Notices of Memoirs — Post-Jurassic Geography. 521 



erosion and the northern portion of the area of sedimentation were 

 elevated to a moderate height and a long period of equilibrium and 

 erosion ensued. This sequence of elevation and of pauses of 

 equilibrium with erosion was repeated until the close of the Kosciusko 

 Period,^ the pauses between the uplifts becoming less important, 

 but the amount of vertical movement becoming correspondingly 

 emphasized. 



At various stages of the process basalts flooded Eastern Australia, 

 especially in areas of older sedimentation. The appearance of the 

 old basalt-covered stream-drifts is suggestive of a temporary subsidence 

 for the plateau areas during the basaltic period. 



Strong streams, such as the Shoalhaven and the Hawkesbury, 

 maintained their general courses against the uplifts along their lower 

 portions. Hence it is inferred that the uplifts were effected slowly ; 

 nevertheless, the periods of equilibrium separating the revivals of 

 elevation were of much longer duration than the uplifts themselves. 



The researches of Dutton, Hayford, Bowie, Gilbert, and others 

 appear to have placed the doctrine of isostatic compensation upon 

 a firm basis; nevertheless, the operation of the adjustments does not 

 appear, as yet, to be understood, and it is probable that cognizance has 

 not been taken of all the factors. 



In the example cited, of the elevation of both the great Mesozoic 

 peneplain and a great portion of the loaded oft'shore area, it seems 

 difficult, under the doctrine of continuous compensation, by erosion 

 and sedimentation, to explain, in the first place, how the positive 

 element could remain, for ages, in the one general position of 

 equilibrium, while the offshore area was being loaded; and, in the 

 second place, how the elevatory movement could have received its 

 initial impetus, especially as the effect appears greater than the cause 

 if it be assumed that the Ci'etaceous sedimentation gave rise to the 

 Tertiary uplifts. On the other hand, the foundering of sub-oceanic 

 areas in the neighbourhood might be adduced as an explanation, but 

 the evidence is not at all conclusive on this point. 



The history of the revivals of elevation during Tertiary time over 

 Eastern Australia indicates crustal adjustment by jumps, and in this 

 case also the increasing amount of vertical movement suggests that the 

 elevations of the plateaus more than compensate for the erosion 

 sustained in these regions during recent geological time. 



The extrusion of basalts is in harmony with the doctrine, but the 

 action appears to liave been catastrophic rather than gradual in 

 nature. 



The sequence of geograpliical forms cited suggests that sedimentation 

 influenced the formation of plateaus only in a minor degree, but, on 

 the other hand, that stresses accumulated gradually within the zone 

 of compensation, until a belt of weakness, or mobility, was established 

 by means of which the ill-adjusted portions were connected. Upon 

 the arrival of such a stage adjustment ensued with relative rapidity 

 with the production of epeirogenic uplifts and depressions. This 

 neither denies the ability of a load, such as a mass of sediments, or an 

 ice-cap, to depress the underlying region, nor does it seek to exclude 

 ^ Closing Tertiary. 



