206 ORIGIN OP LIMESTONE CA VEENS. 



hand; and so come to Pluto's Chasm, an underground ravine roofed with 

 the strata which support precisely similar gulches and chasms opening 

 to daylight, and owing their configuration to the same slow and subtle 

 agencies. Most persons, trying with their gaze to fathom a depth which 

 their candles' beams fail to penetrate, but which, by-and-by, their feet 

 lead them to, are tempted to exclaim, " What mighty convulsions rent 

 these walls asunder!" forgetting the imparted stratum of native rock 

 overhead. But cataclysm, as the all-potent word to explain every hard 

 conundrum of geology, is obsolete. < As in the fable -of -the hare and the 

 tortoise, an agency infinitely slower, a very type of gentleness, has -dane 

 the same work while the convulsion slept. 



Great caves can only occur in a limestone region, and they result from 

 the chemical fact that the carbonates of lime and magnesia are soluble 

 in water containing carbonic acid. "This acid abounds in atmospheric 

 air, and is one of the products of the decomposition of animal and vege- 

 table waters, so that rain-water which has percolated through the soil has 

 usually been enriched with it from both sources. With carbonic acid, 

 then, as the active agent, and water as the carrier, we are able to account 

 for the disappearance of strata however thick, and whether above or 

 below ground. Above ground the result is a lowering of the general 

 level, the deposition of a residual stratum of clay (a constituent, in a 

 finely divided condition, of the Valley limestones), and the formation of 

 valleys where special causes have favored the disintegration of the stone. 

 'Hard' water flows away, and a clay soil is left behind. Below ground, 

 on the other hand, the result is a cave — if there be a fissure in the strata 

 through which the acidified water may make its descent. In the course 

 of time this fissure is worn larger, and the entering water dissolves and 

 bears away with it bit by bit the stratum through which it passes, flow- 

 ing out at some lower level with its burden of lime and magnesia, but 

 leaving the clay behind to plague .the adventurous cave-hunter." 



Given the initiatory crack — common enough in limestones — and it 

 only requires time and abundance of water to hollow out Pluto's and all 

 the other chasms, halls, galleries, and avenues which make up this or a 

 more extensive series of caverns; and when once this work has well 

 begun, other natural agencies contribute their aid to the enlargement of 

 the area and the adornment of its interior. 



From the chasm, where there is a Bridge of Sighs, a Balcony, a 

 Spectre, and various other names and habitations, we recross the Elfin 

 Eamble, walk, wherever dry, on mud or tufaceous floor or ringing rock 

 ((when honeycombed, sounding hollow beneath the tread), and in muddy 



