LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 39 



ture of final consolidation continually lowered. This latter effect 

 is impossible in any eutectic system. Moreover, the temperature 

 of final consolidation of the lower layers is continually raised. 



The effect of rate of cooling summarized. — When the liquid is very 

 quickly cooled, it crystallizes quickly, if at all, and with Httle or no 

 tendency to an offsetting in the composition of the liquid. If it is 

 cooled moderately slowly, zoning of the plagioclase causes a con- 

 tinual enrichment of the residual (interstitial) liquid in albite. 

 If it is cooled still more slowly, sinking of plagioclase causes a similar 

 continual enrichment of the residual (upper) liquid in albite. In 

 favorable cases the final liquid may be more than go per cent albite 

 even although the original mixture were, say, 50 per cent diopside 

 and 50 per cent bytownite. 



CONCLUSIONS TO BE DRAWN FROM THE INVESTIGATED TERNARY SYSTEMS 



If we now combine the information furnished by the investigated 

 systems, important conclusions may be drawn with regard to the 

 crystallization of basaltic magma under various conditions. Instead 

 of the simple pyroxene diopside, present in the mixtures of the last 

 system discussed, we may consider one of the intermediate 

 pyroxenes, which melt with decomposition and the formation of 

 olivine, to be present in addition to plagioclase. Rapid cooling of 

 such a liquid would give merely plagioclase and pyroxene. On the 

 other hand, slow cooling permits radical variation from this simple 

 result. The early formation of oKvine brings about an excess of 

 free siKca in the residual liquid if any process intervenes to prevent 

 the resorption of the olivine by the liquid. The early formation of 

 very calcic plagioclase brings about an enrichment of the liquid in 

 albite if anything intervenes to prevent the continual alteration in 

 the composition of the crystals by interchange with the liquid. 

 Finally the early formation of magnesia-rich pyroxene brings about 

 an enrichment of the liquid in diopsidic pyroxene if similar condi- 

 tions intervene. 



The sinking of crystals affords a means of continually separating 

 crystals from the part of the Liquid in which they formed and is there- 

 fore a process which wiU give the results just outHned. If, there- 

 fore, the mixture of plagioclase and pyroxene referred to were cooled 



