212 N. L. BOW EN 



melting component of the mix-crystal series. As the magma cools, 

 especially if it cools very slowly, these crystals continually change 

 in composition as a result of interchange of material between liquid 

 and crystals, the change being always in the direction of enrich- 

 ment in the lower-melting component. But this change is fully 

 accomplished only when adequate liquid is available. If the 

 crystals are heavy and accumulate toward the bottom, the small 

 amount of liquid there available cannot continue indefinitely to 

 enrich the crystals in the lower-melting component. They there- 

 fore remain very rich in the higher-melting component, increasingly 

 so the greater the preliminary accumulation of crystals. Vogt's 

 important statistical study of the anchi-monomineralic rocks shows 

 quite definitely that in the case of peridotites the ratio of Mg 2 Si0 4 

 to Fe 2 Si0 4 in the olivine increases directly with the proportion of 

 olivine in the rock. 1 The orthorhombic pyroxenes apparently 

 follow a parallel law. 



This is, then, precisely as deduced above for rocks formed by the 

 accumulation of heavy early crystals. If plagioclase were a very 

 heavy or a very light mineral, we should find a similar relation to 

 hold for it, namely, the greater the proportion of plagioclase in a 

 rock, the greater would be the proportion of anorthite in the plagio- 

 clase. But when we turn to Vogt's similar study of anorthosites, 

 we find in them a quite different tendency. The richer a rock is in 

 plagioclase, the greater the tendency for the plagioclase to be, not 

 a very calcic one, but the intermediate one, labradorite. 2 This 

 character of the plagioclase is, it is believed, directly connected with 

 the fact that the plagioclase being precipitated from gabbroic 

 magma sensibly matches the magma in density. It is perhaps 

 slightly lighter than the magma, but usually not sufficiently so to 

 cause it to accumulate locally and to form masses of crystals much 

 enriched in the higher-melting component as do olivine and 

 pyroxene. Instead, it remains practically suspended in the liquid, 

 with probably a very slight tendency to rise at first, and the whole 

 of the liquid is available for the production of the change of com- 



1 J. H. L. Vogt, "Uber anchi-monomineralische und anchi-eutektische Eruptiv- 

 gesteine," Vid. Selsk. Skr. I, No. 10 (1908), pp. 24-25. 



2 Vogt, op. cit., p. 41. 



\ 



