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THE JOURNAL OF GEOLOGY.

The ability of a rock magma to change in chemical composition 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 chemical 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 century, 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

  1. ↑ G. P. Scrope: Volcanos, 8vo, London, 1825.
  2. ↑ Charles Darwin: Volcanic Islands, 8vo, London, 1844.
  3. ↑ Loc. cit.
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