264 John Aitken. — Grit-rocks of North Wales. 



In the southern portion of the area occupied by these grits, pro- 

 bably no one will contend that the Carboniferous series is complete, 

 for, separating the Mountain limestone from the Coal-measures of that 

 district, only one set of beds precisely ' similar in character, and 

 evidently belonging to the same series, is found ; it therefore neces- 

 sarily follows that one of the subdivisions must be abandoned, and 

 the question then arises, which of the two shall be given up, the 

 Millstone -grit or the Yoredale section ? 



It is not unreasonable to suppose that, had the Yoredale rocks 

 been entirely absent from this area, and the Millstone-grits been 

 deposited immediately upon the surface of the Carboniferous lime- 

 stone, some evidence of this interruption to the regular order of 

 things would have been left at the junction of the two, either in the 

 form of an eroded limestone surface, unconformity of strata, or in 

 some other unmistakable way, representing the break in time, and 

 the abrupt transition from a condition of things in which the cal- 

 careous element predominated, to one in which arenaceous rocks 

 were almost exclusively deposited ; yet no such sign exists, the 

 change from one formation^ to the other is quite uniform and 

 gradual, although the passage beds are only thin and feebly repre- 

 sented ; the limestone beds near the top not only become steadily 

 more arenaceous, and split up into thin layers, intercalated with 

 bands of shale, but also exhibit a steady change of colour, from grey 

 through yellow and purplish, into the thin purple sandstones which 

 form the base of the overlying grits, the transition in both cases 

 being perfectly gradual and regular.'^ 



1 had no opportunity of seeing a section where the Coal-measures 

 come on over the grits, and am therefore unable to say whether the 

 same remarks apply in this case or not as to the nature of the 

 transition from one to the other. I, however, observed that, in a 

 horizontal section given by Mr. Davies in the Geol. Mag., Yol. VII. 

 p. 69, Fig. 1, the Coal-measures are shown as resting unconformably 

 upon the top of the Oswestry sandstone ; and I have also in my 

 possession a sketch by the same author, showing in a still more pro- 

 nounced manner the existence of an unconformity between the Coal- 

 measures and the subordinate grits at Sweeney Mountain. If this be 

 so the arguments in favour of removing these arenaceous beds from 

 the Millstone -grit formation, and of including them in the Yoredale 

 series, is greatly strengthened. 



If we next consider the texture and general character of these 

 sandstones we shall also find that they correspond much more 

 closely with the Yoredale than the Millstone-grits. In Lancashire 

 and Yorkshire, and indeed throughout its whole extent on both 

 flanks of the Penine chain, the usual characteristic of the last-named 



^ I here adopt the usual phraseology which represents the Carboniferous Limestone 

 and the Yoredale rocks as distinct formations, this is however merely a conventional 

 division for the convenience of classification, beyond which it has no significance. 



2 The regular passage from the Limestone rocks to the overlying Grit will be 

 rendered apparent by consulting the two sections given by D. C; Davies in the 

 Geological Magazine, Yol. VII., p. 69, Fig. 2, and p. 70, Fig. 3. 



