GKOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 493 



The following is a complete list of the original minerals 

 which we have discovered in the mass divided into two 

 classes: — 



(A) Those forming 10% or upwards of the rock in any 

 part of the mass. (B) Those not forming anywhere as 

 much as 10% of the rock. 



(A) (B) 



Plagioclase (from labradorite to albite) Aegyrine-augite 

 Orthoclase (and soda-orthoclase) Apatite 



Augite (titaniferous) Quartz (derived) 



Diopside Magnetite 



Olivine Pyrite 



Biotite 

 Ilmenite 



The Felspars. 



In the main mass of rock the felspar is nearly every 

 where entirely plagioclase. The crystals are idiomorphic 

 in general form, but uneven on their surfaces of contact 

 with other crystals. Their habit is tabular, parallel to the 

 face 010; and the sections are thus mostly lath-shaped. 

 Twinning on the carlsbad and albite laws affects every 

 crystal, but not very regularly, and traces of pericline 

 twinning are rarely seen. The composition of the plagio- 

 clase cannot be stated with accuracy because the crystals 

 are strongly zoned. The average composition of the plagio- 

 clase is shown by calculation from an analysis (see Appendix), 

 to be that of an acid labradorite with the formula Ab 4 An 3 . 

 A large central area of each crystal section is seen to con- 

 sist of labradorite whose composition is probably about 

 Ab 3 An 4 and this labradorite probably composes nearly half 

 the volume of each crystal. 1 Outwards the crystals then 



1 The composition of the cores of the plagioclase crystals as stated 

 above is arrived at in the following 1 rough and ready manner. Since the 

 more basic core is seen in most crystals to occupy distinctly more than 

 half the area of the section it is assumed that it forms about half the 



F I— Dec. 6, 1911. 



