PRESIDENTIAL ADDKEbS. 59 



or distribution of masses of intrusive rock, but is probable 

 evidence of the great predominance of peripheral over 

 radial contraction of the earth's surface. 



4. The most extensive granite intrusions which have so 

 much controlled the development of the present Main 

 Divide of Eastern Australia group themselves in time 

 around the Permo-Oarboniferous glacial epoch. That epoch 

 was evidently a very critical time in folding, faulting injec- 

 tion of batholiths, and mountain building. 



5. Since the close of Palaeozoic time Australia has been 

 subjected to broad warps, but not to true folding, except 

 in the direction of New Guinea, where Cretaceous, and 

 even early Tertiary strata are highly folded. New Guinea 

 is thus a new fold region: and even in Australia tectonic 

 movements are newer as New Guinea is approached. 



6. In the neighbourhood of large sag basins basic rocks 

 have been erupted, as in the case of the quartz-dolerite 

 (quartz-diabase) sills and dykes of Tasmania, the basalts 

 of the Great Valley of Victoria and the serpentine belt of 

 New England. 



7. At least one important basin which has been an area 

 of subsidence for several geological periods, and therefore 

 probably has been an area where gravity has been in excess, 

 has ceased to subside, and has taken part in the upward 

 warping of Eastern Australia in late Tertiary time. On 

 the isostatic theory of crustal equilibrium this would imply 

 that in sucli an area (as the central coal basin of New South 

 Wales) compensation has now been attained. This may also 

 imply that an extra heavy area, (where gravity is in excess) 

 may eventually become an area of mean density. The rift 

 valley of South Australia from near Adelaide to Lake Eyre 

 is perhaps still subsiding. Bass Strait, and the Snowy- 

 Murrumbidgee rift valley are also probably still subsiding. 

 It would be of considerable scientific interest to have an 



