120 THE PROGRESS OF GEOLOGICAL RESEARCH IN TASMANIA^ 



tures have been mapped in detail, and their relation to the 

 felsitss and keratophyres of the porphyroid igneous complex 

 definitely determined, for the mapping of the structural 

 features has shown that the felsites and kerato- 

 phyres overlie them as effusive lava sheets which 

 have been involved in the same orogenic movement 

 which produced the folds and the schistosity in the schists. 

 It has further been demonstrated that the Read-Rosebery 

 schists conformably overlie the Dundas slates and breccias. 

 The Dundas slates and breccias, the Read-Rosebery schists, 

 and the felsites and keratophyres constitute in fact a con- 

 formable series, having the above ascending order of suc- 

 cession, composed of mixed sediments, pyroclastic accumu- 

 lations, and effusive lava flows. The evidence further goes 

 to show that some at least of these lava flows were sub- 

 marine. 



The Balfour slates and sandstones have been described 

 by L. Keith Ward in Geological Survey Bulletin No. 10. They 

 present many similarities to the Dundas slates, but the re- 

 lationship with this latter series has not been determined. 

 They are wholly sedimentary rocks, the pyroclastic members 

 of the Dundas slates and breccias being absent. They have 

 up to the present yielded no fossil remains whatever. Simi- 

 larity of structural features and the close resemblance be- 

 tween certain rock-types of both series seem to indicate that 

 we here have two members of a great sedimentary system. 



The Mathinna slates and sandstones, which have been 

 described by the late W. H, Twelvetrees in his reports on 

 the Mathinna field, closely resemble the Balfour slates and 

 sandstones in lithological character and structural features. 

 Although widely separated geographically, they are pro- 

 bably parts of the same sedimentary system. Like the Bal- 

 four series, they are apparently unfossiliferous. 



(3). Silurian. 

 In the account of the Geology of Tasmania presented 

 by the late W. H. Twelvetrees before this Association in 

 1902, the whole of the metamorphosed igneous rocks now re- 

 ferred to as the Porphyroid Igneous Complex, together with 

 the Read-Rosebery and Mt. Lyell schists as well as the 

 Dundas slates and breccias, were referred along with bra- 

 chiopod sandstones at Middlesex, Zeehan, and Queen River 

 to the Upper and Middle Silurian. The Gordon River lime- 

 stone, together v/ith the Mathinna slates and sandstones 

 were referred to the liower Silurian. 



