THE PROBLEM OF THE ANORTHOSITES 103 



the formation of a melanocratic fades in the lower part and a 

 leucocratic in the upper. Possibly there may really be formed at 

 the bottom (if there is a bottom) a sheet of pyroxenite corresponding 

 to the early stage of crystalHzation, when this mineral alone is 

 formed. Perhaps somewhere in the upper part or at some other 

 horizon a sheet of labradorite may arise. But that is not the process 

 advocated by Bo wen. 



Let us follow him. The crystallization begins with diopside, 

 and later but while diopside is still forming, labradorite begins to 

 crystallize. If before the crystallization of the feldspar two- 

 thirds of the diopside and all of the magnetite have already crystal- 

 lized, there will remain a leucocratic facies consisting of 80 per cent' 

 labradorite and 20 per cent diopside. But Bowen's scheme is 

 somewhat different. When the feldspar begins to crystallize, it is 

 not labradorite but the more basic bytownite, AbiAn4, that is formed. 

 The specific gravity of this feldspar, according to Bowen himself, 

 is very near to that of the liquid, so that these crystals would not 

 sink in the original liquid. They do sink because the liquid after 

 the crystallization of a certain amount of diopside, becomes of the 

 composition of a syenite, and gets lighter. Indeed, if after the crys- 

 tallization of two-thirds of the diopside, bytownite, AbiAn4, is 

 formed, presumably in such quantity that one-half of the anorthite 

 contained in the primordial magma has been spent, there will 

 remain a mixture of nearly 50 per cent albite, 26 per cent anorthite, 

 and 24 per cent diopside, or a syenite of the following composition : 



Per cent 



SiOz 62 



AI2O3 19.5 



MgO 2.6 



CaO 7.2 



Na.0 8.5 



99.8 



In this way, as we see, a soda syenite is formed, and in the later 

 stages of Bowen's scheme a soda granite may arise. It must be 

 noted that the syenites which are associated with anorthosites are 

 not soda syenites but ordinary potash syenites.^ Bytownite must 



^ The "mangerites" of Kolderup, which accompany the Norwegian labradorites and 

 stand near to the nordmarkites, contain nearlj^ equal quantities of soda and potash 

 (5.51 per cent NaaO and 6.57 per cent K^O). 



