SHARPE OX NORTH WALES. 289 



Cambrian Rocks of the Berwyns. 



The Cambrian rocks form an irregular saddle in the middle of 

 the Berwyns, about fifteen miles long from east to west, and three 

 miles wide from north to south. The upper bed is the slate already 

 mentioned of Nant cwm Dywyll and Blaen Iwrch ; and a similar bed 

 is found to the south of the axis, in the valley of the Rhaiadr, just 

 below Pistill Rhaiadr. It is there vertical, and strikes east ; and the 

 cleavage dips north-east 5°. It is overlaid by Lower Silurian rocks. 



Below the slate is a series of rotten schists, of no very definite 

 character. These run to a great heigiit on both flanks of Cader 

 Berwyn. The summit of Cader Berwyn is a mass of greenstone, 

 having somewhat of columnar cleavage. At the head of the Rhaiadr 

 the schists are altered and disturbed by quartzose rocks, which burst 

 out on the north side of the valley. 



The author is not aware of the existence, in these Cambrian 

 schists, of any organic remains. The position of the beds is shown 

 in Section 1. 



Anticlinal of Lower Silurian and Cambrian rocks south of the 

 Dee. — The Lower Silurian and Cambrian rocks south of the Dee, 

 considered together, are regarded by the author as forming a great 

 anticlinal, whose axis runs due east through Cader BerAvyn. The 

 beds usually strike east and west ; but near the Ceiriog there is a 

 tendency to a south-east strike. The cleavage usually strikes east, 

 and dips north, but at so variable an angle as to lead to the inference 

 that the beds have been disturbed since the cleavage took place. 



The igneous rocks M'hich break through are mostly felspathic; 

 but some peaks consist of greenstone. 



On the north the district is bounded by the Wenlock rocks, along 

 a line the course of which has been already defined. On the east 

 it is bounded by mountain limestone and millstone grit; on the 

 west it is cut off by a great fault which runs along the valley of the 

 Dee from Corwen to Llanderfel, four miles from Bala, and which 

 may be traced beyond Llanderfel southwards, across the moors 

 and up the valley of the Calettwr. The southern boundary of 

 this district has not been examined by the author. 



Lower Silurian Rocks on the Holyhead road, from near the Druid 

 Inn west of Corwen to the Conway river. 



West of the Druid Inn on the Holyhead road, from the point 

 where the Ludlow rocks already described terminate, to the bridge 

 of Maes mawr fechan, is a narrow anticlinal ridge of hard grey 

 Bala grits. On the east side of this ridge the beds dip E.S.E. 40°, 

 on the west side W.N.W. 50°. The cleavage dip on the east side 

 of the ridge is N. 25°. 



At the gorge of Glyn Diffwys is a series of rotten schists, full of 

 Lower Silurian fossils; and between the schists is a bed of dark 

 limestone, which the author considers identical with that of Bala : 

 its position is marked on Mr. Greenough's map. Higher in the se- 

 ries he found some calcareous bands which, he thinks, may represent 

 the Upper Bala or Hirnant limestone. The beds dip in one place 



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