70 D. C. Davies—The Millstone Grit of North Wales. 



Fig. 3. — Section showing the junction of the Millstone Grit with the Carboniferous 

 Limestone, at Carreg-y-big, 3^ miles N. of Oswestry. 



Offas Dyke. 



Quany. 



1 . Massive grey limestone, base not seen. 



2. Dark grey shale 

 8. Grey limestone -with shale 



4. Yellowish brown clay ... 



5. Concretionary limestone 



6. Limestone abounding with encrinite stems 



7. Purplish grey arenaceous limestone, becoming sandy towards tlie 



8. Thin purplish sandstones. Millstone Grit. 



ft. in. 

 6 



top 



7 11 



As might be expected, the sandstones are of various degrees of 

 texture, ranging from fine to coarse-grained, whilst layers of pebbles 

 — varying in size from a pea to an egg — are present in both the 

 lower and upper portions of the group, the calcareous sandstones in 

 the middle being most free from them. All this I need scarcely say 

 is owing to, and denotes the different depths of water in which the 

 whole series was deposited, — from the strong current (tidal or other- 

 wise) — and shallow water in which the pebbles were rolled, to the 

 still deptlis in which the fine grains of sand quietly sank. Both the 

 larger pebbles and the finer sand are of quartz, in its different varie- 

 ties; and although I have examined many blocks of sandstone for the 

 purpose, I have been unable to detect a fragment of limestone, and 

 but rarely any traces of the older slaty and porphyritic rocks of 

 Wales. Both pebbles and sand are, as stated by Sir E. Murchison, 

 in his "Silurian System," cemented together in a paste of felspar, 

 but the distribution of the felspar is very unequal. Thus, in the 

 uppermost beds of the section at Sweeney, we find considerable layers 

 of felspathic clay, succeeded by thick beds of sand, in which it is 

 scarcely present, and some even from which it is totally absent, the 

 sand simply cohering from surrounding pressure, and when this is 

 removed becoming loose sand (white generally), which can be dug 

 out, as it is sometimes, and used in mortar. The various beds are 

 also coloured by metallic oxides, peroxide of iron being largely 

 present in the deep red beds, manganese in the speckled, and copper 

 here and there giving a greenish tinge, as at Sweeney and Trevor. 



In the 8-feet bed, near to the top of the Sweeney section, there 

 are some curious " Pockets," which illustrate both this unequal dis- 

 tribution of felspar, and the varied colouring of the rock. These 

 Pockets differ from all those I have hitherto observed or seen des- 

 cribed, in that they are horizontal, and not perpendicular, extending 

 laterally into the rock, not einaple vertical cavities on its surface. I 



