John Aitken. — Grit-roclcs of North Wales. 265 



rock is that of a strong, massive quartziferous grit, often conglo- 

 meratic and but rarely fine or flaggy. In some cases it is so com- 

 pact that the fracture at first glance presents somewhat of a granitic 

 or sub-crystalline appearance, whilst, on the other hand, the Yore- 

 dale sandstones are generally fine-grained, flaggy, ripple-marked, 

 brown rocks, with which are usually associated beds of black shale, 

 limestone (not unfrequently earthy and somewhat impure), and 

 occasionally, towards the north, with thin seams of coal, the whole 

 forming the Yoredale series.^ 



If we compare the Oswestry sandstones ^ with the grits of Yoredale 

 series we shall find that they fall in very well generally with that 

 formation, whilst they are wholy difierent in character and appear- 

 ance from the Millstone-grits, It will be observed that we shall find 

 the similarity is still more striking by instituting a comparison 

 between the group of beds resting upon the thick, buff, massive 

 limestone, in the Mold district, and the Yoredale rocks, which pre- 

 sents the following succession of strata in ascending order : — Imme- 

 diately over the thick limestone come three or four beds of fine- 

 grained, brown, flaggy grit-rock, separated by beds of black shale, 

 the whole of which gradually become more and more calcareous as 

 we ascend in the series ; upon these rest some black shales, which, in 

 their turn, are overlaid by beds of limestone, about 150 feet in thick- 

 ness (which become highly siliceous at the top), parted by layers of 

 black fossiliferous shales, the whole surmounted by a sandstone rock, 

 ^ which Mr, Morton considers belongs to the Millstone-grit series ; 

 even Mr, Davies, who, although adopting the generally-received 

 classification of these rocks, appears to entertain some doubt as to its 

 correctness, for, speaking of them in the neighbourhood of Mold, he 

 thus expresses himself : — " So rapid is the increase of calcareous matter 

 in the last 15 miles that it is difficult, when looking at the Mold 

 section, to say whether we have, as shown on the Government 

 Survey maps. Millstone-grits with bands of limestone, or limestones 

 interstratified with beds of grit," ^ 



The higher beds of the thick limestone proper become arenaceous, 

 and are charged with corals and shells in such abundance, that in 

 some places the rock appears to be almost entirely composed of their 

 remains. No one can examine a surface of this rock, which has been 



■^ The following passage, extracted from Phillips's Geology of Yorkshire, -will serve 

 as an illustration of this subject. In describing the Yoredale series, at p. 36, the 

 author observes — " The Southern series (Craven) consists alniosts entirely of argil- 

 laceous laminated rocks, locally changing to Limestone and Chert, generally productive 

 of ironstone, and containing marine exuvise. The Northern type (Teesdale) consists 

 of the same argillaceous basis (less calcareous, and of a coarser grain), with the 

 addition of many layers of Sandstone, distinct beds of Limestone, thin coal seams, 

 and sand plants ;" see also section at page 37 of the same work. Also at page 6 of 

 the Memoirs of the Geological Survey of North Derbyshire and the adjoining parts 

 of Yorkshire, by A. H. Green, F.G.S., C. Le Neve Foster, B.A., etc., and J. K. 

 Dakyns, M.A. 



2 See section and description, by A. H. Green, M.A., F.G.S., of the "Lower 

 Carboniferous rocks of North Wales," Geological Magazine, Vol. IV., p. 12; also 

 description by D. C. Davies, Geological Magazine, Vol. VII., pp. 70 to 72. 



3 Geological Magazine, Vol. VII., p. 73, 



