“In this long and winding gallery, fashioned by nature in the marble heart of the mountain, floor, roof, and sides are everywhere intersected by fissures which were formed in the consolidation of the stone. To these fissures and the water which has passed down them, we owe the formation of the cave and its rich furniture of stalactites. The direction of the most marked fissures is almost invariably N.W. and S.E., and when certain of these (which in my geological work I have called master fissures) occur, the roof of the cave is usually more elevated, the sides spread out right and left, and often ribs and pendants of brilliant stalactite, placed at regular distances, convert the rude fissure into a beautiful aisle of primæval architecture. Below most of the smaller fissures hang multitudes of delicate translucent tubules, each giving passage to drops of water. Splitting the rock above, these fissures admit, or formerly admitted, dropping water: continued through the floor, the larger rifts permit, or formerly permitted, water to enter or flow out of the cave. By this passage of water, continued for ages on ages, the original fissure was in the first instance enlarged, through the corrosive action of streams of acidulated water; by the withdrawal of the streams to other fissures, a different process was called into operation. The fissure was bathed by drops instead of streams of water, and these drops, exposed to air currents and evaporation, yielded up the free carbonic acid to the air and the salt of lime to the rock. Every line of drip became the axis of a stalactitical pipe from the roof; every surface bathed by thin films of liquid became a sheet of sparry deposit. The floor grew up under the droppings into fantastic heaps of stalagmite, which, sometimes reaching the pipes, united roof and floor by pillars of exquisite beauty.”
At the time of its exploration, the water stood at a considerably higher level inside than at the present time, and formed deep pools. The barrier of dripstone has been cut through, and the water level lowered, and a passage made for a considerable distance. Inside, the old water line, which separated the subaërial from the subaqueous dripstone, is very distinct, the former being deposited in thick bosses, crumpled curtains, drops, straws, pyramids, and other fantastic drip-structures, while the latter is honeycombed, and composed of rounded, grape-like masses. Between them an ice-like coating of stalagmite forms a dividing line, now supported in mid air, but that formerly shot across the surface of the pools that have been drained, or rested on the mud and stones which had been brought down by the stream in ancient times. In some places it still rests on the surface of the pools.
A stalactitic curtain on the right-hand side presents a very singular appearance, its surface being covered with an abundant crop of tiny club-like bodies about one-tenth of an inch in length, and consisting each of a shining drop of water, enclosing a minute fungus. These may possibly explain in some degree the peculiar fungoid-appearance of certain small bosses of dripstone which I have met with in the caves of Pembrokeshire: for an accumulation of carbonate of lime on such a nucleus would produce the forms which they assume (see [Fig. 17]).
There are also magnificent groups of dripstone, and each joint in the rock is adorned with lines, and pipes, and fringes of calc spar, or widened out into roof-shaped hollows, and traversed by deep, vertical grooves, caused by the passage of water laden with carbonic acid. The general surface of the roof, where the rock is bare, has had its fossils etched out by the acidulated water. In one place you may stand under a branching coral, with its sides and base distinctly marked, and in another fossil shells stand out almost in their original beauty.
Rate of the Accumulation of Stalagmite.
The rate at which the calcareous matter is being deposited at the present time is very easy to be estimated, for that accumulated since the passage was cleared out is white, and contrasts with the dirty, grey-red colour of the older kind. In one case a thickness of 0·24 had been formed in thirty-five years, by the water flowing down the side of the passage excavated by Mr. Farrer, while in another, in about the same time, 0·05 inch had been formed. This would give an annual accumulation of 0·0068 in the one case, and in the other about one-fifth of that amount. This rate does not agree with the rate of increase noted by Mr. Farrer and Professor Phillips in the case of a large stalagmite called the Jockey Cap, on which a line of drops is continually falling from one point in the roof. Its circumference in 1839 measured 118 inches, in 1845, 120 inches, and in 1873, I found it to be 128 inches. The annual rate of increase from 1845 to 1873 is ·2941 inch, and that from 1839 to 1845 is ·2857. I found, however, that the most remarkable increase was that in height. In 1845 its apex was 95·25 inches from the roof, in 1873, 87 inches, which would imply an annual deposit of not less than ·2946. (See [Appendix II].) At this rate it will arrive at the roof in about 295 years. But even this comparatively short lapse of time will probably be diminished by the growth of a pendant stalactite above, that is now being formed in place of that which measured 10 inches in 1845, and has since been accidentally destroyed.
It is very possible that the Jockey Cap may be the result, not of the continuous, but of the intermittent drip of water containing carbonate of lime, and that therefore the present rate of growth is not a measure of its past or future condition. Its age in 1845 was estimated by Professor Phillips at 259 years, on the supposition that all or nearly all of the carbonate of lime in each pint was deposited. If, however, it grew at its present rate, it may be not more than 100 years old; and if it be taken as a measure of the rate generally, all the stalagmites and stalactites in the cave may not date further back than the time of Edward III.
It is evident, from this instance of rapid accumulation, that the value of a layer of stalagmite in measuring the antiquity of deposits below it, is comparatively little. The layers, for instance, in Kent’s Hole, which are generally believed to have demanded a considerable lapse of time, may possibly have been formed at the rate of a quarter of an inch per annum, and the human bones which lie buried under the stalagmite in the cave of Bruniquel, are not for that reason to be taken to be of vast antiquity. It may be fairly concluded, that the thickness of layers of stalagmite cannot be used as an argument in support of the remote age of the strata below. At the rate of a quarter of an inch per annum, twenty feet of stalagmite might be formed in 1,000 years.
The Descent into Helln Pot.
The subterranean passages grouped round Helln Pot, a tremendous chasm near Selside, on the east of Simon’s Fell in Ribblesdale, illustrate in a remarkable degree the mode in which the water is at present wearing away the rock. Those which have been explored constitute the Long Churn Cavern, which is comparatively easy of access through a hole known as Diccan Pot ([Fig. 2], a). On descending into it, the visitor finds himself in the bed of a stream that now roars in a waterfall, now gurgles over the large fallen blocks from the roof, and that here and there has worn for itself deep pools by the mechanical friction of the sand and pebbles brought down by the current. If it be followed down after passing over a waterfall, the light of day is seen streaming upwards beneath the feet from the point where the water leaps into the great chasm of Helln Pot ([Figs. 2], b. 3, a). Above the entrance there is a complicated network of passages, some dry, and some containing streams which have not yet been fully explored.