1869.] HUNT— ON VOLCANOES AND EARTHQUAKES. 389 



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 

 combined with sulphur and with carbon, are products of volcanoes. 

 The combustion of the inflammable gases in contact with air 

 sometimes gives 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. 

 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 stoney materials 

 assume a more solid and crystalline condition, and resemble the 

 older eruptive rocks found in regions not now volcanic. These 

 include granites, trachytes, dolerites, basalts, etc., and are masses 

 of rock which, though extravasated after the manner of lavas, 

 became consolidated in the midst of surrounding rocks, and 

 consequently under considerable pressure. Their presence marks 

 either the lower portions of volcanoes whose cones have been 

 removed by denudation, or outbursts of liquefied 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, therefore, regard the extravasation of igneous matter, 

 whether as lava or ashes at the surface, or as plutonic rock in the 

 midst of strata, as, in its wider sense, a manifestation of 

 vulcanicity, and, for the elucidation of our subject consider both 

 those regions characterized by great outbursts of plutonic rock in 

 former geologic periods, and those now the seats of volcanic 

 activity, which; in these cases, can generally be traced back some 

 distance into the tertiary epoch. To begin with the latter, the 

 first and most important is the great continental region which 

 may be described as including the Mediterranean and Aralo- 

 Caspian basins, extending from the Iberian peninsula east-ward to 

 the Thian-Chan Mountains of central Asia. In this great belt, 

 extending over about 90° of longitude, are included all the historic 

 volcanoes of the ancient world, to which we must add the extinct 

 volcanoes of Murcia, Catalonia, Auvergne, the Vivarais, the Eifel, 

 Hungary, etc., some of which have probably been active during 

 the human period. 



It is a most significant fact that this region is nearly coexten- 

 sive with that occupied for ages with the great civilizing races of 



