520 Notices of Memoirs — Post-Jurassic Geography. 



3. Balcomhimi. — The type locality is at Balcombe's Bay, east shore 

 of Port Philip. The fauua of these beds is richer and more varied 

 than the existing Southern fauna ; its general facies is more com- 

 parable with Northern Australian forms. In the present state of 

 our knowledge it contains rather more than 2 per cent of extinct 

 genera, and even allowing a wide margin for difierences of opinion 

 the living species would barely represent 2 per cent. 



4. Janjukian. — Coastal sections on Bass Strait, parish of Jan Juc, 

 south of Geelong. The fauna from these beds appears to be furthest 

 removed from the living, based on a review of the genera which 

 shows between 5 and 6 per cent extinct, whilst the species only 

 show about 1 per cent living forms. 



When the typical fossils are not obtainable it is not easy to state 

 whether a rock series is Balcombian or Janjukian. To meet this 

 difficulty the wider term Barwonian has been given, as both these 

 horizons are well developed in theBarwon Basin. 



Stratigraphical evidence also exists in confirmation of the above 

 sequence in the Moorabool Valley, in the coastal sections from Poi't 

 Campbell to Cats' Reef and elsewhere. 



(5) The Post-Jueassio Geography of Australia. Notes on the 

 Hypothesis of Isostasy. By E. C. Andrews, Sydney, New 

 South Wales. 



rpHE doctrine of isostasy implies the general correspondence, in 

 X weight, of all vertical columns of unit size composing the Earth's 

 crust to a depth known as the depth of compensation. This depth is 

 taken at 122 kilometres below sea-level by Hay ford.' 



The excess of height of the unit columns, in continental areas, 

 is considered as being compensated by the excess of crustal density 

 in sub-oceanic areas. Isostatic compensation is supposed to follow 

 rapidly upon loading and unloading. Examples of such loading are 

 sedimentation and the formation of a continental ice-sheet, while 

 examples of unloading are erosion and the disappearance of an ice- 

 sheet. The adjustment is considered to be a gradual, rather than 

 a spasmodic, process. Anomalies of gravity, however, are recorded 

 from many localities, and Gilbert^ suggests that the explanation of 

 such is to be sought in nucleal heterogeneity. 



Geography . — East and West Australia form two positive, or buoyant, 

 elements, while the Inland Plains, in the main, represent a negative, 

 or sunken, area. With these three elements should be considered 

 New Zealand, Malaysia, the South Pacific, the Indian, and Southern 

 Oceans. 



During Cretaceous time a great plain of erosion appears to have 

 been formed in the positive elements of Australia, while the extensive 

 epicontinental sea of that period was filled with the waste derived 

 from the neighbouring erosion. Subsequently, both the old plain of 



^ J. F. Hayford, "Figure of the Earth and Isostasy": U.S. Coast and 

 Geodetic Survey, Washington, 1909. 



^ G. K. Gilbert, " Interpretation of Anomalies of Gravity" : U.S. Geological 

 Survey, Washington, 1913. 



