12 DEFINITIONS AND LAWS OF GEOLOGY. 



of water, haviDg an enormous expansive power which is given off as steam at the 

 moment of eruption. Lava is generally a sponge-like mass of myriads of visible ves- 

 icles formed by the sudden exclusion of the water-vapor in the act of solidification. 

 There is abundant evidence of the participation of water and its constituent gases 

 in volcanic phenomena. From the proximity of volcanoes to or occurrence in the 

 sea, it has been supposed their active state is produced by the percolation of sea- 

 water to metallic bases of the earths, or alkalies, at various depths, which bases be- 

 come inflamed and chemical action ensues, producing the eruption. The oxygen of 

 the water is supposed to unite with the metallic base, the hydrogen to unite with 

 sulphur, forming sulphureted hydrogen gas, and with the chlorine forming muriatic 

 acid gas, etc. The gases evolved from volcanoes are muriatic acid gas, sulphur 

 combined with oxygen or hydrogen, carbonic acid gas, nitrogen, and aqueous vapor. 

 Electricity is a factor in all earthquakes and volcanic eruptions. Its action is mani- 

 fest in the atmospheric disturbances, in the undulatory movement on the surface of 

 the earth, and in the speed with which the earthquake wave travels. An earthquake 

 moves in the direction of the wave at a rate frequently exceeding fifty miles in a 

 minute, and when the movement is communicated to the waters of the ocean, the 

 waves follow at a pace hundreds of times slower. Suppose a powerful current of 

 electricity near the surface of the earth, to be broken, and suddenly restored, 

 the shock may be supposed to resemble that of an earthquake. Fusion might 

 result in consequence of such restoration. The crystallization of stratified rocks 

 might break such electrical currents, if any exist in the earth, or it might disturb 

 the equanimity of the electricity if it exists in a passive state, to the same extent as 

 if it were a broken and restored current. In other words, subterranean electric cur- 

 rents, if once excited, may melt the rocks and produce the heat necessary, when as- 

 sisted by the presence of a sufficient quantity of water, to produce volcanic eruptions. 

 Such are some of the theories to account for the instigating or proximate causes of 

 earthquakes and volcanoes. 



The mouth of a volcano is called a crater, though the pit on Kilauea has been 

 called a caldera. If steam alone escapes through a vent, it is called afumarole ; but if 

 sulphurous vapors also escape, it is called a solfatara. When hot springs deposit lime, 

 it is called tufa; but if the deposit is silicious, it is called sinter or geyserite. Lava 

 consists of silica, alumina, lime, magnesia, soda, potash, and iron oxide. If the 

 silica is in excess, it is tracliyte, and belongs to what lithologists call the acidic group, 

 from the large quantity of silicic acid it contains ; but if there is a large proportion 

 of soda or potash and lime or magnesia, and not more than 50 per cent of silica, it 

 is a basalt, and belongs to the basic group, from the larger quantity of alkaline and 

 earthy bases it contains. Trachyte is a grayish igneous rock, of rough fracture 

 owing to the grains of glassy feldspar which mainly constitute it. Basalt may be 

 light -colored crystalline or granitoid, or dark colored, compact, massive, like dolerite ; 

 but in addition to labradorite and pyroxene, it contains chrysolite in disseminated 

 grains. When lava becomes glassy, it is called obsidian. 



§ 18. The most important change taking place upon the earth is in constant 

 operation at the bed of the ocean. Near the shore it is a littoral deposit; farther 

 away it is a chalky deposit, consisting of foraminifera and shells, and in deeper water 

 it is a red, silicious clay. The character of the deposit is dependent upon the depth 

 of the ocean, except where washings from land affect it. The depth of the pure 



