Composition of Igneous Rocks. 413 



solutions plays a more prominent part in the production of 

 variation in igneous rocks than the tendency to heterogeneity 

 as emphasized in the case of Soret's principle. Suppose a 

 magma of homogeneous composition to become cooled along 

 the border so that some of the constituents begin to crystallize 

 out on the wall rock. The first to separate would be the 

 minerals characteristic of basic borders. The deposition of 

 these minerals from the magma would introduce heterogeneity 

 into the magma. Diffusion would tend to restore homogeneity, 

 thus causing a movement toward the border of the substance 

 necessary for the continued formation of the minerals which 

 had separated. The result is the same as that attained by 

 Soret's principle, but the causes are quite different — the one is 

 the tendency to homogeneity, the other to heterogeneity. 

 Harker's view is based chiefly on the fact that the basic borders 

 are composed of the minerals which are among the first to 

 crystallize from molten magmas. Such early crystallizing 

 minerals are — olivine, various pyroxenes, biotite, basic plagio- 

 clases, iron oxides and sulphides. On the other hand acid 

 plagioclase, orthoclase, quartz and muscovite, which characterize 

 more acid rocks, are generally absent from the basic borders. 

 The basic minerals crystallize along the border just as a crystal 

 grows when suspended in a saturated salt solution — by appro- 

 priating the salt within its " court" (Hof), which in turn is 

 constantly being replenished by diffusion (the tendency to 

 uniformity) from the more distant parts of the solution. 



Some homogeneous salt solutions, if allowed to remain at 

 constant temperature for a long time, become gradually more 

 concentrated in the lower strata.* It is very probable that 

 similar concentration occurs in complex silicate magmas, par 

 ticularly near the temperature of solidification. An eruptive 

 magma would therefore tend to become acid above and basic 

 below. Since the margin becomes cooled comparatively 

 quickly, the rock which solidifies along the border will have 

 had very little time to become differentiated and will conse- 

 quently possess nearly the same composition as the original 

 homogeneous magma. Differentiation and solidification pro- 

 gress simultaneously, and hence the rocks farthest from the 

 border will have suffered the greatest differentiation and will 

 vary most in chemical composition. In the upper horizons of 

 the eruptive there would be a gradual increase of acidity 

 toward the center; the middle horizons would show very little 

 differentiation while sections through the lower portions of the 

 reservoir would show an increased basicity toward the center. 

 All of these cases are well known in field explorations. 



* Gouy and Chaperon, Ann. chim. phvs., 1887, p. 387. 



