PRE- CAMBRIAN ROCKS OF BANGOR. 



139 



results of his investigations led him to infer that there were at St. 

 David's two distinct formations underlying the Cambrian, and, as 

 he considered, unconformable to it and to one another. 



In the 'Records of the Rocks' (1872, p. 28 et seq.) the Rev. W. 

 S. Symonds expressed the opinion that many of the crystalline 

 rocks of North Wales and other districts " belong to a more re- 

 mote age than that of the Cambrian formations, and that hereafter 

 they will be classed as Laurentian or, at all events, as Pre-Cam- 

 brian." 



Section near Bangor. (Length of section 2 miles.) 



N.W, Bryniau. 



B.E. 



Menai 

 Straits 



a. Mountain Limestone. 



b. Black slate. 



c. Banded slate. 



d. Greenish grit and subordinate 



purple and green slate. 



e. Grit and conglomerate. 





/. Greenish slates, horiistonea, 



breccias, &c. 

 g. Schists. 



Referring to the above section it will be seen that black slates, ex- 

 posed in various places between Llandegai and Pentir, S.E, and S. 

 of Bangor, are followed in descending order to the N.W. by black- 

 and-grey sandy shales with wavy white lines, and beds of gritty 

 sandstone, and, on the whole, agreeing in lithological character with 

 the Lingula Mags. The black slates above are on the strike, and 

 probably about the horizon of the black slates exposed in a road- 

 cutting and trial-shaft for coal about three quarters of a mile S.S.E. 

 of Caernarvon, which are highly fossiliferous, and clearly of Arenig 

 age (see Marr, Quart. Journ. Geol. Soc. vol. xxxii. p. 134). In the 

 Bangor section the details of this part of the series have not yet 

 been made out ; so we can say nothing of Menevian. Below the 

 Lingula Flags there is a considerable thickness of dark-greenish, 

 gritty, chloritic rocks, in the lower part of which purple slates are 

 here and there intercalated. Wherever the section is clear we find 

 at the base of these a grey coarse grit, with hyaline quartz gene- 

 rally conspicuous, all weathering yellowish. This grit is almost 

 always, partly or wholly, replaced by a coarse conglomerate, with 

 pebbles up to the size of a man's head, but generally about the size 

 of pigeons' and hens' eggs. The matrix is the same as the grit. 

 These descriptions are from observations with the naked eye only. 

 It is probable that the microscope would disclose many varieties of 

 character, dependent on what part of the underlying series each bed 

 was derived from. This section agrees in its general features with 

 that seen along the shores of Llanberis and Llyn Padarn. 



