VOLCANOES, THEIR ACTION AND DISTRIBUTION. 55 



ment, and are nearly all situated along three well-marked bands and 

 the branches proceeding from them. The volcanoes of the eastern 

 coast of Africa, with Mauritius, Bourbon, Rodriguez, and the vents 

 along the line of the Red Sea, may be regarded as forming a fourth 

 and subordinate band. Nearly all of them are situated near the limits 

 which separate the great land and water masses of the globe, either 

 on the parts of continents not far removed from their coast-lines, or 

 on islands in the ocean not very distant from the shores. Two con- 

 spicuous exceptions to this rule are the volcanoes of the Thian-Shan 

 range in Asia, in the center of the largest unbroken land-mass of the 

 globe ; and the group of the Sandwich Islands, almost in the center of 

 the largest ocean, and rising almost from the greatest depths of that 

 ocean. Geological researches have, however, shown that the Thian- 

 Shan Mountains in Pliocene times stood on the southern borders of a 

 great inland sea. A regular parallelism seems to exist between vol- 

 canic bands and the great mountain-chains ; and the researches of Mr. 

 Darwin have shown that " nearly all the active volcanoes are situated 

 upon rising areas, and that volcanic phenomena are conspicuously 

 absent from those parts of the earth's crust which can be proved at the 

 present day to be undergoing depression." 



Inferences are sometimes hastily drawn from the fact that most 

 volcanoes are near the ocean which the facts themselves will hardly 

 warrant. Thus, it is frequently assumed that we may refer all the 

 phenomena of volcanic action to the penetration of sea-water to a mass 

 of incandescent lava in the earth's crust and to the chemical and me- 

 chanical actions which result from the meeting. This argument, how- 

 ever, as Mr. Scrope has shown, involves a reasoning in a circle. "It is 

 assumed, on the one hand, that the heaving subterranean movements 

 which give rise to the fissures by which steam and other gases escape 

 to the surface are the result of the passage of water to the heated 

 masses in the earth's crust. But, on the other hand, it is supposed that 

 it is the production of these fissures which leads to the influx of waters 

 to the heated matei'ials. If it is the passage of water through these 

 fissures which produces the eruptions, it may be fairly asked, What is 

 it gives rise to the fissures ? And if, on the other hand, there exist 

 subterranean forces competent to produce the fissures, may they not 

 also give rise to the eruptions through the openings which they have 

 originated ? " 



Many of the various theories which have been proposed to account 

 for volcanic action depending upon the supposed jDresence of active 

 forces within the earth ; upon the contact of water with the hot solid 

 or liquid matter of the interior of the globe ; upon the chemical ac- 

 tions that may be taking place within the earth ; upon the heat that 

 may be developed by the contraction of the earth's crust ; or upon the 

 occlusion of gases by the metallic elements of which many suppose the 

 core of the earth to be composed have a certain probability. They 



