OF ARTS AND SCIENCES. 339 



pose all the superincumbent mass of rock, more or less. In 

 craters of depression, as water can more copiously fall in, this 

 decomposition of compound aggregates proceeds with great 

 rapidity, and to great depths. The changes resulting from 

 the chemical action thus established are not merely mechani- 

 cal ; salts more or less soluble form, and are dissolved in 

 the escaping waters, while the rocky masses are reduced to 

 their insoluble, finely divided, proximate elements. Few of 

 the compound silicates resist this action ; thus soda felspar, 

 which generally is found in trachytes, and in the laboratory, 

 decomposes slowly, but rapidly yields its silicate of soda to 

 the acid and aqueous vapors, and solfa-tara is soon reduced to 

 a mixture of silicic acid and pipe-clay. From this brief state- 

 ment of facts, it becomes apparent that every volcanic focus 

 becomes covered to a great depth with finely divided materials, 

 resulting from rock decomposition, and any succeeding erup- 

 tion must be preceded by a removal of this matter in the way 

 of upheaval. The narratives of the eruptions of "Vesuvius, 

 so remarkable for graphic description, show that, after periods 

 of repose, llie first efforts of*reawakened vigor are expended 

 on the materials covering the surface. One of the most in- 

 structive examples, also, is that of the eruption, so called, of 

 the volcano Morne Ronde, on the island of St. Vincent, in 

 1792, the marks of which I have examined. This, as is well 

 known, is one of the volcanic vents of the Windward West 

 India Islands, and its resumption of activity was preceded by 

 no preliminary efforts. The inhabitants of the island were 

 roused from their slumbers at about 2 A. M. by a terrific 

 convulsion, the earth swaying under their feet, while the 

 atmosphere, suddenly displaced, was rushing in opposing cur- 

 rents from all directions, attended by deafening reports. By 

 one sudden explosion, the top of the mountain, about three 

 thousand feet high, lost several hundred feet in height, while, 

 as afterwards appeared, a cavity of about eight hundred feet 

 deep was formed. The larger masses of the covering mate- 

 rial rolled down its base, while the more finely divided part 



