CHAMBERS'S INFORMATION FOR THE PEOPLE. 



been so long exposed to the wasting of the ele- 

 ments, that no actual crater can now be traced. 

 Volcanoes for the most part occur in lines, and 

 not far from the sea ; partly along the margins of 

 the great continents, partly in chains of oceanic 

 islands. The most remarkable example of the 

 marginal arrangement is seen in the long chain of 

 volcanic vents that studs the western border of 

 America from Tierra del Fuego to Mexico. Of 

 insular volcanoes the most numerous and energetic 

 occur in the Indian Archipelago Sumatra, Java, 

 and the adjacent islands. Along the island of 

 Java there runs a band of not less than forty- 

 five volcanoes, most of which have been seen in 

 eruption. 



But the forces, whatever they are, that produce 

 volcanoes and earthquakes do not always act in 

 this violent paroxysmal way. They are constantly 

 producing movements and changes on a vastly 

 greater scale than anything resulting from earth- 

 quakes and volcanoes, but so gently and insensibly, 

 that it is only by the results which they bring 

 about in a long course of years that we can detect 

 their operation. Without shaking or rending the 

 earth, they succeed, in the end, in elevating vast 

 tracts of land above the sea, or in depressing them 

 beneath it. The raised beaches which form so 

 marked a feature in the scenery of many parts 

 of the coasts of the British Islands, are evident 

 proof of such a gradual upheaval The Scandi- 

 navian peninsula, except a part of its southern 

 end, is even now slowly rising ; in some districts, 

 at the rate of two or three feet in a century. But 

 the most marked elevation of a continuous kind is 

 that which is taking place along the western coasts 

 of South America. Evidences of the opposite 

 movement a gradual sinking of the land are 

 also abundant. The extreme south of Sweden is 

 undergoing sensible depression ; and submerged 

 forests on several parts of the British coasts point 

 to a like operation. The most striking proof, 

 however, of subsidence of the earth's crust is fur- 

 nished by the coral islands of the Indian and 

 Pacific Oceans. The characteristic form of the 

 coral island is a ring of white rock enclosing a 

 lagoon of still water, the walls of the ring rising 

 for the most part perpendicularly from immeasur- 

 able depths. Now the polypes that form these 

 rocks can only live at a moderate depth, and it 

 has been satisfactorily established that these ring- 

 islands had their beginning in fringing reefs of 

 coral such as are now seen off the shores of 

 existing land. Let us suppose that one of these 

 fringing reefs encircles some oceanic island, and 

 that the sea-floor in that region is being slowly 

 depressed. As the downward movement con- 

 tinues, the corals keep building up the reef to 

 about the level of the waves. In this way the 

 space of water between the island and the coral 

 ring is greatly increased, while, of course, the area 

 of the island itself is correspondingly lessened. 

 The downward movement continues the island 

 grows less and less, until its last mountain-top 

 sinks beneath the sea. Over the submerged 

 island there now stretches a smooth sheet of 

 green water known as a lagoon. Encircling it 

 is the circular reef of coral, or atoll, along the 

 outer margin of which the restless waves of the 

 ocean are ever surging. Soil gradually forms on 

 the reef, seeds borne to it by the waves or carried 

 by birds take root, and the ring of coral reef 



58 



becomes a habitable spot for man. Such is the 

 history of the growth of the coral islands. They 

 have been built round the summits of a sinking 

 continent, over whose mountains and valleys the 

 great ocean now rolls. 



What may be the nature of the force which 

 produces these upheavals and subsidences, those 

 quakings and eruptions, how it originated and 

 how it is kept up, are questions to which, as yet,. 

 no very definite answer can be given. It is clear, 

 indeed, that heat plays a large part in producing 

 these changes. The temperature of the earth is 

 found always to increase as we descend into the 

 crust ; and if the rate of increase which has been 

 observed were to continue without any modi- 

 fication, we should reach the melting-point of even 

 the most refractory substances at the depth of a 

 few miles. But, for reasons already given, the old 

 notion that there is still a central liquid part in 

 an incandescent state, with a thin crust over it 

 liable to be shaken and brokerj. through by the 

 commotions of the fluid interior, cannot now be 

 held. 



That the interior of the earth, however, whatever 

 be its composition, is intensely hot, is indicated 

 by all the evidence we can gather on the subject. 

 Some portions must be liquid, as is shewn by the 

 discharge of fluid lava at a white heat from 

 volcanic vents. There seems, indeed, to be good 

 reason to believe that though the main mass of 

 the interior may now be solid, there nevertheless 

 exist within it large lakes or reservoirs of melted 

 rock, and that volcanoes serve as the orifices of 

 communication between these areas and the sur- 

 face. When the water which is everywhere tra- 

 versing the upper layers of the crust reaches these 

 heated spaces, it is converted into steam, which 

 exerts an enormous expansive force. The abund- 

 ance with which steam is given off during vol- 

 canic eruptions has long been familiar, and serves 

 to indicate that steam may be the agent more 

 immediately employed in forcing melted lava to 

 the surface. During the changes which are in 

 progress underneath, a mass of water will some- 

 times be suddenly precipitated into an area of 

 intensely heated rock, and its instant expansion 

 will produce a sensible concussion or earthquake 

 above ground. 



The constant transference of materials from the 

 interior to the surface, whether by the action of 

 volcanoes or by that of springs, must necessarily 

 produce cavities within the crust. When, for 

 example, we contemplate such a mountain as 

 Etna, and reflect that all its vast piles of lava, 

 scoriae, and ashes have been abstracted from the 

 interior of the earth, we see how real and important 

 is this transference of material, and how easy it is 

 to conceive of the formation of large hollow spaces 

 beneath the surface. Again, the amount of solid 

 material removed by springs, though it does not 

 stand up before us as an enduring monument 

 like Etna, is probably in reality greater in any 

 one year over the whole globe, than all the lava 

 and ashes which have been erupted by volcanic 

 action during the same period. By some springs, 

 such as those of a thermal kind, the quantity 

 of these materials carried off in a single year 

 would, if collected and rendered visible, make 

 huge mounds or even small hills. Alike, there- 

 fore, by the action of volcanoes and the subter- 

 ranean circulation of water, cavities must be 



