304 Maw — Distribution of White Clays and Sands 



directed seems requisite on any mechanical theory. The effect of 

 ice, which has been adduced to explain the excavation of rock lake- 

 basins, complete on aU sides, seems scarcely applicable to the 

 present case, as the walls of the pocket exhibit no cAddence of 

 abrasion or striation, but present a smooth mammillated surface, 

 such as would be produced by gradual dissolution, and resembling 

 that of underground cavities in the limestone, which are generally 

 considered to be due to this process. 



Another point to be noticed is, the absence of rocky fragments or 

 debris, and the striking dissimilarity of the contents of the cavities 

 to the Glacial beds that overlie them. Had the pockets been 

 excavated by Glacial abrasion, the entire clearing out of all debris 

 from the bottom of a deep cul-de-sac would be highly improbable, 

 and, furthermore, had the pockets been at any time opened and 

 exposed, the subaerial accumulation of debris from the sides of the 

 Limestone hills, on which the pockets are frequently placed (as for 

 example at Nant y Gamer, near Llandudno), would soon have filled 

 them up. As a rule, however, the pockets are occupied with the 

 white clays and sands, free from stones and limestone fragments, 

 and resting on these are the Glacial drifts, entirely dissimilar in 

 colour and mineral character, and containing both local and foreign 

 boulders. 



A point of analogy -with the sand pipes of the Chalk is the tendency 

 to a vertical disposition, or a conformity to the general shape of the 

 pockets of the strata occupying them ; instead of the beds lying 

 horizontally or nearly so, as they would have done from direct de- 

 position in the containing cavities, they are in some cases disposed 

 vertically or more generally with a steep inclination, dipping towards 

 the centre ; they are also frequently disturbed with singular con- 

 tortions and full of little faults and slips, which, from the formations 

 being strictly confined within the limits of the cup-shaped cavities, 

 appear, at first sight, difficult of explanation : if, however, the gradual 

 dissolution of the underlying limestone is taken into consideration, the 

 singular arrangement of the beds is at once accounted for. The strata, 

 with a disposition originally more horizontal would, in gradually 

 sinking, conform themselves to the changing outline of the slowly 

 deepening cavity. This sinking and dislocation is evident in nearly 

 the whole of the examples before referred to. Mr. Binney describes 

 the occurrence of an almost vertical mass of pebbly gravel in the 

 midst of a mass of pijoe-clay occupying a pocket at Caldon Hill 

 Limestone Quarry ; and Mr, Brown, in his paper on the Drifts of the 

 Weaver Hills (see p. 201), states that the white clay and sand 

 deposits below Caldon Low, in the same neighbourhood, betrays its 

 existence by deep sinkings in the surface of the ground. The Welsh 

 deposits, near Llandudno and on Holywell Mountain, appear also to 

 have sunk down since their original deposition, rendered evident by 

 their tendency to a vertical or steeply concave arrangement, and 

 accompanied by the dislocations before referred to. The Irish beds 

 exhibit similar evidences of altered arrangement. Mr. Blake ob- 

 serves that the dip of the stratified clay varies with the angle of the 



