302 



CAVERNS IN LIMESTONE. 



ern there is a wide opening into an upper cavern, the top of which 

 is not visible from below, even with the illumination of fireworks, 

 which those who show the mine generally take with them. 



By the ceaseless action of such internal currents of water, falling 

 into original fissures, or descending through soft strata in mountains 

 of compact limestone, it is easy to conceive that caverns of great ex- 

 tent may be excavated. A very few years since, a miner, in driving 

 an adit or passage into the heart of the well-known rock called Mat- 

 lock High Tor, discovered a large cavern and a lake in the middle 

 of the mountain. Many of the coves or caves in Craven, in York- 

 shire, were originally caverns, the roofs of which have fallen in ; 

 they have streams of water rushing into them, forming subterranean 

 cascades. The cavern called Weather Coat Cove, and the rocks at 

 Gordale Scar, offer illustrations of the effects of subterranean cur- 

 rents. Where springs of water of considerable magnitude rise at 

 once to the surface, it is obvious that they are not the result of slow 

 percolation through porous strata, but that they are the outlets of in- 

 ternal streams or rivers. The river Air rises at the foot of a per- 

 pendicular limestone rock, called Malham Cove, in Craven ; it is a 

 broad, powerful, and permanent stream, before it receives any tribu- 

 tary rivulets from the adjacent valleys. It is generally believed that 

 the subterranean stream which gives rise to the river Air, is con- 

 nected by internal passages widi Malham Tarn, a mountain lake, sit- 

 uated at a considerable distance. Perhaps the spring at Holywell, 

 in Flintshire, may be cited as offering a similar proof of underground 

 rivulets. 



The reason why subterranean streams of water, and extensive 

 caverns, should occur chiefly in districts where compact transition or 

 mountain limestone is the prevailing rock, will admit of an easy ex- 

 planation. Slate rocks are almost always intersected by perpendicu- 

 lar fissures, which carry off the water, and prevent its accumulating 

 in large streams ; ^and the secondary strata in England are generally 

 too soft, or too much broken, to form the roofs of extensive caverns, 

 or the beds of subterranean rivers. In the vicinity of the Alps, 

 where the secondary limestones are extremely hard and compact ; 

 they contain caverns, and afford a passage for subterranean currents. 

 A considerable cavern has, however, been recently discovered in 

 mica-slate and common slate, in the Isle of Thermia, one of the 

 Cyclades, at the height of 1400 feet above the level of the sea.' 

 M. Virlet, who visited the cavern, attributes the excavation to sub- 

 terranean streams of water, as there is a considerable deposition of 

 mud and bluish clay at the bottom of it. — Seance du Fci'., 1832, de 

 la Societe Geologiqne de France. 



It is admitted by M. Desnoyers, in the report from which this ac- 

 count is extracted, that the existence of such a cavern in rocks of 

 mica-slate and slate, is a new fact in geology. There are several 

 thermal springs in the island, which indicate the action of sul^terra- 



