126 THE PROGRESS OF GEOLOGFCAL RESEARCH IN TASMANIA, 



tonics of the Island. Such generalisations must, however, 

 be drawn with care in view of the relatively small areas 

 in which structural geology has been investigated in detail. 



The most recent summary of our tectonic geology is that 

 included in Professor Sir T. W. Edgeworth David's Presiden- 

 tial Address to the Linnean Society in 1911. Since that 

 date, however, considerable progress has been made towards 

 supplying data for a more detailed general survey. The 

 writer has dealt v/ith this problem of the mapping of tec- 

 tonic lines in connection with his work on the Metallogenic 

 Epochs of Tasmania, and has summarised the conclusions 

 which are justifiable on the evidence at present available. 



It has been demonstrated that there have occurred in 

 Tasmania at least three, and possibly four, distinct periods 

 of orogenic movement, and one period of block faulting on 

 a huge scale. To these diastrophic movements must be added 

 the intrusion of the diabase, which, although unaccompanied 

 by horizontal thrust, must on the evidence recently obtained 

 by P. Bo Nye in the Miiilands have been characterised by 

 vertical upthrusting on a very large scale. 



The first definitely fixed orogenic period was that which 

 followed the Pre-Cambrian sedimentation. If L. K. Ward's 

 deductions in regard to the subdivision of the Pre-Cambrian 

 are correct an earlier disturbance must be admitted. The 

 direction of the tectonic lines of this Epi-Algonkian orogenic 

 revolution swing in gentle curves from N.N.W. in the South, 

 through N.N.E. near Cradle Mt., back to the N.N.W. to the 

 Northwards, and ultimately end on the north coast with a 

 N.N.E trend. The overfolding is towards the East. There 

 was apparently no batholithic end-point to this Epi-Algon- 

 kian orogenic movement. 



The period of sedimentation and contemporaneous ef- 

 fusive igneous activity v/hich characterised the Cambro- 

 Ordovician was followed by a pronounced orogenic revolution. 

 The Epi-Cambro-Ordovician trend-lines are N.N.W. in the 

 southern portion, bending to due N. in the neighbourhood 

 of Rosebery, but resuming the general N.N.W. direction 

 north of that locality. No overfolding has yet been observ- 

 ed. A batholithic end-point characterised the close of this 

 orogenic period. 'The intense alteration and mineralogic 

 reconstitution, which was characteristic of this orogenic 

 movement, gave rise to the fissility of the Dundas slates 

 and the schistosity of the Read-Rosebery and Mt. Lyell 

 schists. The development of the schistose structure was 



