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rocky shore, or filled with heaped-up pebbles, if it opened on to a shingle beach. By its form and by its contents we could generally make a shrewd guess whether it was a sea-cave or

We should ask whether the parts where the cave expands are those on which the sea would act with greatest force and efficiency, or whether the shape could be better explained by reference to torrents coming in the other way.

We should examine the contents to see whether in their character or arrangement they indicated the action of the in-rushing water, or whether they are such as could never have survived the scour of tidal and wind-driven waves. When we have to inquire into the origin of caves in inland cliffs and on mountain-sides, now far above the sea, where many of the traces above-described may have been long removed by denudation, there are further tests to be applied. There we should have regard to their manner of occurrence and their place in the physical geography of the neighbourhood. A sea-cave does not necessarily, or even commonly occur in the line of drainage from the uplands, but in the higher cliffs and headlands between the valleys that run down to the sea.

Whereas the caves due to subterranean watercourses lie in the lines of drainage; and the caves due to sub-aërial waste coincide in distribution with the outcrop of the beds that readily lend themselves to that kind of weathering

Moreover, allowing for the possibility of unequal elevation of different parts of a coast-line, we can still generally find sufficient evidence to show whether the rock in which the cave occurs forms part of an old sea-cliff or of an escarpment.

We must remember also that during the formation of a sea-cave the base of the cliff is being swept by the sea. Sometimes an inland stream washes the base of a rock in which a watercourse cave has its outfall, but generally in the case of inland-formed caves a vast mass of talus is being formed along the base of the cliff in which the cave occurs. The scour of floods may keep the mouth open, but as the water is being drained off to other and lower levels, this sweeping of the cave mouth ceases, and the cave deposits show interbedded fallen rock and transported earth and stones, and often the remains of animals.

As a general statement we may say that a typical sea-cave runs into a cliff which rises vertically from the level of the

*

* Cf. Whitaker, Q.J.G.S. vol. xxiii. c. 186, p. 265. Geol. MIug. vol. iv. 1367, pp. 457, 443.

floor of the cave, or is even undercut a little, because the talus has always been removed from the base, so that the fragments broke away all over the face of the cliff from top to bottom, and the base sometimes was even undermined by the waves.

In the case of an inland cliff, on the contrary, the fallen rock is not removed, so that only the upper part of the cliff above the sloping mass of talus is exposed to the action of the weather. The exposed part is' reduced in height as the talus grows, so that the cliff keeps on receding above only, as the talus keeps covering up more and more of the lower part.

The form that a chalk cliff would eventually have behind the talus bas been calculated by the Rev. O. Fisher.*

Of course, the sea-cliff, when removed inland by elevation, gets, after a time, eaten back by sub-aërial weathering, and covered over by talus like any ordinary escarpment.

Gaping fissures of such a character that they could in any case be looked upon as caves are very rare, but the faultbreccia that commonly fills such cracks is easily removed, and the various denuding agencies are apt to follow fissures, and thus caves be formed along them. The unequal flow of lava curling and coiling over the half-cooled mass of earlier flows sometimes leaves openings like caves.

It is said that some of the caves in volcanic districts are opened out by the various acidic vapours which act on the micaceous and other schistose rocks which have been already fissured by the earthquakes so frequent in those countries ; as, for instance, in the case of some of the caves of Corinth and the Cyclades.T

These are, however, few and unimportant, seldom occurring where a cave would be much frequented by man or the lower animals. The caves, and those

which generally have proved of greatest interest, are the old subterranean watercourses so frequent in limestone rocks. The way in which these caves are formed is well known, but many of the phenomena connected with them appear to be less clearly understood, and so we hear of various startling theories propounded which, on inquiry, turn out to be based on a wrong interpretation of the mode of formation of the deposits found

commonest

* Geol. Mag., vol. iii. 1866, p. 354. See also Davison, Geol. Mag., vol. 1886, p. 65.

+ Virlet, Bull. Soc. Géol. de France, t. ii. p. 329.

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in such caves. It is to these questions I especially invite attention to-night, and in the selection of examples in illustration I shall be chiefly guided by the desire to make clear the distinction between the age of the caves and of the cave deposits, and the mode of formation of the cave earth and laminated clays, stalagmitic floors, and broken-up travertine breccias, stream-gravel, and angular talus.

First, I would just remind you that these caves are formed in a rock which can not only be mechanically broken up and carried off, but can also be dissolved in water and carried away in solution wherever water can pass. Even pure water can take up two grains per gallon of carbonate of lime, of which these rocks are largely composed. But pure water is very rarely found in nature. The rain generally takes up some carbonic acid from the air, and when it falls on the ground gets a great deal more from the decomposing vegetation; and water with carbonic acid in it acts rapidly upon the limestone rock, carrying off part of it as a bicarbonate of lime, while the earthy part is washed away in mechanical suspension till it settles down in some pool of still water as mud, often forming a considerable part of the cave-earth which fills all the interstices of the broken rock. As may be seen by the analyses of hard waters, it is not uncommon to get 25 grains per gallon of carbonate of lime in the water of limestone districts, and this means the never-ceasing operation of the agencies which tend to form caves.

So, of course, the most favourable conditions for the formation of such caves are,-First, a limestone into which the water can trickle down along joints and fissures, and find its way out at some lower level. Secondly, an area over which the rain can gather into streamlets and collect from vegetation the acids which will help it to dissolve the rock. The crack into which the water first finds its way may be very small; the water soon opens it out, acting first chemically, then mechanically, on the surrounding rock. When the sand and broken rock get a free passage, mountain torrents, full of débris torn from the hill-side or washed out of ancient boulder-clays, are precipitated into the chasms, which take the place of the half-opened joint, and the work goes on apace.

It is quite clear that in such circumstances it must often happen that, as the clay or shale on the hill-side is being denuded away, the water must find its way into the jointed limestone further and further back continually, and, in the deep recesses of the mountain, new channels must often carry off the water that once ran higher up. Thus, the higher outfalls are left dry, and then they are in a state for man and beast to inhabit. Sometimes, however, when all the hill is full after some great thunderstorm, water spurts out of every joint and spouts in torrents from each cave, and until the cave is quite beyond the chance of such catastrophes, we cannot hope to find a clear, continuous record of its old inhabitants.

To give an example of a cave now being formed in one part and periodically modified in another, I will carry you to the flanks of Ingleborough, where the conditions are peculiarly well suited for the formation of caves and for the examination of all the accompanying phenomena. Many of you are familiar with the form of the grand bluff known as Ingleborough,--the most conspicuous feature as you look north from Lancashire towards the borders of Yorkshire and Westmoreland. Its flat cáp of millstone grit; its steep slopes of rapidly-crumbling Yoredale shale, here and there braced up by throughs of sandstone, or grit, or limestone; its great table of mountain limestone, on which these all stand ; and its base of Cambrian and Silurian, altogether combine to furnish some of the most charming bits of scenery and most interesting bits of geology in the kingdom. On the S.E. slopes of Ingleborough is a great hollow space where the water runs off the impervious Yoredale shale and the patchy drift down to the basement table of mountain limestone. The drainage area is about a square mile, and the stream is usually small and generally lost at once in the first open joints of the limestone that it gets to. But a flush of rain-water soon fills these crevices to overflowing, and the surplus water rushes on 100 yards or so to a great chasm, known as Gaping Gill Hole, into which it plunges with a roar. The air dragged down, tangled in the water, ascends in a current, carrying mist and spray far above the chasm's brink. I have watched this wonderful abyss many a day of storm and sunshine. No one has ever been to the bottom of it; but I can tell you something more about it that bears directly on the subject we are considering.

In that country, so favourable for the formation of all the various kinds of swallow-hole, cave, and keld, I once had the good fortune to witness one of those grand storms which in a few minutes change the face of nature, and in a few hours leave a mark that ages may not efface.

I had climbed some way up Ingleborough. It was a glorious July morning. Myriads of insects were busy with their own various pursuits. The haymakers were hard at work ; more hurried, perhaps, as the weatherwise saw thickenings towards the south, and felt the sultry heat that warned them there might be a storm. I turned now and then as I got higher, and saw the mist gather on the southern horizon. Soon it took shape and formed in the eddies as the rapidlyrising wind crept on. Two principal masses of cloud came crowding up, converging on Ingleborough, from Lancaster and Clitheroe. I had once before seen that kind of sky in South Wales, and, a few hours after, thirty-eight bridges were carried away in our county. So warned, I hurried homewards, and it was well I did. The clouds appeared to me to be rolling on in vertical planes. I ran, and only just got in to my inn before the worst was on us. Drenched haymakers, who had lingered too long in some insufficient temporary shelter, kept coming into the village. The storm burst with all its fury on the south-eastern flank of Ingleborough.

The stream that drains that area runs through the village of Clapham. The valley is dammed close above the village, to form a small tarn. This soon felt the flood, but, of course, the equalising effect of a lake upon the stream below it prevented our realising the tremendous rainfall for a time; because, before the stream could be raised six feet as it flowed out of this lake, the whole area of the lake had to be raised to that extent. But very soon this was done and the arch was filled, and a great spout of turbid water was projected forward on to the rocks at the base of the dam above the church. I went up the valley round the lake towards the celebrated Ingleborough Cave. It was a striking scene. Water spurted out of every crack and joint in the rocks, but the united subterranean watercourses could not carry it all, and the overflow from the drift-covered country above the usual outfalls rushed down the valley, carrying mud and boulders with it in its headlong course.

The stream below the cave runs over bare limestone for a considerable distance, and the noise made by the boulders, as they were rolled along the rocky floor, was so great that my companions thought the thunder-storm was beginning again, and hurried home. I went on to the great cave. Here I saw a wonderful sight. The lower cave was full, and the water was spouting out of the upper cave, which is usually dry, as you pour water out of the mouth of a kettle; and well it might, for, if the swallow-hole that feeds it was full to overflowing, it had had the pressure of more than eleven atmospheres upon it.,

This was one of the most instructive geological phenomena it has ever fallen to my lot to witness. Here I saw what was, to all intents and purposes, a local cataclysm Gentle slopes of pasture, where usually no stream ran, were suddenly gashed by a torrent, and the débris swept far away across the

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