202 Maw— Pockets in Mountain-limestone. 
241, several of which are described as being 40 feet in diameter, 
and 20 or 80 feet deep; and Mr. Prestwich has expressed to me his 
opinion that the Llandudno ‘pockets’ may also be eroded swallow- 
holes. 
The absence of horizontal stratification, and the general tendency 
to a vertical disposition of the materials forming the contents of the 
pockets, would appear analogous to the vertical and concentric ar- 
rangement of the strata filling up the sand-pipes of the Chalk noticed 
by Mr. Prestwich (Philosophical Transactions, p. 300, part 2, 1864). 
The gradual dissolution of the limestone and the slowly lowering 
into the cavity of previously existing superincumbent beds would 
easily account for their vertical arrangement, which could not have 
been produced had the contents been directly deposited in the cavities 
from watery suspension. 
Iam indebted to my friend Mr. Blake, of Newton Abbot, for a 
careful account of a group of from ten to twenty similar ‘ pockets’ 
in the Mountain-limestone at Ballymacadam, near Caher, Co. Tip- 
perary, which he says are accompanied by fissures or swallow- 
holes, that carry away the surface-water, and are supposed to have 
outlets near the River Suir, about two miles below. The Bally- 
macadam ‘pockets’ contain white and grey Potter’s Clay, inter- 
stratified with Lignite similar to the Bovey-Tracy deposit; and Pro- 
fessor Jukes informs me that they are supposed to be of similar 
(Miocene) age. 
As to the age of the Llandudno cavities, in the absence of organic 
reraains any estimate must be speculative; but, judging from their 
relative positions on the hill-side, J am under the impression that 
they were formed subsequently to the range of Mountain-limestone 
assuming its existing outlines. Their excavation was, however, 
clearly antecedent to the Boulder-clay Drift, which spreads over the 
surface, but otherwise forms no part of the contents. 
Such steep cavities, however formed, could not have remained 
long empty; and, when once filled, would not be easily denuded; so 
that the age of the contents of the ‘pockets’ must approximately 
represent the period of their excavation. Indeed, had they remained 
long exposed as open cavities on the steep hill-side, subaérial débris 
from the limestone hills must have been largely associated with the 
material that now occupies them. 
A great, and probably the principal erosion of the Mountain-lime- 
stone, which may have given the hills much of their present contour, 
took place soon after the close of the Carboniferous period, as frag- 
ments of it enter largely into the Permian Breccia; and Professor 
Ramsay states that in the Vale of Clywd the New Red Sandstone 
lies very unconformably on the eroded Carboniferous Limestone. 
As far, therefore, as the evidence of superposition goes; the Llan- 
dudno deposits might be of any age younger than the Permian and 
older than the Boulder-clay. All the materials comprising the beds 
are, however, very unlike in mineral character any part of the Trias, 
Lias, and Oolite ; and on these grounds their probable age would be 
restricted to the Cretaceous or ‘Tertiary periods. 
