(JoTTOi!i.—GeoJ(>f/if of Signal Hill, Dunedin. 

 The chemical classification is as follows : — 



125 



The " norm " is seen to correspond roughly to the " mode," or actual 

 mineral composition. 



It is interesting to compare the actual proportions of feldspar to feld- 

 spathoid in the three types of phonolite : (1) In the Logan's Point phonolite, 

 where feldspar and nepheline each constitute about one-third of the rock, 

 silica is relatively high and alumina low, while alkalies are high ; (2) in the 

 nephelinitoid phonolite, where nepheline constitutes the greater portion of 

 the rock, silica is lower, alumina is higher, and alkalies are about the same ; 



