Alnoitic Rocks at Isle Cadieux, Que. 15 



moreover, a decidedly appropriate one in this particular 

 province; indeed nephelite is frequently an important 

 groundmass constituent in some of the alnoites of 

 Harvie. 



Petrologists will have no difficulty in accepting the 

 formation of biotite from other femic minerals (in this 

 case principally chrysolite) because it is an action with 

 which they are already acquainted. The formation of 

 monticellite from other femic materials (in this case prin- 

 cipally augite) has not been noted elsewhere. Assuming 

 that it is desirable to describe the action in a few words, 

 this may be done roughly by stating that the monticellite 

 is formed as a result of desilication of augite by the 

 alkalic liquid. We thus find an explanation of the fact 

 that where no augite was present in the original material 

 no monticellite was formed. 



The liquid that accomplished this action was in all 

 probability a nephelite-rich liquid closely related to that 

 which formed the nephelite aplites of this province, and 

 in order to throw some light on equilibrium in such liquids 

 the results of experimental study of related liquids will 

 now be presented. 



Experimental Studies of Belated Mixtures. 



The system CaO-MgO-Si0 2 has been investigated by 

 Ferguson and Merwin and their results are expressed 

 in the equilibrum diagram, fig. 1. A liquid made up of 

 50 per cent forsterite and 50 per cent akermanite begins 

 to crystallize with deposition of the olivine, forsterite, 

 an end member of the chrysolites ; when the temperature 

 falls to 1450° monticellite begins to crystallize; and at 

 1436° akermanite, an end member of the melilites, begins 

 to crystallize and monticellite redissolves until finally 

 monticellite is entirely replaced by akermanite and the 

 whole mass has completely solidified. In this case, then, 

 we have the succession, forsterite ( chrysolite )-monticel- 

 lite-akermanite (melilite), the akermanite having a replac- 

 ing relation to monticellite. This liquid and the adjacent 

 liquids which show the same effects are, of course, them- 

 selves very rich in lime and magnesia, but when other 

 components are added the same relations must persist at 

 first though they will be modified and may finally disap- 



