LESSONS IN GEOLOGY. 



241 



LESSONS IN GEOLOGY. VI. 



VOLCANOES-IGNEOUS AGENCY. 



THB universal action of water in to level. If the world continue 

 for tln< requisite number of ages, and if no other cause interfere 

 jvith tin- di'tfrading and filling up whidi is carried on by 

 riiii-ilrii>, rivur, or ocean current, in duo time our 

 world will be a level plain. This, however, can never 

 be the case, for there exists a force which constantly 

 opposes the action of water. The Creator " hath not the 

 one over against the other," and hero, as in every domain 

 of Nature, is a finely adjusted balance the aqueous 

 agency on the one hand, and the igneous agency on the 

 other ; the one wearing down, the other elevating; the 

 OIK* tilling up and making the surface even, the other dis- 

 rupting and throwing the existing arrangement into dis- 

 order. 



The action of heat is exerted in three ways (1) in 

 volcanoes, (2) earthquakes, and (3) the gradual upheaving 

 or subsiding of portions of the earth's crust. We are 

 not able to trace the action of heat so certainly as that 

 of water, for the simple reason that wo can watch the 

 one from first to last ; whereas we can only study the 

 effects of thermal action, and thence attempt to divine 

 the causes. 



Wherever there is room for speculation, no lack of 

 speculators has been found ; and upon almost every 

 question upon which a doubt could exist and there are 

 many in this part of the subject the geological world has 

 been divided. We shall notice the prominent points of 

 the various controversies as we discuss the subjects as 

 they present themselves. 



1. Volcanoes. We shall find that rocks which owe ^ 



their origin to igneous action may be safely divided 

 into plutonic and volcanic rocks. In the former class 8 



are arranged all rocks of the primary formations, such 

 as granites, etc., which are the offspring of a much more general 

 And widely extended action than those igneous rocks which date 

 from a much later period. These rocks, which ore found within 

 limited ranges, have been ejected from a crater, that is, a hole 

 is formed from the surface to the bowels of the earth beneath, 

 .up which is thrown molten 

 lava and ashes, or scoria:. 

 This matter is cast up and 

 falls around the opening, 

 forming a hillock, the top of 

 which is a cup-like forma- 

 tion, at whose base is the 

 mouth of the volcano. A 

 .mountain thus entirely 

 formed of material ejected 

 from the centre of action be- 

 .neath, is called a crater of 

 .eruption. However, another 

 method of construction of a 

 volcanic mountain can be 

 imagined, where some violent 

 upheaving force lifts up the 

 surface into a bubble-like 

 mound ; the apex, being natu- 

 xally the weakest part, gives 

 way, and from the opening 

 thus formed the internal fires 

 belch forth, and the eruption 

 proceeds, the walls of the 

 bubble falling down from the 

 mountain. A volcano thus 

 produced is styled a crater of 

 elevation. These two theories 

 .have been energetically sup- 

 ported by eminent geologists. 

 Scrope, Provost, and Lyell are 



adherents of the eruption theory, whilst Humboldt, Elie do Beau- 

 mont, Dufrenoy, and others, maintain the theory of elevation, 

 We shall find that there is some truth in each theory, but that 

 no volcano owes its origin entirely to elevation or eruption, both 

 processes having taken port in the erection of the mountain. 

 It is supposed that all the volcanoes in the world are members 



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of four groups. The Andes, which edge the western shore of 

 the American continent, present in their chain many active, and 

 numerous extinct, burning mountain!. Thin is the beet <faflyi 

 volcanic group. A second in recognised as compruing the 

 Aleutian Islands, Japan, the Moluccas, and the volcanoes of the 

 East Indies. The third group includes those of the 

 Pacific Archipelago ; while the fourth is represented M 

 stretching from Central Asia to the Azores, and include* 

 the volcanoes of the Greek islands, and those of Southern 

 Italy. 



A remarkable fact will at once strike the observant 

 reader, namely, that all volcanoes are in the immediate 

 neighbourhood of the sea. 



There is no exception to this rule. In Central Asia 

 there is said to be a volcano on the north declivity of 

 the Thian-Shan Mountains, but its last eruption is referred 

 to the seventh century. There are not wanting evidence* 

 that in that part of Asia large bodies of water existed, 

 and even now the largo lake Issik-Kul is in the neigh* 

 bourhood of the quiescent mountain. This apparent ne- 

 cessity of being m immediate proximity to water, caused 

 Bischoff to propound a theory, which is generally ac- 

 cepted: that the immediate cause of volcanic action is 

 duo to the fact that water percolates through fissures in 

 the surface down to the hot regions beneath; here it is 

 mode steam at a high pressure, and thus forces for itself 

 a passage through the superincumbent crust, urging with 

 it molten matter, and the debris caused in the upheaval 

 of the strata, to produce a vent. Whether water be a 

 requisite to volcanic action or not, one thing is certain, 

 that no volcano can exist without heat. 



Many geologists hold that the centre of the earth is 

 still composed of molten matter, and that the lines traced 

 by the volcano-chains mark the direction of vast fissures 

 in the earth's crust up which, by the pressure of steam, the 

 lava is forced. A grave objection at once appears to the 

 acceptance of this assumption. If this bo the case, according 

 to the laws of hydrostatics, the pressure exerted at one point 

 by the expanding steam must be felt by the whole liquid mass, 

 for liquids transmit pressures equally in all directions ; hence 

 the same force which throws into action one volcano, must also 



cause all the neighbouring 

 cones to erupt. This argu- 

 ment finds a remarkable ex- 

 ample in one of the Sand- 

 wich Islands. Manna Loa is 

 a volcano frequently active ; 

 there is a crater near its sum- 

 mit, 10,000 feet above the 

 level of the ocean ; 6,000 feet 

 up one flank is another crater, 

 Kilauea. It often happens, 

 that while Loa is in action, 

 the lava in Kilauea is molten, 

 yet undisturbed. It seems 

 an inevitable conclusion, that 

 if these craters both derive 

 their lava from the same re- 

 servoir, the force which pro- 

 pels the molten matter to the 

 higher crater must cause a 

 jet of lava to be thrown from 

 Kilauea to a similar height. 



That simultaneous dis- 

 turbances would take place 

 in each crater, if their ducts 

 led to the same reservoir, 

 may be fairly inferred from 

 the fact, that we have nu- 

 merous accounts of volcanic 

 action occurring at the same 

 moment at many distant 

 points: for example, a severe earthquake visited Chili in 1835; 

 at the same moment the shock was felt over a wide area ; the 

 two volcanoes, Yantales and Osorno, burst into action ; and at 

 Juan Fernandez, 720 miles distant, a submarine eruption took 

 place. Thus the commotion in some deep-seated reservoir 

 affected a tract of country 900 mile Ictjr rnd 600 broad. 



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