THE CAVERNS. 
141 
But there is another great class of caverns very different from 
any of these. Limestone, like granite, is a brittle rock. It is, 
however, softer than granite, and is chemically acted on by 
water, both more rapidly and more completely. Thus, not only 
by the sea-side and within the action of the tidal wave, but in 
every place where there is a continuous mass of limestone com- 
pact enough to hold together, there are numerous crevices in 
the rock arising originally from fracture or contraction, and in- 
creased during elevation, but finally enlarged and converted 
into caverns by water. 
In England such caverns occur’ in the compact limestones 
of the coast of South Wales, in many parts of North and South 
Devon, in Derbyshire, and in Yorkshire. In France and 
Belgium many others might be cited ; in Central Germany 
there is a well-known and remarkable district honeycombed to 
a marvellous extent by them, while in Carniola the caverns of 
Adelsberg, in Greece a multitude of grottos sacred in classic 
story, and in America the Mammoth caverns of Kentucky, may 
be mentioned rather as a few examples selected at random from 
a countless host, than as giving any outline even of the chief 
localities of such phenomena. So frequent, indeed, are caverns 
in some kinds of limestone, that their absence, rather than their 
presence, is remarkable. But, on the other hand, there are 
some limestones that seldom contain them. Such are the 
Portland stone and some of the other oolites of the British islands, 
although corresponding rocks in other countries abound with 
them. Such, also, is chalk, which is too soft to admit of large 
halls and intricate passages, although noble vaults receding 
from the sea are not unknown. 
Limestone caverns, commencing from or tenninating in sea- 
side cliffs, close to the ordinary sea level, are not on the whole 
the most common. In this respect, therefore, they vary essen- 
tially from those in granite, and the picturesque effects obtained 
from them are also very different. Much more frequently they 
open out on cliffs or escarped faces of rock, far away from the 
sea • often, indeed, in river banks, but even then at levels far 
above the present water-line. Their openings, again, are not 
always large in proportion to the size of the cavern, nor have 
the entrances of important caverns of this kind any relation 
to the magnitude and extent of what is within. In all these 
respects they have their own marked peculiarities, and these 
depend partly on original formation, and partly on subsequent 
and long- continued wearing by running water. 
Water-action, in the case of these great open spaces within 
limestone rock, is a force that requires explanation; for, although 
the result is clear enough, the exact course of proceeding is 
less manifest. 
