92 GEOLOGY OF MT. FLINDERS AND FASSIFERN DISTRICTS, Q., 



Ohemical classification : Class i., Order 5, Rang 2, Subrang 3. 

 Magmatic name : Pulaskose. 



F1.13. Loc: one of the northern foothills of Mt. Flinders. 

 It overlies the light-coloured trachytes, and is probably the last 

 flow. 



Handspecimen : dark aphanitic rock with the characteristic 

 fracture and aspect of the andesitic and phonolitic trachytes(even 

 fracture). 



Texture : holocrystalline, microcrystalline, even-grained; fabric 

 intermediate between typical trachytic and typical pilotaxitic. 



Composition : felspar forms about 75 %, ^girine 20 %, and 

 magnetite 5 %. The felspar consists chiefly of acicular microiites, 

 but partly also of allotriomorphic grains. Carlsbad twinning is 

 frequent; albite twinning occurs occasionally, and shadowy extinc- 

 tion from crowding or ultramicroscopic twinning is very frequent. 

 The refractive index is almost that of canada balsam. These 

 properties indicate that the felspar is chiefly anorthoclase (soda- 

 microcline ?), but true soda-orthoc ase and true albite are present 

 in smaller amount. The segirine is not quite fresh, being some- 

 times in an incipient stage of decomposition to chlorite, and shows 

 a tendency to develop uralitic cleavage. The pleochroism is not 

 very strong, being a (near c') bluish-green, b brownish-green, and 

 C yellowish-green; and absorption a>-b>-f. The extinction angle 

 is about 3° behind. The double refraction is 0*050. It is, there- 

 fore, an segirine-augite. Magnetite is abundant in rounded 

 grains. A colourless, dull grey mineral, with low double refrac- 

 tion and a refractive index slightly above Canada balsam, occurs 

 interstitially between the felspar, and is probably nepheline. 



Order of consolidation : the felspar is studded with minute 

 included rods of segirine The magnetite is corroded; felspar 

 frequently penetrates segirite in an ophitic manner. The order 

 therefore appears to be — 



Magnetite 



^girine 



Felspar (idiomorphic variety) 



Felspar (allotriomorphic) 



Nepheline 



