ON MESOZOIC DOLERITP] AND DIABASE IN 



TASMANIA. 



By W. H. Twblvetrees, F.G.S., and W. F. Petterd, 



C.M.Z.S. 



The following Notes lay no claim to be an exhaustive 

 description of our familiar '• diabase" or "dolerite" rock, 

 which plays such an important part in the geology and 

 physical configuration of our Island. The present object is 

 rather to place upon record some inferences drawn from the 

 examination of numerous microscopical sections of speci- 

 mens collected or received from all parts of Tasmania. It 

 is by accumulating the results of observations that stepping 

 stones are formed to more complete knowledge. A glance 

 at Mr. R. M. Johnston's geological map of Tasmania, issued 

 by the Lands Office, will show the share this rock takes in 

 the structure of the Island. It occupies the whole upland 

 area of the Central Tiers. On the northern face of the 

 Tiers — the Western Tiers as they are here called — there is 

 a tongue of the rock prolonged northwards past IMount 

 Claude. At their north-west corner it forms or caps 

 mountains, such as Cradle Mountain (the highest in Tas- 

 mania), Barn Bluff, Mount Pelion West. Eldon Blufi: 

 forms a narrow western extension. Mount Sedgwick is a 

 western out-lier ; Mount Dundas another. In that part of 

 the island it is also found at Mount Heemskirk Falls, and 

 on the Magnet Range, two miles north of the Magnet 

 Mine. Mounts Gell and Hugel are also western out-liers. 

 Its south-west boundary is Denison Range, with Mount 

 Field West, Mount Mueller, and Mount Picton. Southerly 

 we find it in the Hartz Mountains ; the rock goes down to 

 the south as far as the Rivers Huon and Esperance, and 

 even further south it is found on the isolated peaks of La 

 Perouse and Adamson's Peak, and a narrow fringe of it 

 runs along the coast-line south of the Huon to South East 

 Cape and South Cape. On Bruni Island it is present in a 

 very massive form. It is found on Tasman's Peninsula, 

 and in the whole of the south east of the Island it cuts up 

 the sandstones and shales of the Permo-Carboniferous and 

 Trias-Jura countrJ^ Mount Wellington and other heights 



