392 



and fusion of normal sedimentaries (sandstones, clays, limestones, 

 dolomites, salt-beds, &c.), appears to the present writer highly 

 improbable. The effect of denudation and deposition is to separate 

 the chemical constituents of igneous rocks and banded gneisses into 

 portions which are soluble and others which are insoluble under 

 ordinary surface conditions. The alkalies, lime, magnesia and iron 

 largely pass into solution. The most important insoluble constituents 

 formed in connection with the breaking up of the crystalline rocks 

 are kaolin and quartz. Of the soluble constituents iron is readily 

 precipitated as ferric oxide wherever oxygen obtains access to the 

 solutions. It may also be separated as carbonate under those peculiar 

 and little-known conditions which result in the conversion of beds of 

 limestone into ironstone. Magnesia may assume the solid form again 

 by the obscure process generally known as " dolomitization." Organic 

 life restores to the solid crust of the earth the lime which is removed 

 in solution. The alkalies are, however, restored to the land under very 

 local and exceptional conditions, and it is doubtful whether the total 

 alkalies present in beds of rock-salt, carnallite, &c., are com- 

 parable in amount with the alkalies which have been removed in 

 solution during geological time. To produce, therefore, an igneous 

 rock by the melting of sedimentary rocks, it would in general be 

 necessary to collect together samples of the necessary ingredients from 

 different portions of the earth's crust. The local melting of sedimentary 

 rocks could only produce normal igneous products under exceptional 

 circumstances. 



Another argument against any speculation which would connect 

 the igneous and banded crystalline rocks on the one hand, with the 

 normal sedimentary rocks on the other, in any kind of cycle, may be 

 drawn from the fact that the latter appear to be insignificant in bulk 

 when compared with the former. If the sedimentary rocks were totally 

 removed from the planet its importance as a member of the solar 

 system would not be appreciably affected. 



Admitting, then, that we are profoundly ignorant as to the precise 

 conditions under which molten masses originate, let us consider certain 

 facts which throw some light on the phenomena accompanying 

 consolidation. 



Any homogeneous liquid mixture containing two or more definite 

 chemical compounds may be termed a solution, no matter at what 

 temperature the mixture may commence to solidify. It is customary 

 in the case of solutions to regard one of the constituents as the solvent 

 medium. Thus, in the case of a solution containing salt and water, at 

 ordinary temperatures the salt is said to be dissolved in the water. This 

 distinction between " the dissolver " and " the dissolved," though very 

 apparent in such a case as that which has just been mentioned, where 

 the phenomena of solution are studied at temperatures considerably 

 below the fusing point of one of the constituents and above the fusing 

 point of the other, become less marked when studied at temperatures 



