president's address SECTION c. 71 



of tuffaceous sandstones, as, for instance, in the Ipswiah 

 Coal Measures, and Mr. A. W. Howitt records a similar 

 occurrence in homotaxial rocks in Victoria, These state- 

 ments argue that there must liave been a certain amount of 

 contemporaneous volcanic activity in Australia in early 

 Mesozoic time. Lava sheets, however, belonging to this 

 period seem to be wanting, as far as the author is aware. 



In Tasmania, however, perhaps in Mesozoic time, a little 

 later than that during which the Hawkesbury series of 

 Sydney and the Ipswich Coal Measures of Queensland were 

 formed, volcanic energy developed itself on a grand scale, 

 and has left lasting monuments of its powerful work in the 

 shape of the Tiers which form such an important physical 

 and geological feature in the south-eastern and northern 

 portions of Tasmania. 



The age of these diabasic greenstones is, at present, by no 

 means settled. The author is inclined to think, for reasons 

 which will be given in a subsequent paper, that the greater 

 part of them are later than the Mesozoic coal-fields of 

 Jerusalem, Seymour, &:c., and than the coal-field of New Town, 

 near Hobart. If this view as to the age of the Tiers is 

 correct, their eruption may have been directly due to the 

 prolonged and heavy sedimentation which produced the 

 Permo-Carboniferons marine sediments of the estuary of 

 the Derwent, &c. The thickness of the latter must amount 

 to several thousand feet. It may be argued that such a vast 

 amount of time has elapsed between the deposition of the 

 uppermost of the marine sediments of the Permo-Carbon- 

 iferous and the formation of the New Town Coal Measures 

 that it is extravagant to assume that any volcanic action 

 developed subsequent to the formation of the latter is at- 

 tributable to heavy sedimentation in Permo-Carboniferous 

 time ; but the author has the authority of Mr. R. M. 

 Johnston for saying that there is a conformable upward 

 passage from the top of the Permo-Carboniferous Marine 

 Beds into the New Town Coal Measures, and the flora of the 

 latter is certainly characterised by a commingling of Aus- 

 tralian Lower Mesozoic plants with those of Upper Palaeo- 

 zoic affinities, though, on the whole, the facies of the flora 

 is decidedly Mesozoic. 



The suggestion, therefore, may not be unreasonable, that the 

 extrusion of the basic rocks composing the Tiers may have 

 been due to heavy sedimentation. 



With the exception of some comparatively insigHifisant 



