BY DR. W. N. BENSON. 33 



The petrological character of the dolerites is of in- 

 terest. Tliey are of medium grainsize, and consist pre- 

 dominantly of plagioclase and pyroxene. The plagioclase 

 rorms small, more or less, idiomorphic tabulse, somewhat 

 zoned, the central portion having the composition of 

 bytownite. The pyroxenes are more varied, a rhombic 

 and two types of monoclinic pyroxene are present. In a 

 rock from the lower portion of the dolerite on Cradle Mt. 

 (1458) there is a normal, more or less, ophitic augite (som.e 

 times subidiomorphic), with the usual large optic axial 

 angle, associated with, and frequently including prisma- 

 tic crystals of enstatite. In a rock from the summit of 

 the mountain, however, the monoclinic pyroxene, which 

 IS partly subophitic, partly subidiomorphic, has two dis- 

 tinct types, namely, those grains which have the normal 

 optic axial angle (which are in the minority), and those 

 which are approximately uniaxial, indicating that they 

 contain a large excess of magnesian silicate, i.e., are mag- 

 nesium-diopside, or the augite-enstatite of Wahl. This 

 mineral has been previousl}^ recorded in the dolerite of 

 Cataract Gorge by Osann (^). and is known to be fairly 

 common in other occurrences of dolerite in Tasmania. (^^) 

 In both these rocks, there is a small amount of magnetite 

 and of very finely crystalline intersertal granophyre, dotted 

 with crystallites of magnetite. The former of these 

 rocks contains grey felspathic veins at first thought 

 to be granophyre. They prove to have a 



highly ophitic to poikilitic texture. The pyroxenes are 

 sometimes roughly prismatic, ophitic or broken up into 

 isolated patches, which are in optical continuitv over 

 quite large areas. The pyroxene is quite fresh, usually 

 uniaxial, but sometimes of the normal character. There 

 are, in addition, small prisms of enstatite. The felspar is 

 slightly zoned, has the general composition Ab^Ano, and 

 forms a few small phenocrysts. Between the tabulse is a 

 small amount of minutely cr\^stalline granophyre. A few 

 large grains of magnetite are also present. 



Two inches from the chilled margin of the dolerite of 

 Barn Bluff the rock is very fine-grained, with an interser- 

 tal structure. It contains small phenociystic laths of 

 plagioclase and larger prisms of augite, more or less con- 

 verted into chlorite and carbonates. At the margin itself, 

 the gi'ainsize is extremeh^ minute, and the texture appears 

 to be subvariolitic. Both these rocks contain vesicles 



(3) Ueber einen Enstatitaugit-fiihirenden Diabas von Tasmanien. 

 Centbl. fur Min.. 1907. pp. "(05-11. Translation by W. H. Twelvetrees Ann. 

 Eep. Dept. Mines, Tas., 1907. 



(lO) J. A. Thomson. Joum. and Proc. Roy. Soc. N.S.W., 1911, p. 30b. 



