300 THE REVOLUTIONS OF THE CKUST OF THE EARTH. 



opposes the opinion that the ancient Plutonic rocks were produced by 

 the simple escape of fluid matter through fissures in the crust, and that 

 all the ancient eruptions took place under the sea. He believes that 

 there have always been islands, which, like the islands of Oceanica in 

 our day, are centers of great eruptions. As to the sedimentary strata 

 which cover ancient craters, and which d'Ormalius mentioned in favor 

 of his view, a depresssion of these islands below the level of the sea 

 would perfectly account for the marine deposits upon the trachytes and 

 the basalts. Taking the lava as an essential character of the volcanoes, 

 d'Ormalius, moreover, denies their existence, on account of the entire 

 absence of lava in geological eras anterior to the Quaternary period. 

 This fact does not prove the absence of eruptions, but merely a change 

 in the eruptive matter, which might result from the greater depth of 

 the strata from which the lavas proceed and from the increased pres- 

 sure they must support, which cannot but affect the nature of the igne- 

 ous rocks. Ami Boue thought eruptive craters existed as far back as 

 the Secondary period, and even before the Carboniferous period. 



The formation of volcanic cones and the craters they contain has long 

 been a subject of discussion. De Buch advances the theory of the form- 

 ation of craters by gaseous emanations, proceeding from the interior of 

 the globe. These emanations, he says, raise the sedimentary strata like 

 a bubble or blister, which, in bursting, forms a circular cavity by the 

 accumulation of the debris. The maps of Teneriffe and of Palme, with 

 which he suj>ported his theory, led to a no less important discussion in 

 regard to the marine formation of the basalts, according to the Nep- 

 tunian theory of Werner. The views of De Buch prevail in the cele- 

 brated descriptions of Etna and of Vesuvius by the French savans Elie 

 de Beaumont and Dufresnoy. Sir Charles Lyell combats this theory in 

 his Principles of Geology, to-day universally esteemed. Fr. Hoffmann, 

 who saw arise from the bosom of the sea the island of Ferdinandea, op- 

 posed to the theory of De Buch the formation of craters by accumula- 

 tion of scoria, sand, and volcanic ashes.* But let us remark, in passing, 

 these hills of sand and ashes are sometimes hundreds of feet in height, 

 such as those formed by a very violent volcanic action in the middle of 

 a great plain in the State of Nicaragua,! or I may say even such as the 

 great Chimborazo, if we admit the consolidation of sand and cinders or 

 ashes into solid rock. 



The effects of volcanic action are very great, and, as we shall show, 

 they may contribute to the modification of the crust of the globe. 



On the 12th of July, 1831, suddenly arose, near the bank called Nerita, 

 between the Pantellaria islands and the coast of Sciacca, in Sicily, a 

 volcano, from out of a sea from a hundred and forty to two hundred 

 meters (450 to 650 feet) in depth. It was formed of scoria ; was about 

 three leagues in extent, and abcnt sixty yards in height. It disap- 



*D. H. von Decbeu, L. v. Bach, Sein Einfluss auf die Entwickelung der Geognosie, Bonn, 

 1 Sr,:i, pp. 14, 15. 

 \ Ramon «le la Sagra, Comptes Bendus, t. Ixvi, p. 481. 



