CHEMICAL EVIDENCE 485 



The rocks concerned are all igneous and the syenite and anorthosite 

 seem surely derivatives from the same parent magma and of no great 

 difference in age; hence they present no striking differences in compo- 

 sition. The granite-gneisses are in all probability of much greater age 

 and from a different magma, but are nevertheless related igneous rocks 

 and have a composition quite like that of the granites which did arise 

 from the anorthosite-syenite magma. Because of this close chemical 

 relationship the chemical evidence is likely to be much less positive in 

 character than would be the case were the rocks possessed of more promi- 

 nent differences in composition. 



It is also the case that the rock of the syenite and anorthosite bathy- 

 liths is so variable that it is an impossible matter at the present time to 

 arrive at any exact notion regarding their average composition. 



Furthermore, if the syenite has digested material from the anorthosite 

 the field relations make it clear that the rock mostly concerned is the 

 gabbro border along with a wholly uncertain amount of anorthosite 

 gabbro, and the precise proportions and composition of this mixture is 

 again an uncertain matter. 



