THE VALE OF CLWYD. 19 
the water; and above it there is often a sandy bed, as if the 
calcareous material of the rock had been partly removed by 
chemical action, This bed must also have been originally 
more sandy than the rest of the limestone. The common 
minerals are ca/c7fe, barytes, and white and violet fuorspar. ; 
THE WEATHERING OF THE MOUNTAIN LIMESTONE. 
One of the most marked features of the Mountain Limestone 
is its mode of weathering. Partly because of a somewhat 
concretionary nodular character pervading the whole of the 
rock, but more on account of the readiness with which it yields 
to the action of carbonated water, so that every little temporary 
growth of moss or lichen begins the work, and we find all over 
exposed surfaces pits, like old worn pholas borings or small 
joints, invisible in the newly broken rock, but picked out by the 
weather so as to look at first like glacial grooves, but soon 
opened out into great scars. Combinations of all these kinds 
of weathering, on a larger scale, give us caves, which sometimes 
appear to run into solid rock like enormaus pholas borings; at 
other times these caves obviously follow the pre-existing fissures 
and joints. 
Extensive tracts of limestone are weathered along the bedding 
planes, and flake off in slabs from a few inches to a foot 
in thickness, which are eaten out into round holes or sinuous 
openings, so as to take all sorts of fantastic forms. These are 
much sought after for ornamental rock work. They may be well 
studied on the wooded limestone hills N. of Denbigh. On the 
smaller phenomena of pitted surfaces have often been founded 
theories of submergence to explain the supposed pholas borings, 
but a careful study of their manner of occurrence will show that, 
although they do exactly resemble weathered pholas borings, 
they are most commonly produced by the action of rain-water, 
helped by the additional carbonic acid derived from the atmos- 
phere and from decaying vegetation. On the Orme’s Head 
they may be seen in parallel vertical bands, running down the 
cliff for 40 or 50 feet, where the drip of water from above has 
kept up a dotted growth of vegetation, while on the inter- 
mediate dry portions there are none. The cliffs have been 
perishing too rapidly to allow of the possibility of these pits 
having been there since the last submergence, nor could we even 
on that hypothesis explain their continuous occurrence for such 
a height, except we suppose the emergence to have gone on so 
rapidly that the lower ones were being formed after the upper 
ones had been raised above the level of the sea, and yet that 
they kept up the arrangement in parallel vertical bands. 
In the quarry between Pontnewydd and the cave now known 
by that name, there is a band of red clay, chiefly the earthy 
residuum of the decomposed limestone, drifted into the opening 
fissure between two beds of limestone. The under surface of 
