.'50 ON MBSOZOIC DOLERITE AND DIABASE. 



Augite. — The augite crystallises after the felspars, some- 

 times enclosing them, sometimes wrapping them partially 

 round or moulding itself on their ends. This gives rise to 

 the structure called ophitic or diabasic. The structure has 

 been surmised to have originated in rocks which consoli- 

 dated under hj'drostatic pressure, for instance, beneath the 

 ■ocean ; but this is purely hypothetical, and does not 

 account for the same structure in the middle of thick sub- 

 aerial lava flows. The augite is nearly colourless, or of 

 an extremely light-brown tint ; never the violet tinge 

 •which characterises the augites of Tasmanian Tertiary 

 basalts. This colour character is occasionally 'rather use- 

 ful in distinguishing the mesozoic from the Tertiary 

 •dolerites. Where the augites are fragmentary and small 

 and the felspars much reduced in size, and the I'ock 

 :assumes an intersertal structure, as at Killafaddy, Tasman 

 Peninsula and some other localities, doubt sometimes 

 :arises, on inspection of microscopical slices, as to whether 

 we are looking at dolerite or basalt. In such cases the 

 :absence or rarity of olivine, which at most only occurs 

 sporadically in the Mesozoic rock, is a useful guide. The 

 Tertiary basalts of the island invariably contain a plentiful 

 amount of olivine. 



The augite has not been converted into diallage. Twin 

 •crystal sections, parallel to the clinopinacoid, exhibit fine 

 oblique striae, which mvist be parallel to the basal plane, 

 :and not the orthopinacoidal lamination of diallage. In 

 rsections parallel to the orthopinacoid the striae are at right 

 rangles to the vertical axis. 



Olivine.— l^hiQ is not an abundant accessory. From 

 most parts of the rock it is entirely absent. When it does 

 •occur, as at Killafaddy, Ross, Hobart, Bothwell, West 

 Devonport, &c., it appears to be idiomorphic. It is then 

 ■one of the early minerals in the rock, most likel.y second 

 in point of time only to the apatite and iron ores. It 

 ;appears preferentially in the finer grained varieties and 

 those which show an approach to intersertal structui-e. 



Apatite. — Occurs as slender needles in the felspars and 

 in the unindividualised groundmass when this is present, 



Ilmenite and Magnetite. — The iron ores in most diabases 

 .are ilmenite and titaniferous magnetite. Ilmenite cannot 

 be recognised in our rock in any definable forms, though 

 many of the shapeless grains may be that mineral. On 

 the other hand the forms of magnetite can be discerned 

 very well. In a section of the interesting rock at the 

 Jlobart Railway Station, which is of a porphyritic nature. 



