Subjacent to the Boulder-clay. 303 



separate districts, and the peculiar dark sooty-like substance is 

 common to the Welsh, the Irish, and the Staffordshire deposits. 



In connection with their infraposition to the Boulder-clay, and, 

 therefore, the possibility of tKeir Tertiary age, their mineral re- 

 semblance to some of the Lower Tertiary beds ought not to be lost 

 sight of, though this fact be insufficient to draw conclusions from 

 without the evidence of fossils. 



The evidence bearing on the question of the original extension of 

 the beds beyond their present limits, appears somewhat conflicting : 

 on the one hand, the general limitation to the Mountain Limestone 

 in far removed localities, and the various levels at which the small 

 and isolated patches occur, seem to point to a local, and to a certain 

 extent subaerial agency, the similar mineral character being de- 

 pendent on an identity of local circumstances and similar sources of 

 component materials. Qn the other hand, these deposits are not 

 invariably confined to the Mountain Limestone, and in the case of 

 the white clays resting on Lower Silurian rocks, west of Conway, 

 the distance of transport from the nearest Carboniferous Limestone 

 and Millstone Grit seems scarcely compatible with mere subaerial 

 agency. 



Again, when occurring on the Mountain Limestone, the deposits 

 invariably occupy deep depressions on its eroded surface, a contour 

 implying the removal previous to their deposition of the whole of 

 the Millstone G-rit in each individual locality ; but as the sands and 

 chert-beds of the Millstone Grit appear to have largely entered into 

 the composition of the deposit, a certain amount of transport 

 appears indispensable. The Millstone Grit, in many parts of North 

 Wales, is in a very soft and friable condition ; but the local debris, 

 resulting from its subaerial decay, does not resemble the deposits 

 under consideration, in which the sorting and separating agency of 

 water appears manifest in the interstratiflcation of various coloured 

 sands with laminated-clays and tough pipe-clays in well-defined 

 beds. This arrangement could^ not have been produced without the 

 agency of some body of water, though it is probable that the several 

 masses of the deposit may have been accumulated within separate 

 and limited areas. 



With reference to the cavities in the Limestone containing the 

 clay- and sand-beds, and the probable process of their excavation, I 

 would refer to a suggestion I made in describing the pockets at 

 Llandudno (Geological Magazine, May, 1865), that they were 

 gradually formed by the slow dissolution of the Limestone, and that 

 this may have taken place subsequently to the deposition of the mass 

 of the materials occupying them, after the manner of the excavation 

 of the sand-pipes in the Chalk and Coralline Crag, into which pre- 

 viously existing superincumbent beds appear to have been gradually 

 lowered. 



The evidence in support of this view is various ; the point that 

 first suggests itself is the difficulty in accounting for the excavation 

 of a deep cul-de-sac, complete on all sides, by any ordinary process of 

 either marine or subaerial denudation, — as some force vertically 



