SHARPS ON NORTH WALES. 



287 



identical with the dark roofing-slate worked near Llansaintfraid glyn 

 Ceiriog. 



The high ridge of Cyrn-y-brain consists entirely of Lower Silurian 

 rocks. These are bounded on the north, nearly along the line of the 

 road to Wrexham, by the mountain limestone of Llandegla and the 

 millstone grit of Moel Grugog. To the west the beds of Cyrn-y- 

 brain are cut off' by the continuation of the Nant Morwynion fault. 



Lower Silurian Rocks south of the Dee. — The dark grey roofing- 

 slate, the uppermost of the Lower Silurian beds, is worked in the 

 vale of the Ceiriog, at the east end of Cefn Canol, west of Llansaint- 

 fraid. The same bed is worked on the right bank of the Ceiriog, 

 at the northern foot of Fron Frys. 



At Cefn Canol and at Fron Frys the planes of bedding and clea- 

 vage meet at the same angle, viz. 15° ; but if the position of the 

 two planes at Fron Frys be compared with their position at Cefn 

 Canol, it will be found that each of them has been shifted 22|° in 

 azimuth and 20° in inclination to the horizon ; and this disturbance 

 must have taken place since the planes of cleavage were formed. 





Strike. 



Difference 

 in strike. 



Inclination to Horizon. 



Difference 

 in Incli- 

 nation. 



Fron Frys. 



Cefn Canol. 



E. 

 E. 



Fron Frys. 



Cefn Canol. 



Cleavage 



E.22i°S. 

 E.22i°S. 



22i° 

 22i° 



N.N.E. 45° 

 N.N.E. 30° 



N.25° 

 N. 10° 



+ 20° 

 + 20° 







Angle at which T 

 the planes of 

 hedding and 

 cleavage meet.. 









15° 



15° 









This bed of dark roofing-slate is continued to the north-west, until 

 it is cut off" about half-way between Llangollen and Corwen by the 

 great fault of the Dee. South of the Ceiriog and below the dark 

 roofing-slate of Fron Frys, is a series about 300 feet thick, consisting 

 of rotten schists, full of organic remains ; and including two beds of 

 limestone, which have been described by Mr. Bowman and Professor 

 Sedgwick. They dip N.E. 25°. 



The lower bed of limestone is not very continuous, but thins off 

 into numerous alternations of thin semi-calcareous bands. The fossils 

 of these beds are much injured by cleavage. In the escarpment 

 west of Fron Frys the author found the following species, besides 

 others mentioned by Professor Sedgwick : — 



Orthis costata. Turbinolopsis elongata. 



basalis. pluriradialis. 



n. s. Favosites fibrosa. 



Atrypa globosa. Retepora, 2 species. 



To the south occurs a great outburst of felspathic rocks, which 

 Mr. Bowman has described; and on this line of section the Bala grits 

 appear to be wanting. On following the line of the strike of tiie Fron 

 Frys beds to Corwen, where there is no outburst of igneous rocks, one 

 meets to the south of that town with a full development of the lower 



VOL. II. — PART I. u 



