OF SOME TASMANIAN ROCKS. 57 



No. 4. — Hydrated Olivine Basalt. 



(From Native Point, Perth.) 



A rock of abnormal physical character, inasmuch as it is 

 invariably heavy from the absorbed moisture, and soft to a 

 degree. It is pale brown in colour, showing a variety of 

 tints between almost yellow to a fairly-dark shade. On 

 exposure to atmospheric action, it commonly fractures m 

 all directions, and finally breaks up into fragments. It 

 closely resembles the tuffaceous substance known as palago- 

 nite. It was obtained in sinking- holes in the locality men- 

 tioned. 



Microscopical Characters. 



This structure is that of a normal basalt. The porphyritic 

 mineral is olivine, and augite in the form of grains and 

 minute prisms is embedded in a plexus of narrow lath-shaped 

 felspars. There is a glassy base, and large patches of zeo- 

 litic substance (chabazite ?) and vesicles crowded with 

 minute spherulites. Magnetite is present in small quantity. 

 The twinned felspars give extinction angles up to 27°, and 

 are probably labradorite. 



The most important mineral is the olivine, which exhibits 

 interesting alteration features. The crystals have the 

 irregular form$ which intra-telluric minerals receive from 

 the attacks of the magma at the crisis of eruption, and are 

 invariably margined with a deep orange or brownish-red 

 border, consisting of fine fibres perpendicular to the contour. 

 The interior is of a citron-yellow colour, and both the in- 

 terior and the border have assumed a pleochroic nature. 

 The former is serpentinous (sometimes chloritic), and the 

 latter, in all probability, is a hydrated ferric oxide. No 

 fresh olivine remains in the rock. The change sometimes 

 proceeds until the precipitate of ferric oxide colours the 

 whole crystal, and occasionally we see it result in laminae 

 with the cleavage lines, pleochroisms, and red and green 

 interference colours of biotite. This mineral is very similar 

 to the hydrated silicate of iron, lime, magnesia, and soda 

 called " iddingsite," butl its general features indicate that it 

 is a pseudomorph after olivine. For a discussion of this 

 kind of replacement, see H. H. Arnold-Bemrose on the 

 Microscopical Structure of Carboniferous Dolerites and 

 Tuffs. Q.J., Geol. Soc, 1894. p. 617. 



