11 



exposed at One Tree Point. The overlying basalt marked c, 

 is essentially a dark, close-grained, feldspar basalt, with 

 abundant grains and crystals of olivine. It is full of small 

 fragments, scarcely altered, of the surrounding stratified 

 rocks. The rock itself is in every respect very similar to 

 the basalts at Deloraine, Breadalbane, Table Cape, Geilston, 

 CaniiDbell Town, Eingarooma, Latrobe, Fingal, and various 

 places in Victoria, in nearly all of which j^laces it overlies 

 fluviatile and lacustrine formations similar to that at One 

 Tree Point. Towards the south the basalt seems to have 

 flowed in repeated layers at different times. This may be 

 inferred also from the marked difference in the form and 

 density of the various sheets. The flows of basalt to the 

 south, which I assume to be the more recent, are very unlike 

 the massive beds at the northern point of junction with the 

 underlying clays. At certain points towards the creek 

 southward, they are highly vesicular, and present very much 

 the appearance of pumice stone, although of a more dense 

 character. This also is the character of the soft underlying 

 feldspar basalts of Beauty Bay, Lindisfarne Bay, and 

 Cornelian Bay, good sections of which may be seen at low 

 water all round the sea margin. 



The feldspar basalt on the northern and western shores of 

 Prince of Wales' Bay is identical with that at Cornelian Bay, 

 but the cavities are filled with lime, and is thus converted 

 into a true amygdaloid. It is very curious that all the 

 great basaltic sheets of Tasmania and Australia of tertiary 

 age should be so closely associated with the great fresh- 

 water systems of the period ; and it is also worthy of note 

 that the mode by which they were ejected was probably by 

 fissure-eruption, and not by the more familiar type of 

 conical volcano. The immense level sheets of basalt in the 

 Midland and Northern districts of Tasmania, together with 

 the vast plains composed of similar basaltic sheets in 

 Victoria, support the views recently advanced by Professor 

 Geikie,* who states positively upon the subject, that the 

 volcanism of earlier periods as exhibited in the great sheets 

 of the Western States of America, Abyssinia, India, and 

 elsewhere, can with difficulty be explained by reference to 

 any modern volcanic phenomena. 



The persistent horizontality of the tertiary basalts in 

 Tasmania and Victoria, also offer strong reasons in favour of 

 the fissure -eruption theory. 



With respect to the flows at One Tree Point there is 

 another interesting feature, which, if correctly inter2)reted, 

 proves that there were repeated flows of basalt also at the 



* Nature, November, 1880. 



