OF SOME TASMANIAN ROCKS. 55 



magma-basalts, and described by Boricky in his work on the 

 basalt rocks of Bohemia (i). Such rocks occupy a position 

 between the basic and ultra^basic jocks,and Rosenbusch has 

 given the name of limburgite toj those with abundant 

 olivine, with the intention of detaching them from basalts, 

 and of emphasising their position as extreme members of 

 the nepheline and melilite effusive series. Limburgite has 

 been recorded from Cape Verde, Kilimanjaro, and Madagas- 

 car, besides the European occurrences. Judd and Cole ( ^) 

 describe it from Lamlash (Holy Isle), Arran ("). 



The constituents of the Burnie I'ock are olivine, augite, 

 and magnetite, in a brown glass devitrified by the develop- 

 ment of globulites and crystallitic rods. 



Augite is in colourless crystals, porphyritically dispersed, 

 and, as numerous small laths and prisms, vertical section* 

 give an extinction angle up to 36°-40°. 



Olivine is abundant and fresh, giving numerous char- 

 acteristic hexagonal sections in the zone (010), (001). Its 

 crystals are often corroded and scattered, and cracks intro- 

 duce inclusions of the base. 



Magnetite is present in fair quantity in well-form€>d 

 crystals and minute grains. 



The base is a brown glass with globulites, belonites, and 

 microlitic laths of augite. Some of the rods may, perhaps, 

 be incipient felspars. The globulites cluster more densely 

 round the borders and in the neighbourhood of the larger 

 crystals, forming semi-opaque aggregations. Amygdaloidal 

 cavities are discernible, some beautifully fringed with zeo- 

 lites, some with an isotropic periphery and a faintly-polaris- 

 ing crystallitic centre. 



No. 3. — Basaltvitro'phyre* (Glassy Basalt). 



(From Sheffield.) 



This is, microscopically, one of the most attractive rocks 

 occurring in this State. It is usually intensely black, 

 although rarely of a dark grey-brown colour, with a shining 

 vitreous lustre, having commonly numerous veins and 

 patches of milk-white to glassy zeolitic magma, which, in 



(') Petrographische Studien an den Basaltgesteinen Bohmens, 1874, 

 pp. 63-60. 



(*) On the Basalt Glass of the Western Isles of Scotland, Q.J., Geo. 

 Sec, 1883, p. 459. 



* As Pitchstone, " The Geology and Palaeontology of Queensland and 

 New Guinea," Jack and Eiheridge, 1892. 

 Minerals of Tasmania, 1896, page 68. 



