364 



C. A. SUSSMILCH AND W. G. STONE. 



The groundmass is microcrystalline with a pilotaxitic 

 fabric in some sections. It is essentially a matrix of minute 

 felspar laths with microscopic augite grains in which are 

 embedded the phenocrysts. It is for the most part much 

 altered and inclined to opacity. In some sections secondary 

 silica is plentifully scattered through it in microscopic 

 aggregates. 



(ii.) Phase in which hornblende occurs. 

 In slide x 64, which is a good example, the hornblende is 

 abundant in beautifully fresh crystals of a green colour, 

 exhibiting strong pleochroism. Twinning is common, and 

 some good zoning is present. The augite is generally above 

 the average in size for this phase, one crystal measuring 

 3 mm. in diameter. It shows slight alteration to horn- 

 blende in places. 





Augite-andesite phase (x 32). 









Chemical 



Composition. 









Per cent. 





Per cent. 



Si0 2 ... 





53-62 



CI 



trace 



A1 2 3 





12-10 



S(FeS 2 ) 



trace 



Fe 3 3 



... 



2-00 



Cr 2 O s 



absent 



FeO ... 





6-30 



NiO and CoO 



trace 



MgO... 





7-07 



MnO 



0-17 



CaO ... 





11-64 



BaO 



0-04 



Na 3 





1-84 



SrO 



f trace 



K 2 ... 





1-52 



Li 2 



absent 



H 2 O(100° 



C) 



0-22 



v fl o 8 



0-02 



H 2 O(100° 



C.+ 



) 1-86 









co 2 ... 





1-12 





100-33 



Ti0 2 ... 





0-50 









Zr0 2 ... 





absent 



Specific gravity 



2-892 



P 2 5 ... 





0-29 



Trace = less than 0-01% 



so 3 ... 



... 



0-02 



f Spectroscopic reaction only, 



