the group to a bright blue in the 

 upper part. They weather to a bright-green, sometimes becoming 

 almost bleached. Joint-surfaces may be somewhat iron-stained. 

 The more flaggy strata are often highly micaceous. The beds 

 have yielded the characteristic Lingulella davisii at numerous 

 localities. 



The highest beds of the group are dark-blue slates, which 

 eventually become lithologically similar to, and form a transition 

 into, the Dolgelley Beds above. Lingulella davisii is particularly 

 abundant in these highest beds, which are exposed on the roadside 

 at Pont Aber-Gvrynant. This highly-fossiliferous band is very 

 characteristic of the top of the Ffestiniog Group, and it can be 

 traced right round the Harlech Dome. 3 



The Ffestiniog Beds occupy a belt of country, averaging half 

 a mile in width, all along the south side of the Mawddach 

 Estuary. The ground occupied by their outcrop is generally very 

 rough, and remains uncultivated on account of the resistant nature 

 of the numerous grit-bands ; most of the ground is densely wooded. 

 East of the Gwynant the Ffestiniog Beds give rise to hills 600 to 

 S00 feet high with a frontage (scarp) towards the estuary. West 

 of the Grwynant they occupy lower, but still rough and densely 

 wooded ground, with numerous precipitous little hills. 



The thickness of the Ffestiniog Beds exposed along the Maw- 

 ddach Estuarv amounts to rather more than 2000 feet ; but the base 

 of the group is nowhere to be seen, as it lies buried beneath the 

 alluvium of the estuary. The total thickness of the Ffestiniog 

 Group in the Dolgelley district is estimated by Dr. Andrew at 

 3000 feet. 3 



(ii) The Dolgelley Beds. — The Dolgelley Beds represent 

 here, as elsewhere, a much more uniform and deeper- water group 

 of deposits than the Ffestiniog Beds. They consist of dark-blue 

 to black mudstones, which are so highly pyritous that joint-surfaces 

 become covered, especially in moist places, with a thick coating of 

 iron-rust. Although the rocks are so uniform and fine-grained, 

 the effects of cleavage are not usually sufficient to obliterate 

 the bedding, which is shown in crag-exposures by a series of 

 bedding-joints ; nevertheless, small pieces do not split readily along 

 the bedding. Occasional thin and pale-coloured seams of gritty 

 material nuiy be intercalated among the normal fine - grained 

 dark mudstones. Near the centre of the group comes the well- 

 known band with a black streak. About this horizon the mud- 

 stones usually show some lamination. The colour then becomes 



1 Q. J. G. S. vol. lxvi (1910) p. 150 ; also A. R. Andrew, Geol. Mag. 1910, 

 p. 165 ; compare Bernard Smith, ' Ball or Pillow-form Structures in Sand- 

 stones ' Geol. Mag. 1916, p. 151. 



2 W. G. Fearnsides, Q. J. G. S. vol. lxi (1905) p. 612 ; and ibid. vol. lxvi 

 (1910) p. 152. 



3 Geol. Mag. 1910, p. 166. 



