ENDLiCH.] ORIGIN OF ERUPTED ROCKS. 265 



have been advanced. They have been regarded as portions that were, 

 by some means, separated from the central fluid or plastic mass. Pro- 

 cesses of chemical decomposition and formation of new compounds have 

 been quoted as causing a sufficiently high degree of temperature to pro- 

 duce viscidity of the rocks. Pressure unequally distributed has been 

 adduced as a factor capable of producing the same result. 



So far as my investigations have led me, I have arrived at the con- 

 clusion that the formation of reservoirs containing liquefied rocks or, 

 using the general term, lava is the result of primarily irregularly dis- 

 tributed pressure and chemical action. In order to present this view 

 more clearly, I shall present a synopsis of the methods employed in pro- 

 ducing the obvious result of volcanic eruption. First, I shall endeavor 

 to point out those facts connected with pressure, and then the character 

 of the chemical changes which the rocks undergo. 



Charles Babbage conceived, in 1834,* that, in consequence of the 

 changes going on at the earth's surface, changes produced by the re- 

 moval and redeposition of portions of the solid rocks, the surfaces of 

 "equal temperature" within its crust must be continually changing. As 

 the result of such changes he concludes that "rents may be formed, 

 mountain-chains raised, and even continents elevated." Eecognizing 

 very clearly the direct influence of such a change of equilibrium, Bab- 

 bage has probably overestimated the extent of the results which might 

 be produced. Two years later Sir John Herschell expressed similar 

 views more fully in a letter to Charles Lyell.t He says: "Supposing 

 the whole (earWs crust) to float on a sea of lava, the efl'ect (of accumw 

 lated sediment) would merely be an almost infinitely minute flexure of 

 the strata; but supposing the layer next below the crust to be partly 

 solid and partly fluid, and composed of a mixture of fixed rock, liquid 

 lava, and other masses in various degrees of viscidity and mobility, 

 great inequalities may subsist in the distribution of pressure, and the 

 consequence may be local disruption of the crust where weakest, and 

 escape to the surface of lava." Essentially the same idea as expressed 

 by Babbage, the view of Herschell presumes conditions which may par- 

 tially be granted, and the results he deduces therefrom are in propor- 

 tion to the original force employed. He states further that in case the 

 process of sedimentation be continued to that point until "some support 

 gives way," a portion of the solid crust breaks down, thus forming one 

 or more venl^ for the ejection of liquid lava. Into the opening thus 

 formed the liquid will rise by simple hydrostatic pressure. With in- 

 creasing height of the column or sheet the pressure diminishes until a 

 point is reached where the "ignited water" can become steam. Then 

 the joint specific gravity suddenly diminishes, and there is violently 

 forced upward a mixture of lava and steam. 



In this way Herschell explains the production of volcanic eruptions as 

 directly the result of pressure caused by accumulations of sediment. 

 One of the main conditions essential to this explanation is the breaking 

 of " some support." Though undeniably this may be supposed to occur, 

 it is an occurrence that would probably result in a violent, short erup- 

 tion, and not in one continuing for ages. Acceptable as this expla- 

 nation of the origin of volcanic eruptions is, it has a bearir)g most di- 

 rectly upon a short, destructive series of ejections, rather than a long, 

 continued one. 



T. Slerry Hunt has, for a number of years, paid much attention to 

 thisquestion, and has furnished many valuable contributions thereupon : 



*Proc. Geol. Soc. of London, 1834, vol. ii, p. 75. 

 t Proc. Geol. Soc. of London, 1836, vol. ii, p. 548. 



