204 CHEMISTRY OF THE EARTH. 



activity ; from which it is reasonably inferred that the phenomena of 

 earthqnakes and volcanoes have a common origin. The discharge 

 through openings in the earth's crust, of ignited stony matter, generally 

 in a fused condition, and the disengagement of various gases and vapors, 

 accompanied by movements of elevation or subsidence of considerable 

 areas of the earth's surface, sometimes rapid and paroxysmal, and at- 

 tended with great vibratory movements, are evidences of a yielding crust 

 of solid rock resting upon an igneous and fluid mass below. To the 

 same conditions are also to be ascribed the slow movements of portions 

 of the earth's surface, shown in the rise and fall of continents in regions 

 remote from centers of volcanic activity. The unequal tension of the 

 yielding crust and the sudden giving way of the overstrained portions 

 are probably the immediate cause of earthquake phenomena; the seat 

 of these, according to the deductions of Mallet, is to be found at depths 

 of from seven to thirty miles from the surface. 



§ 49. Abrief description of the phenomena of volcanoes will here be nec- 

 essary. Volcanoes are openings in the earth's crust through which are 

 discharged solid, liquid, and gaseous matters, generally in an intensely 

 heated condition. Sometimes the ejected material is solid, and consists 

 of broken, comminuted rock, or the so-called A'olcanic ashes. Oftener, 

 however, it is discharged in a more or less completely fused condition, 

 constituting lava, which is sometimes fluid and glassy, but more fre- 

 quently pasty and viscid, so that it flows slowly and with difiticulty. 

 The ejected materials, whether liquid or solid, build up volcanic cones 

 by successive layers — a fact which has been established by modern 

 observers in opposition to the notion come down from antiquity, that 

 volcanic hills are produced by an uprising or tumefaction of previously 

 horizontal layers of rock by the action of a force from beneath. First 

 among the gaseous products of volcanoes is watery vapor ; water ap- 

 I)ears not only to be involved in all volcanic eruptions, but to be inti- 

 mately combined with the lavas, to which, as Scrope has shown, it helps 

 to give liquidity. The water at this high temperature is retained in 

 combination under great pressure, but as this j^ressure is removed passes 

 into the state of vapor, a process which explains the swelling up of lavas 

 and their rise in the craters of the volcanoes. Besides watery vapor, 

 carbonic and hydrochloric acid gases, and hydrogen, both free and com- 

 bined with sulphur and with carbon, are products of volcanoes. The 

 combustion of the inflammable gases in contact with air sometimes 

 give rise to true burning mountains — a name which does not properly 

 belong to such as give out only acid gases, steam, and incandescent 

 rocky matters, which are incombustible. 



§ 50. The escape of elastic fluids from lavas gives to them a cellular 

 structure, but when slowly cooled under pressure, as seen in the dykes 

 traversing the flanks of volcanoes, the stony materials assume a more 

 solid and crystalline condition, and resemble the older eruptive rocks 

 found in regions not now volcanic. These include granites, trachj-tes, 

 dolerites, basalts, &c., and are masses of rock which, though extrava- 

 sated after the manner of lavas, became consolidated in the midst of 

 surrounding rocks, and consequently under considerable pressure, (§ 37.) 

 Their presence marks either the lower portions of volcanoes whose 

 cones have been removed by denudation, or outbursts of liquefled rock 

 which never reached the surface. The escape of such matters, and the 

 formation of volcanic vents, are but accidents in the history of the 

 igneous action going on beneath the earth's surface. We shall, there- 

 fore, regard the extravasation of igneous matter, whether as lava or 

 ashes at the surface, or as plutouic rock in the midst of strata, as, in its 



