Subjacent to the Boulder- clay. 305 



outline of the containing limestone basins, and that the beds seem 

 rather to line than fill the pockets. Also that the surface of the 

 ground is sometimes depressed conformably with the shape of the 

 basin. Mr. Wynne, in the descriptiye letter-press accompanying 

 quarter sheet, No. 45, S.E. of the Irish Survey,, observes, with 

 reference to these beds, that " where the lignite appears at the sur- 

 face, it seems to have a dip at a high angle southwards." This, I 

 apprehend, must be an inclination from' the circumference of one of 

 the pockets. One of the conditions requisite for the gradual dissolution 

 of the limestone — the existence of underground outlets for the dis- 

 charge of the water, removing the lime in solution, is always present. 

 They are noticed by Mr. Blake and Mr. Wynne, in connection with 

 the limestone pockets in Tipperary ; and by Mr. Brown, at the seat 

 of these deposits on the Weaver Hills. In North Wales there is 

 evidence of the existence of underground outlets in the fact that the 

 water freely drains away from the pockets or basins, and Swallow- 

 holes are occasionally visible on parts of the Mountain Limestone 

 range not obscured by the deposit. In the mining operations of the dis- 

 trict it is not at all uncommon to break into these cavernous openings 

 in the Mountain Limestone. At Glan Alyn Mine, near Mold, a large 

 cavern occurs at a considerable depth, and discharges a portion of the 

 mine water; and at the Britannia Mine, near Llanarmon, a cave 

 lined with stalactitic deposits was found at a depth of 25 yards, also 

 affording a passage for the water. 



Perhaps the most striking fact bearing on this point is the occur- 

 ence of great masses of redeposited lime, as Tufa (see Geological 

 Magazine, June, 1866), at the foot of the limestone range in which 

 the pockets occur. Near Oaerwys, in Flintshire, in the valley con- 

 necting Mold with the Vale of Clwyd, many hundred thousand cubic 

 yards of Tufa have been deposited immediately adjacent to a cavernous 

 channel in the limestone cliff directed to the neighbourhood of the 

 pockets. 



A deposit of Tufa has also been noticed near Llangollen, and I 

 am informed by Mr. Beckett, of Wolverhampton, that other masses 

 also occur, somewhat further removed from the Mountain. Limestone 

 range : viz., on the eastern side of Wepre Brook, in the parish of 

 Hawarden, in Flintshire, about half a mile below Euloe Castle, where 

 it occupies a considerable breadth of steepish bank, and is remarkably 

 full of calcified ferns and other local plants ; Tufa also occurs on the 

 banks of the river Alyn, at Gwersyllt, near Wrexham. It has been 

 already noticed that mixed and interstratified with the sand; occupy- 

 ing the cavities, occur beds and patches of a very white and smooth 

 clay, resembling Kaolin in appearance, the origin of which, from 

 the mere mechanical degradation of previously existing beds, seems 

 difficult to account for, as in its purity and whiteness it is unlike 

 anything that could have been derived from the wearing down of the 

 Carboniferous beds. The question suggests itself whether it may 

 not be the insoluble matter contained in the limestone, left behind 

 after the dissolution of the carbonate of lime and iron. The purest 

 limestone contains a small quantity of silica and alumina, that 



VOL. IV. NO. XXXVII. 20 



