VOLCANIC THEORIES. 



dust ejected in a state of incandescence. There is, strictly 

 speaking, no combustion in or over the crater, and therefore the 

 term burning mountain is physically incorrect. 



75. The production of volcanoes, or of volcanic craters, which 

 remain passive, as above explained, is by no means confined to 

 those parts of the solid crust of the earth which lie above the 

 waters of the ocean. The bottoms of seas, and even of lakes, are 

 subject to such convulsions still more frequently than the land. 

 When the bottom of the sea is thus up-heaved, islands are 

 produced, the form, character, and magnitude of which depend on 

 that of the up-heaved mass. The form of Palma, one of the 

 Canaries, and the peaks which it throws up to the height of 7000 

 feet, and that of Nisyrus in the .2Egean, present examples of this. 

 The volcanic origin of the latter was known to the ancients, which 

 gave rise to the fable that Neptune, when pursuing Polybotes, one 

 of the giants that fought against the gods, followed him across 

 the sea as far as this land of Cos, where having torn off a part of the 

 island, he hurled it upon him, and buried him under it. This 

 fragment of Cos was called Nisyrus. 



When the dome up-heaved from the bottom of the sea breaks 

 at the summit, so as to form a crater of elevation, a part of the 

 annular rampart is sometimes destroyed, so that the sea enters, 

 and an enclosed bay is formed, where innumerable tribes of coral 

 animals build their cellular dwellings. 



It happens also frequently that the craters of elevation on land 

 become filled with water, so as to form lakes surrounded by a 

 rampart. 



76. Various theories have been proposed to explain the pheno- 

 mena of volcanoes, and to solve the questions, What is it that 

 burns ? What excites such prodigious degrees and quantities of 

 heat? heat sufficient to fuse not only the metals, but the most 

 refractory earths, imparting to masses of fused earth a heat which 

 many years are required to dissipate. We have throughout these 

 pages assumed, that the origin of all these phenomena is the 

 fluid incandescent matter contained within the solid crust of the 

 earth, such being, after much discussion, the explanation now 

 generally accepted by geologists. Among the hypotheses which 

 have been proposed, and which received and merited much con- 

 sideration, though now put aside, was the chemical theory of 

 Sir Humphry Davy, in which the evolution of heat and light in 

 the depths of the volcanic craters, was ascribed to the chemical 

 action of the most oxydable metals, such as potassium, sodium, 

 calcium, &c. It is a fact, familiar to all that have studied the 

 elements of chemistry, that a piece of potassium immersed in 

 water will instantly become decomposed, combining with its 



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