BY R. M. JOHNSTON, F.L.S. 193 



structure, and texture. Some of the characteristics are 

 due to having been subjected to different degrees of pres- 

 sure and various modes of coohng, but more frequently 

 to the effect of long continued action of decomposing 

 agencies. The structure of the rock is mainly sphseroidal, 

 polygonal, or hexagonal, concretionary, and much jointed, 

 although sometimes showing a very perfect columnar or 

 prismatic appearance as at Emu Bay and other places 

 along the North- West Coast ; and the texture varies from 

 compact to vesicular, vesicular-amygdaloid, or spongy 

 (basaltic pumice) ; the latter form, as at One-Tree Point, 

 Hobart, often so light that it floats in water, and, but for 

 its black or brownish colour, might readily be mistaken 

 for true trachyte-pumice. In Tasmania, as in the older 

 eruptive basalts of Victoria, the three sub-species of basalt 

 (1. Dolerite, Dolerite- Lava; 2. Anamesite, Anamesite 

 Lava ; 3, Basalt, Basalt Lava) may graduate vertically 

 or laterally into each other. Professor Ulrich's description 

 of these three sub-species of basalt, with respect to Victoria, 

 serve equally well for Tasmania, and may be of much 

 value to local geologists. He states : — 



" Proceeding now to a special description of the older 

 and newer basalts relative to their lithological character, 

 bearing in mind their essential mineral composition, viz. — 

 " augite," " labradoi'ite," and " titaniferous iron," — we 

 have in Victoria the three established sub-species, — viz. 

 dolerite, anamesite, and basalt, — besides an infinite number 

 of varieties dependent on texture and other characters. 



(1) "Dolerite, Dolerite Lava. — A crystalhne-granular, 



dark and light-bluish or greenish-grey rock, in 

 which crystals of its principal components (augite 

 and labradorite) can be distinctly recognised by 

 the naked eye. This species has not as yet been 

 observed within the older basalt areas, and is also 

 rare within those of the newer. 



(2) "Anamesite, Anamesite Lava. — Of a bluish or 



greenish, often brownish grey or black colour, 

 and distinctly recognisable, yet so finely crys- 

 talline-granular a texture, that the component 

 minerals (augite and labradorite) cannot be clearly 

 distinguished without the aid of the magnifying 

 glass, and then only by their difference in colour. 

 It is apparently the most prevailing. Its frac- 



