Yol. 56.] INTRUSIVE DIABASE IN TASMANIA. 333 



21. On an Intrusion of Diabase into Permo-Carboniferous Bocks 

 in Frederick Henry Bay (Tasmania). By T. Stephens, Esq., 

 M.A., F.G.S. (Read December 20th, 1899.) 



[Plate XXII.] 



The most striking features in the geological structure of by far the 

 greater part of Tasmania are the lofty mountain-ranges, which, to 

 the casual observer, appear to consist solely of diabasic greenstone, 

 or dolerite, as it is variously termed. Tasmania is in extent nearly 

 as large as Ireland, and this rock occupies the summits of all the 

 mountains in the eastern half of the island, extending well into the 

 western half in the central and southern districts, and reaching a 

 maximum altitude of 5069 feet above sea-level. But the diabase 

 is not confined to the mountain-ranges. It crowns almost every 

 considerable hill within the area described ; it is exposed at 

 numerous points in the river-beds and valley-bottoms ; it fringes the 

 margin of basaltic lava-flows of Tertiary age ; and its presence has 

 been detected elsewhere by boring through overlying sedimentary 

 strata. 



The oldest sedimentary rocks exposed in the south-eastern 

 quarter of Tasmania belong to that division of the Upper Palaeozoic 

 Series which in Eastern Australia and Tasmania is classed as 

 Permo-Carboniferous. The question of the relations of these 

 sediments to the diabase has often been raised. Were they quietly 

 deposited during a long unbroken period against and around vast 

 masses of igneous rock previously cooled and denuded ? Or is the 

 diabase an intrusive rock, the existence of which in the shape of 

 vast dykes and sills has been disclosed through the removal of 

 overlying strata by the ordinary processes of denudation ? The 

 former theory seems to have been widely accepted as the basis of 

 the geological history of the eastern half of Tasmania, though not 

 without occasional adverse criticism from competent observers who 

 had noted evidence to the contrary at isolated points. My own 

 opportunities of observation over the whole area iu question have 

 long since led me to conclude, firstly, that the central plateau is not 

 a vast boss of ancient volcanic rocks, but rather a ring or network 

 of massive dykes and sheets of diabasic greenstone, which traverse 

 all the sedimentary rocks of pre-Tertiary age ; and, secondly, that 

 there is no known instance in which the diabase of Eastern 

 Tasmania can be proved to be anterior to the deposition of the 

 Permo-Carboniferous Series. 



The question is one of considerable importance, even from an 

 economic point of view, for the principal coal-seams crop out 

 on the flanks of lofty ranges in which the diabase is a pre- 

 dominant feature. The problem is one of those which can never 

 be fully solved without a systematic geological survey of the 

 whole country, and this, unfortunately, has not yet been under- 



Q. J. G. S. No. 222. 2 a 



