iS94. VARIATION OF IGNEOUS ROCKS. 137 



Italian volcanic rocks, suggested that the original paste was acid in 

 composition, and that it became converted into basic rocks by the 

 fusion of the Apennine limestone which supplied it with lime, mag- 

 nesia, and iron. Herrick, Clarke, and Deming, although they do not 

 go so far as Ricciardi, believe that basic rocks may be made more 

 acid by the influence of the enveloping or " country rock," as they 

 call it. They make no definite statement as to whether this is by 

 fusion or chemical interchange. 



The feeble point of Ricciardi's hypothesis is that, however 

 magnesian or ferriferous be the Apennine limestone (for the numerous 

 analyses of which we are indebted to that author and Abich), the 

 amount of lime would be excessive in proportion to the other bases 

 taken up by the lava. 



For years I have been strongly impressed by the probability that, 

 after all, we may have only one fundamental paste. It is not unlikely, 

 admitting either a fluid shell between the earth's crust and nucleus, or 

 even large isolated portions of such a shell, that this, during long 

 ages since our globe consolidated, has been modified in composition 

 by the rounding off of projecting inequalities of the undersurface of 

 the crust or roof where thickenings and crumplings had taken place 

 while cooling. Not only might the primitive cooled fusion-crust be 

 reabsorbed, but the earlier sedimentary deposits might be likewise 

 consumed, so that anterior to any fissure-filling a large extent of 

 igneous paste would have changed in composition ; but in such condi- 

 tions I have little faith. Physicists will have it that our earth is practi- 

 cally a rigid mass ; that pressure raises the temperature of solidification ; 

 and that therefore igneous rock at any considerable distance from 

 the surface is solid, although at a very high temperature. In what way, 

 therefore, can the "focal diffusion" and " segregation " processes 

 have gone on ? So far we have no evidence of segregation in a 

 pure solid, but only in those solids wetted or permeated by a liquid. 

 One escape from this difficulty is suggested by the last remark — 

 namely, that perhaps this solidified mass of incandescent rock is com- 

 posed of quite different minerals to those which reached the surface, 

 undergoing liquefaction on the way ; and that some silicates remain 

 fluid and act as the diffusion agent. In fact, there is a complete 

 analogy to what occurs in a marl, limestone, or clay, when water serves 

 as the medium for the concretion of limestone, pyrite, flint, and other 

 nodules ; or for the more complete crystallisation of pyrites, selenite, 

 hauerite, etc. But in all such cases the segregation is diffuse ; by 

 which I mean that in a stratum of marl we do not find the lime at 

 one locality or part of the country and clay at another, but we 

 encounter the resulting concretions scattered through the clay. If 

 we imagine this to be suddenly fused, the resulting rock would have 

 the composition of the original mud, and supposing that all issued by 

 one vent, or cooled in place, we should not have a limestone rock 

 here and a clay there. 



