varying amom: 

 clinic pyroxen< 

 their position i 



s of plagioclase felspar, rhombic or mono- 

 or both, and olivine may be classified by 

 . the following diagram.. (Fig. 6.) 



fa 



P? di ™ 



sides opposite to 

 angles F, P and O, 

 respectively. The 

 chemical composi- 

 d tion varies regu- 

 larly round the 

 F. P. 0.— Felspar, pyroxene, olivine. triangle. 



a. — Anorthosite. g. — gabbro and norite. 

 p.— Pyroxenite (diallagite, websterite, enstatolite). 

 1. — Lherzohte, wehrlite, hartzburgite. 

 d. — Dunite. t. — troctolite, allivalite. 

 o.— Olivine gabbro and olivine norite. 



In passing through the points F P O the percentage of 

 alumina and lime, and the ratio Fe:Mg steadily decrease, 

 the sum of Fe and Mg rapidly increases, the percentage of 

 silica rises slightly from F to P, and falls rapidly from F 

 to O. 1 



The plutonic inclusions of Dundas lie within the triangle 

 a p I or along the line p d. Rocks lying in the lower triangle 

 a I d have not as yet been found in the quarry. This corres- 

 ponds to the general rarity of rocks of this mineralogical 



(0). 



