DIFFERENTIATION PHENOMENA OF THE PROSPECT INTRUSION. 1 L7 



usually understood, but rather of gravitational differenti- 

 ation in the original uncombined (or partly combined) 

 constituents of the earth's "photosphere" — to use the 

 solar analogy — followed by partial mixture and chemical 

 reaction caused by crust movements. According to this 

 theory certain gneisses of mountain ranges would be some 

 of the first consolidated parts of the earth's crust; and 

 siliceous magmas (as distinct from mixtures of liquid metals) 

 would be produced by earth movements causing silicification 

 of the metallic substratum, and by mixture subsequently 

 from time to time, though naturally most abundantly in 

 the earlier part of the earth's history. It should be noted 

 that there is no reason, on this hypothesis, why any great 

 body of magma forced into a new position in the earth's 

 crust should not be of varying composition in different 

 parts ab initio. 



Differentiation I regard as a generic name for a number 

 of very different processes by which all the various original 

 siliceous magmas are subject to a further change, some- 

 times slight, sometimes radical in character. The origin 

 of any particular rock may, then, have been simple, or it 

 may have been complex. Some rocks have probably con- 

 solidated from an original magma, little changed since 

 oxidation and silicification were completed; others may 

 have been produced from a magma which was differentiated 

 several times whilst completely liquid, and perhaps altered 

 by assimilation, a final differentiation taking place during 

 crystallisation. It is thus possible that rocks virtually the 

 same in composition may have arisen in many different 

 ways, some by very round-about processes of differentiation 

 and mixture. The only test of derivation from a common 

 magma, or at least, from magmas some parts of which had 

 a common origin sometime in their history, is consanguineity 

 — the presence throughout the series of rocks of some 



