Maw—Pochets in M ountain-limestone. 201 
the ‘pockets,’ the contents of which appear to be of a totally dif- 
ferent age and character. 
A reference to Section fig. 1, Pl. VII., will best explain the form, 
size, and relative position of these ‘ pockets,’ and the arrangement of 
their contents. Three of them are at present known to exist, 
namely, the two larger ones at Nant-y-Gamer, shown in Section 
fig. 1, and another smaller cavity, about 32 feet in diameter, at Fferm, 
half a mile to the south. The surface gives no indication of their 
existence, and I have not ascertained how they were discovered; 
but for the last fifteen years workings have been successively carried 
on by Mr. Thomas Smedley, of Holywell, Mr. Jabez Jones, and 
Mr. R. F. Steble, of Liverpool, for the white siliceous sand and white 
clay used in the glass-works ; and these have well exposed their 
structure and contents. 
When I first visited Llandudno, in 1853, but one of the ‘ pockets’ 
was known to exist. It is now worked out and abandoned; and the 
second, about 180 feet lower down the hill, is being emptied by 
means of an adit, driven in from the hill-side to about its centre. 
The ‘pockets’ or cavities range from 180 to 280 feet above the 
sea-level ; they are from 100 to 170 or 180 feet in diameter, about 
110 or 120 feet deep ; and, judging from the upper one, which is 
tolerably well exposed, their form very much resembles that of an 
egg-cup, and is altogether independent of the structure of the 
Mountain-limestone strata, which the sides intersect at right angles. 
In aspect they are very much like an old quarry-face, but rather 
smoother, presenting no hard angularities of fracture, but a mammil- 
lated surface, indicating that their excavation must have been by 
the gradual dissolution of the limestone. 
On removing the Drift and Mountain-limestone débria, which was 
nearly 40 feet thick over the upper ‘pocket,’ beautifully white 
siliceous sand is reached, forming the bulk of the contents of the 
cavities. It is partially discoloured by ochreous stains, which almost 
disappear in drying; and the sand is occasionally intersected by 
carbonaceous markings. ‘These are the only indications of organic 
remains I could detect ; but they are so obscure that no definite 
structure can be made out. 
Here and there the sand graduates into patches of very white 
clay, which is for the most part rather sandy, but occasionally very 
smooth and soft, resembling the finest kaolin in texture and appear- 
ance. Patches of smooth red clay also occur in the white sand. 
The exact form of the bottom of the ‘ pockets’ has never been 
ascertained ; the upper cavity was deserted before the contents were 
completely removed ; and the lower ‘pocket’ is not yet worked 
out ; so it is uncertain whether the cavities terminate with a rounded 
blunt bottom, or are prolonged into narrow outlets for the discharge 
of the water which would appear to have been essential in their 
excavation. 
Relative to the method by which these curious cavities have been 
formed, I would observe that they may have some analogy to the 
swallow-holes of Tertiary age in the Chalk mentioned by Mr. Prest- 
wich in vol. x. of the Journal of the Geological Society, pp. 222 and 
