D. C. Davies—The Millstone Grit of North Wales. 71 



give a representation of some of those (Soo Woodcut, figs. 4-7). 

 At first sight thoy bear some roseml)lanco to the concretions which 

 are so abundant in tlie Wonlock Shale of North Wales, but there is 

 this difference, that whereas the Wonlock Shale examples are con- 

 cretions of a harder nature than tho surrounding rock, and. are 

 formed around a cr^'stalline central nucleus, or cavity, filled with 

 crystalline matter, these in tho Grit are cavities witli no perceptible 

 outlet or inlet, filled with loose brown sand, gatliered around a 

 nucleus of white or yellowish clay, the surrounding rock being 

 nearly white. Around the outer margin, as in figs. 4 and 7, there 

 are dendritic markings ; and sometimes, as in figs. 6 and 7, dark linea 

 of mineral matter enter the surrounding rock. ^ 



Pockets in the Millstone Grit at Sweeney, near Oswestry. 

 Fig. 4. Fio. 5. 



Fig. 6. Fig. 7. 



Fig. 4. Front view of Pocket, 2 feet 6 inches in diameter, with marginal rim {a) of dendritio 

 martings of manganese, and centre (i) of yellow sand. 



5. Side view of the same Pocket, showing the comparative depth to which it penetrates the 



rock m, (c) central nucleus of felspathic clay. 



6. Front view of another Pocket, with end section of central nucleus of yellow clay (c) and 

 radiating lines of manganese. 



7. Front view of irregular shaped Pocket, with ochreous centre (c) and dark variegated 



margin with dendritic markings, [m) matiix of pinky-white sandstone, bed No. 7 of 

 Fig. 1. 



My description of the beds so far has been drawn principally 

 from the sections south and west of Oswestry, but in many respects 



^ It is difficult to account for the origin of these Pockets, and it is with diffidence 

 that I \enture to offer the following suggestion, viz. : That in the centre, where we 

 now find the clay, there was originally a concretion of mineral matter ; next, by 

 electro-chemical agency, such as is supposed to have aided in the formation of some 

 mineral veins, this mineral matter was disseminated around to the boundary of the 

 present pocket, and the felspathic clay — the cementing matrix of the rock — concreted 

 into the central cavity thus made by the dispersion of the mineral base, leaving the 

 sand, which had become discoloured by the dispersion of the mineral matter, simply 

 cohering by the force of the surrounding rock. 



