46 JOURNAL OF GEOLOGY— SUPPLEMENT 



and NaAlSijOg, however small, there is a certain corresponding 

 concentration of KAlSi04, NaAlSi04, and SiOa. During the slow 

 crystallization of the basaltic magma, with the continual increase 

 in the concentration of KAlSisOs, NaAlSijOg, and the promoting 

 agent water, there is a corresponding increase in the concentrations 

 of KAlSi04, NaAlSi04, SiOa, and others, until finally some of these 

 exceed their saturation limit. SiOz then separates as quartz; 

 KA.lSi04 with HAlSi04, certain complex ferromagnesian molecules, 

 and a limited amount of NaAlSi04 separate as a solid solution 

 making up the mineral biotite. The molecules which separate are 

 not necessarily the most concentrated; certain others may be much 

 more concentrated, but correspondingly more soluble. Neither is 

 it necessary that the molecules separate in the stoichiometric pro- 

 portions represented by the reactions given. They are formed in 

 the liquid in these proportions, but the extent of their separation 

 from the liquid is determined by their solubility relations. The 

 relative amounts of quartz and biotite contained in a given rock 

 have, therefore, no necessary relation to the proportions of the 

 various molecules indicated in the foregoing reactions. Indeed, 

 quartz, the one product of these reactions, usually is greatly in 

 excess of biotite, a grouping of several of the other products. This 

 means that there is a relative storing up in the liquid of the mole- 

 cules, other than SiOz, which result from these reactions. The 

 molecules stored up are, then, principally the aluminous ortho- 

 silicates of the alkahs, a fact of great significance in its bearing on 

 the origin of nephelite syenites. 



THE CRYSTALLIZATION OF BASALTIC MAGMA 



Let us now review briefly the crystallization of a magma of 

 basaltic composition. If the magma is cooled very rapidly, it 

 crystalHzes rapidly, giving simply a diabase. If there is rather 

 slower cooHng, there may be a Umited amount of sinking of crystals 

 of the early-formed pyroxene and more calcic plagioclase, and a 

 consequent enrichment of the uppermost hquid in alkaline feldspar, 

 giving a diorite as a fight, upper differentiate. If the cooling is still 

 slower, there is more prolonged opportunity for the sinking of crys- 

 tals and the liquid may be sufficiently enriched in alkahne feldspar 



