Lavis — On the Structure of Rocks. 117 



hold, on account of Sorby's discoveries, that the fluid portion of 

 the earth's interior is an igneo-aqueous solution. We must first 

 prove that granite, or at least that studied by Sorby and others, 

 was not an intrusive rock in porous strata. In other words, it 

 must be proved that granite is the primitive rock cooled without 

 the intervention of secondary water. 



It, therefore, on being transferred from great to lesser pres- 

 sure, would only exert that small amount of expansion which is 

 proper to its chemical components, which would therefore undergo 

 no change of state, but remain as liquids under normal atmo- 

 spheric pressure at the earth's surface. In fact, whatever expan- 

 sion tended to take place in transferring the volcanic magma from 

 great depths to the surface would be more or less balanced by the 

 corresponding loss of heat, and consequent tendency to contract as 

 a result of that, so that only a change in volume would take place, 

 if any, in proportion to the different power of the two agencies to 

 accelerate or diminish contraction. This theory of the solution of 

 water in lava, and not lava in water, is incidentally mentioned by 

 the Eev. 0. Fisher. 1 



Extrusion of Igneous Matter through Dry, or nearly Dry, Rocks 

 to the Surface. — Should such volcanic magma in its native state 

 reach the surface, it might overflow without any explosive mani- 

 festations whatever, and consequently no cone of scoria or other 

 fragmentary materials would be formed around the exit, and the 

 locality of this would be only detected on a plain by the possible 

 formation of domes, or mamellons, where the lava was sufficiently 

 viscous. Neither should we expect such an exudation of fluid- 

 rock to be accompanied by mechanical vibrations other than that 

 dependent upon the formation of the fissure, or duct, by which the 

 lava escaped, and which formation would be dependent upon 

 causes extraneous to the actual expulsion of the fluid magma. 

 That such favourable conditions may sometimes occur, so that the 

 actual dyke may traverse strata that are not water-logged, we 

 cannot deny, and possibly some of the great basalt plains of 

 America and elsewhere may so have originated ; yet, geology 

 teaches us to consider such to be rather the exception than the 

 rule. 



1 Physics of the Earth's Crust, 1881, p. 190, 



