Belt— On the " Lingula Flags." 539 



most abundantly at the base and in the upper beds. Above Dol- 

 goed, in thin shaly slates, it occurs in thousands, all drawn out 

 and distorted by slaty cleavage ; but the best specimens have been 

 found on the left bank of the Mawddach below Hafod-fraith. In the 

 Lower beds, at Cae-gwernog above Llanelltyd, where it was first 

 found by Mr. Salter in 1864, and near Dolgoed, Olenus trimcatus, 

 Ang. accompanies the Agnostus, but it does not follow it into the 

 higher beds, where it is replaced by a closely-allied species, 0. cata- 

 ractes, Salter, of which I have obtained fragments from the rocks by 

 the roadside, a little north of Llanelltyd, and in the blue slates 

 below Hafod-fraith. 



The total thickness of the Upper Maentwrog beds in the Dolgelly 

 and Maentwrog districts is about 1800 feet. 



The Maentivrog Group, comprising the Upper and Lower Maent- 

 wrog beds, contains altogether, in the district under consideration, 

 about 2500 feet of strata, and is, as has been already mentioned, 

 well defined, both lithologically and palgeontologically. The beds 

 are everywhere much jointed and weather to a rusty colour, and 

 form hills covered with angular ferruginous debris, so that even at 

 a distance the experienced eye can distinguish them from the hard, 

 shelving flags of the Festiniog Group, or the low rounded hills 

 formed from the much softer slates of the Dolgelly Group. 



In the Dolgelly district the slates and shaly flags of the Maent- 

 wrog Group occupy all the ground between the Menevian beds and 

 the river Mawddach, from a little above Barmouth up to Llanelltyd, 

 where they cross the river and occupy both its banks as far as 

 Tyddyngwladis Mine. A little below the Tyddyngwladis lode they 

 are thrown entirely to the eastern side of the river, partly by 

 intrusions of diabase and partly by faults. Between Cwmhesian 

 Mine and Hafod-fraith they again cross the river and range north- 

 ward, towards Trawsfynydd. 



In the Maentwrog district these beds are finely developed, and 

 splendid sections of them are shown in the Waterfall Valley, and in 

 the valley running from Tafarn-helig to Caen-y-coed. In the Caen- 

 y-coed quarries (now abandoned) the topmost beds are well ex- 

 hibited, and from this locality the finest specimens of 0. cataractes, 

 Salter, have been obtained by Mr. Homfray, who has also found with 

 it, fragments of a crustacean allied to Eymenocaris. 



The rocks of the Maentwrog Group are of little economic im- 

 portance. Some rough slates and slabs have been worked in them 

 at Caen-y-coed and above Llanelltyd, but the fine-grained beds are 

 too much jointed, and the coarse -graiaed too rough for profitable 

 working. The gold-mines of Hafod-y-morfa, Cefn-deiiddwr, and 

 others of less importance have been opened in these beds. The 

 auriferous quartz veins also contain ores of copper, lead, and zinc, 

 but only in small quantities. 



FESTiNioa Group. 



Lower Festiniog Beds (No. 5 in Section). — Lying conformably upon 

 the Upper Maentwrog beds are a thick series of micaceous, grey 



