86 ALFRED C. LANE 



less in combination with other elements commonly occurring 

 in rocks, and yet that the eutectic ratio of alkali :silica :: 1:12 

 would hold true. Possibly it must be in combination with H^O 

 or some other oxide. 



The first element, of course, to be considered would be the 

 lime. What composition of albite and anorthite would g'ive the 

 same eutectic ratio? A little simple algebra proves that it is the 

 commonest labradorite (Ab^ AUg). Now, this feldspar is, as we 

 all know, one of the commonest in rocks, and more easily fusible 

 than either albite' or anorthite, so that it is strictly eutectic. In 

 other words, the diagram seems to show that there is probably a 

 eutectic series from a micropegmatite with quartz about equal to 

 albite or orthoclase down to labradorite. If this theory is true, 

 then there would be a tendency in rocks where the alkali-silica 

 ratio was less than 1:12 to have the silica crystallize out first in a 

 porphyritic way as quartz; and, on the other hand, if the alkali 

 ratio were a little greater, there might be a tendency to have the 

 feldspar crystallize out early, so as to bring the residual magma 

 down to the eutectic ratio. A large excess of alkali would bring 

 it near another eutectic. Corresponding to these would be a 

 more or less quartzose marginal zone where the crystallization 

 first began. That this is liable to be modified more or less by 

 irregular changes in temperature, pressure, and environment, I 

 need hardly add. 



But what about the other elements, in particular the iron and 

 magnesia? If silica were there in excess of the eutectic ratio, 

 they would combine with it very easily and, such compounds 

 being relatively insoluble in the magma, crystallize and separate 

 out, in sharp form embedded in the eutectic compound. In har- 

 mony with this, we find some of the few analyses which are 

 markedly below the eutectic ratio containing considerable quan- 

 tities of lime, magnesia, and iron. They may be aggregates of 

 these earlier eliminations of the magma — divergent splits. Some 

 of them are such, I judge from the description. Moreover, the 

 minerals formed in such magmas must have high silica ratios, be 

 augite or enstatite rather than olivine. 



'According to the latest researches of Doelter and others this is not so — for albite 

 is 10° to 15° more fusible. 



