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Extract from beginning of chapter:

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While at work on the introductory portion of [Igneous Rocks II], I was led to publish under the guise of a paper on “Problems in Petrology” (Iddings, 1911), read before the American Philosophical Society in April 1911, a reply to some criticisms of the quantitative system of classification of igneous rocks. Harker had made these criticisms in his book on The Natural History of Igneous Rocks, chiefly regarding [the system's] artificial character as contrasted with the idea of a “natural” system, cherished by some petrographers.

The argument was based on the existence of definite quantitative relations between mineral composition and texture on the one hand, and chemical composition of magmas and physical conditions attending their eruption and solidification on the other. These relations rest on the obedience of the component elements to the laws of physical chemistry, and must be capable eventually of definite mathematical expression, although the relationships and the number of variable factors may place the problem beyond the range of exact mathematical formulae.

Rock magmas being merely differently mixed solutions of inorganic compounds or salts, there are no inherent or inherited characteristics of form, organism, or immaterial traits, as in living beings. Petrographers, by using such terms as consanguinity, 1 parent magmas, families of rocks, and minerals of first and second generation, have conveyed the idea that there exist among igneous rocks genetic relations analogous to those sustained by living organisms.

In fact, Harker (1909, p. 362) has stated that the mutual relationships of igneous rocks will

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