Feb. 1894. VARIATION OF IGNEOUS ROCKS. 135 



With volcanic rocks the question is somewhat different. The 

 fundamental paste of these is composed almost entirely of oxides — 

 some basic, others acid, but the complete intersolution or miscibility 

 of which, in a fused state, has never been disproved, nor does it seem 

 likely to be, as they all have a common tie in oxygen. When they 

 cool, then other conditions are introduced, which do not concern us 

 at present. Soret's principle will, no doubt, be urged against this 

 statement ; but in that case we have to deal with experiments on 

 bodies much less nearly related, and under conditions of far less 

 viscosity than those of an igneous paste, and with extremes of 

 temperature far different to those likely to occur in masses of the 

 enormous dimensions of the earth. Neither must we neglect the 

 influence of tidal action, convection currents, and many other 

 mechanical disturbances, such as shearing between the different 

 fluid shells in the direction of the earth's rotation, supposing such con- 

 tinuous fluid shells to exist within our earth at present. In the case of 

 localised reservoirs, there would be the absorption of water, vesicula- 

 tion in the paste, curreats sent up by the injection of earth fissures, or 

 eruption of some of the material on the earth's surface, and other 

 mechanical disturbances. 



The oxides fused together in the manufacture of glass, which 

 vary still more in their relative specific gravities than those which 

 constitute igneous rocks, are not known to separate in the glass pots, 

 however long they are left quiet. It may be objected that the depth, 

 time, and even, perhaps, temperature are hardly comparable in the 

 two cases, but we must not forget that even the most basic glasses 

 are markedly viscous at high temperatures. These views may con- 

 veniently be styled the " differential paste and alloy hypothesis." 



Mr. C. E. Button^ says, "We know of no natural processes 

 capable of separating the more acid parts of such a magma except 

 the chemistry of the atmosphere acting at temperatures far below 

 the melting-points of the silicates." He urges the following 

 chemical argument against such a hypothesis : — The separation of 

 a magma into two or more degrees of acidity is disproved by the low 

 percentage of silica in basalt not being confined to the felspar and 

 augite, but being also in the base, while the high percentage in rhyolite 

 is in the felspar, and still more in the base. Hence the segregation 

 must have affected the base more than the crystals. 



The only way out of this difficulty is to suppose crystal segrega- 

 tion and refusion without subsequent mixing. I do not deny the 

 possibility of such a process occurring occasionally ; but to imagine 

 that under every volcanic region, and under a large number of 

 individual vents, there is an apparatus as complex as a chemical 

 factory or an iron foundry, can only be relegated to those fantastic 

 theories of volcanic magic that pass for science. 



1 "Geology of the High Plateaus of Utah," 4to, Washington, i8So, p. 124. 



