50 ON MESOZOIC DOLERTTE AND DIABASE. 



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

 times enclosing them, sometimes wra})pin^ them partially 

 ronnd or monldini2: itself on their ends. This ii^ives rise to 

 the structure called ophitic or diabasic. The structure has 

 been surmised to have originated in rocks which consoli- 

 dated under hydrostatic pressure, for instance, beneath the 

 ocean : but this is purely hypothetical, and does not 

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

 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 rock 

 assumes an intersertal structure, as at Killafaddy, Tasinan 

 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 

 Tertiar}' 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, exhil:)it fine 

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

 and not the orthopinacoidal lamination of diallage. In 

 sections parallel to the orthopinacoid the striae are at right 

 angles to the vertical axis. 



Olivine.— ^\n^ 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 likely 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 structure. 



Ajjatite. — 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 discerjied 

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

 Hobart Railway Station, which is of a porphyritic nature. 



