REPORTS OF RESEARCH COMMITTEES. 311 



These peculiar geographical features appear to be due to earth 

 movements of the same age and dating from, the' earlier Tertiary 

 to' the present, the great heights of the plateau prcduced thus 

 being determined by a movement during what is known as the 

 Kosciusko Period, representing either the close of the Cainozoic 

 or a very late stage in that period. The movement was not made 

 at one step, but in many stages, the area of greatest stability being 

 in South-Western Australia, and the area of maximum instability 

 being on the outer island-rings, where later movements have raised 

 Pleistoceinei coral reefs variably such that they occur at all levels 

 to 2,000 feet above sea-level, while in the same region other coral 

 reefs of the same age appear to have been faulted or warped simul- 

 taneously to great depths below sea-level. In the earlier stages 

 these uplifts were punctuated with long pauses during which wide 

 valleys were formed, many representing incomplete peneplains. 

 Elevations after planation were preceded by move,ment9 cf depres- 

 sion of short duration. 



During the later revivals of movement the periods of standstill 

 separating the vertical movements became less and less in duration. 

 This marked .a condition of increasing instability for the period 

 considered. During the whole Kosciusko Period the vertical move- 

 ments were more pronorinced in amount in directions north and 

 east along radii as from a south-western centre near Perth. This 

 marks a condition of increasing crustal instability as frora Perth 

 towards the Solomons and to New Zealand along radii from South - 

 Western Australia. The finest recent example of this is shown 

 by the great movements, up-and-down, of the Pleistocene Coral 

 Reefs in the outer or eastern island rings, and the very gentle 

 movements shown along the continental margin at the same time. 

 These movements appear to have been rhythmic in space and 

 time, the upward movements being arranged on curves, convex to 

 the Pacific, and the downward movements in corresponding 

 troughs. These gaps separating the island areas appear to have 

 an indirect relation, structurally, toi the main breaks in the 

 Eastern Australian plateaux, such as those of the Darling Downs, 

 of Cassilis, and of the Charters Towers-Townsville area. 



Moreover, each main detail in this wonderful unit has had a 

 history similar tO' that of the whole. Thus the Mundi Mundi fault 

 at Broken Hill, seen first b}'^ Benson, has had a conplex history. 

 The main direction of the fault-zone is north and south ; the 

 to'pography to-day is the algebraic sum of the revivals of faulting 

 on an old zone of weakness, the revived topography in the earlier 

 stages being profoiundly modified by erosion between successive 

 movements of faulting, while the later movements mark an ever- 

 growing instability of the crust in that region. 



