836 THE JOURNAL OF GEOLOGY. 



The ability of a rock magma to change in chemical composi- 

 tion in different parts, so as to crystallize into different mineral 

 combinations which correspond to mineralogically diverse rocks, 

 does not appear to be limited to small volumes of magma, but 

 shows itself on quite different scales; sometimes confined to a 

 narrow dike, at others acting throughout a large mass thousands 

 of feet in diameter. That which is seen to have taken place 

 within a comparatively limited volume of molten magma might 

 be reasonably assumed to be possible within much greater 

 volumes. Nevertheless it does not necessarily follow that it has 

 done so ; conditions which may have brought about the change 

 in one case may not exist in another. 



The probability that such changes have taken place in great 

 reservoirs of molten magma, and have brought about the chemi- 

 cal and mineralogical differences among igneous rocks, finds its 

 support in other evident relationships than those of facies and the 

 gradual transitions in mineral composition between the kinds of 

 rocks. The nature of this evidence is twofold and consists, first, 

 in the existence of associations of various kinds of igneous rocks 

 in volcanic regions ; and second, in chemical and mineralogical 

 diversity between different associations of rocks, that is, between 

 groups of rocks belonging to different regions. The association 

 of various kinds of rocks in particular volcanic districts, and 

 their constant recurrence in company with one another in widely 

 distant parts of the world impressed itself upon the minds of 

 Scrope, 1 Darwin 2 and Dana 3 in the first half of the present cen- 

 tury, and led them to the opinion that the various kinds of lavas 

 thus associated must have originated from some common source, 

 that is, from a common molten magma, by some process of 

 separation or differentiation. 



Subsequently, as the chemical and mineralogical constitution 

 of rocks became more readily determinable, it was discovered 

 that there were chemical and mineralogical characteristics of 



Z G. P. Scrope : Volcanos, 8vo, London, 1825. 



2 Charles Darwin: Volcanic Islands, 8vo, London, 1844. 



3Loc. cit. 



