166 CAVES AND CAVE DEPOSITS. 
So, of course, the most favourable conditions for the forma- 
tion 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 hillside 
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 continually further 
and further back into the jointed limestone, and, in the deep 
recesses of the mountain, new channels must often carry off the 
water that once ran at higher levels. Thus, the original out- 
falls 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 that grand bluff—the most conspicuous feature 
as you look north from Lancashire towards the borders of York- 
shire and Westmorland. Its flat cap of millstone grit; its 
steep slopes of rapidly-crumbling Yoredale shale, here and there 
braced up by /hvoughs 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. Buta flush of rain-water soon fills these crevices 
to overflowing, and the surplus water rushes on 1oo 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 
