138 



Prof. J. Prestwich. 



through, and is held in, the numerous fissures and cavities by which 

 it is traversed. These fissures are due to contraction on cooling-, 

 and to the fractured state of the lava produced by the split- 

 ting caused by subsequent disturbances, whilst larger cavities are 

 produced by other causes. Of these, the two most important are — ■ 

 1st, the escape of vapours while the lava is consolidating. Sometimes 

 the hardened outer crust of the lava is raised in great blisters, which, 

 on the escape of the vapour, are sufficiently solid to retain their posi- 

 tion, and remain like so many empty beehives on the surface of the 

 lava streams. The Grotta delle Palombe, on Etna, which, according to 

 Waltenhausen, has a length or depth of about 500 feet, and a height 

 in places of from 70 to 80 feet, and the great ice cave near the top of 

 the Peak of Teneriffe, described by Piazzi Smyth,* and so large as 

 to contain a lake of water of considerable size, are attributed by them 

 to the escape of elastic vapours. 



2nd, the escape of lava from a lava stream after the exterior of it 

 has become solid, when an empty . shell in the form of a cave or 

 tunnel is left. These tunnels or caverns are of common occurrence, 

 and often of large size. Scrope observesf that " among the lavas of 

 Etna, Bourbon, Iceland, St. Michael, Teneriffe, and many others, 

 caverns of very large dimensions are thus formed beneath the surface 

 of a lava stream, and often imitate in their extent and windings the 

 well-known caves worn by water in limestone rocks." Phillips 

 and others notice the occurrence of similar tunnels in the lavas of 

 Vesuvius, but they are all small. 



In the great volcanic mountains of South and Central America, 

 Humboldt long ago inferred that large cavities filled with water must 

 exist in consequence of the ejection of water, with small fishes and 

 tufaceous mud, from fissures caused by the earthquake shocks which 

 precede the eruptions of the volcanoes in the Andes. J 



A French geologist, M. Virlet d'Aoust, has, moreover, given particu- 

 lars of two great tunnel-caverns of Central America, § which will serve 

 to indicate the magnitude of some of these subterranean reservoirs. 

 The first is that known as the Cueva de Ghiuacamote, near Perota, 

 which he was assured extended several leagues in length (!) He found 

 ib to be a cavern of great size, and divided into compartments by 

 falls of the roof. The floor is covered with a sandy gravel, and the 

 side walls, here as in the cave of Custodio, exhibit grooved lines 

 covered with slight calcareous incrustations indicative of old water- 

 levels. The other is the Brena de Custodio, in the State of San-Luis 

 Potosi, of which he says that it forms a perfect semispherical tunnel 



* " Teneriffe," p. 352. 



t " "Volcanos," 2nd Edit., p. 79. 



X " Cosmos," Sabine's Translation, vol. i, p. 230. 



§ " Bull. Soc. Geol. de France," 2nd Ser., vbl. xxiii, p. 34. 



