﻿634 MR. W. G. FEARNSIDES ON THE GEOLOGY OF [Aug. I905, 



passes close by Amnodd-wen, and gives the summit of Arenig 

 its characteristic double peak. It has a throw of about 300 feet, 

 and appears on the surface in turD as a gully and with fault- 

 cliffs on either side successively. It seems to run right across the 

 district, and is recognizable on both sides of the much later north- 

 and-south fault ; it has a notable hade to the north-east. Another 

 important fault belonging to the same series is that which forms 

 the waterfall at Pont Aberderfel. It passes close behind the Ehyd- 

 y-fen Hotel, through Bwlch Llestri and on to Nant-yr-Achab. It 

 is worthy of notice, because near Tai Herion it brings Asajphellus- 

 Flags against Extensus-'Ela%s and Calymene- Ashes, and, making 

 there no feature, led the older geologists to record Arenig and 

 Tremadoc fossils from the same locality. 



The second great series of faults is represented only by one great 

 example, which traverses the entire district, and cuts the country 

 and all its structures completely in two. It was traced by Sir 

 Andrew Eamsay in a direction a little west of north and east of 

 south, for more than a dozen miles. It has a downthrow to the west, 

 and at one place brings the Ashes of the Upper Series against the 

 Lower Dolgelly, or even the Ffestiniog Beds of the Lingula-'Elsig 

 series. It is much later than all other structures in the district, 

 and from the frequent stories of earth-rumblings which one hears 

 at the farms along its course, one must conclude that it is probably 

 still in a condition of instability. 



In addition to these faults, there are also a couple of rather gentle 

 anticlinal folds which affect outcrops in the Tryweryn Valley, and 

 again in the neighbourhood of Llechwedd Erwent. These would 

 seem to be somewhat earlier than even the oldest faulting, and pro- 

 bably belong to the time of the intrusion of the andesitic dolerites. 

 They have a Caledonian trend, and have taken a pitch corresponding 

 to the general dip of the beds of the whole country. The more 

 southerly example is cut off along its north.- western flank by a 

 fault. 



III. Conclusions. 



This brings us to an end of our consideration of the solid geology 

 of Arenig Fawr and Moel Llyfnant, and I may now briefly summarize 

 the results which have been attained. 



The rocks are divisible into an Upper and a Lower Series, which 

 are separated by an unconformity. The Lower Series belongs to the 

 accepted Upper Cambrian rocks of North Wales, and includes 

 representatives of the Ffestiniog, Dolgelly, and Tremadoc groups, 

 the last of these being incomplete upwards. The Ffestiniog and 

 Dolgelly Beds are quite like the corresponding beds of both 

 the Tremadoc and the Dolgelly districts, while the character of the 

 Tremadoc is intermediate between that of the beds of Penmorfa 

 and those of Sheinton, and is quite unlike that of the. ashy beds 

 described as Tremadoc at Dolgelly. 



