242 N.. L. BOW EN 



stantially where formed. If liquid they could not fail to be repre- 

 sented very prominently in all anorthosite terranes, for the liquid 

 would be freely intruded into overlying rocks at every disturbance 

 experienced by them. 



SUMMARY 



Anorthosites are made up almost exclusively of the single 

 mineral plagioclase, and in virtue of this fact they present a very 

 special problem in petrogenesis. The conception of the mutal 

 solution of minerals in the magma and the lowering of melting 

 temperature consequent thereon is no longer applicable. Yet 

 anorthosites give no evidence of being abnormal in the matter of 

 the temperature to which they have been raised, in other words, 

 they give no evidence of having been raised to the temperature 

 requisite to melt plagioclase. A possible alternative is that they 

 may never have been molten as such, and are formed simply by 

 the collection of crystals from a complex melt, probably gabbroic 

 magma. This possibility is in harmony with the expectations that 

 grow out of experimental studies and for this reason a consideration 

 of the likelihood that anorthosites have originated in the stated 

 manner becomes imperative. 



A consideration of the method whereby accumulation of plagio- 

 clase crystals might take place leads to the conclusion that the most 

 promising is the separation by gravity of the femic constituents 

 from gabbroid magma, while the plagioclase crystals, which are 

 basic bytownite, remain practically suspended. Then, at a later 

 stage, when the liquid has become distinctly lighter, having attained 

 diorite-syenite composition, the plagioclase crystals, which are now 

 labradorite, accumulate by sinking and give masses of anorthosite, 

 at the same time leaving the liquid out of which they settle of a 

 syenitic or granitic composition. 



Some of the consequences of this manner of origin of anortho- 

 site are as follows. Typical anorthosite, very poor in bisilicates, 

 should not occur as small dikes, for a mass of accumulated crystals 

 should have little invading power. A proportion of about 15 or 20 

 per cent bisilicates or other foreign material such as orthoclase and 

 quartz should be necessary for the formation of small dikes. 

 Typical anorthosite should for like reasons not occur as an effusive 



