160 THE BUILDING OF EASTERN AUSTRALIA 



During the Tiiassic, the Sydney basin not only lagged 

 behind in the uplift, but was subsiding at intervals, so that 

 during this period it was a shallow sea in which the 

 Narrabeen shales, the Hawkesbury sandstones, and the 

 Wianamatta shales were laid down. During the Narrabeen 

 interval volcanic eruptions of an andesitic nature showered 

 out ashes, which helped to build up the chocolate shales. 

 The abundance of fossil leaves and wood in this formation 

 indicates shallowness, proximity to land, and possibly 

 nearly fresh water conditions, indicating that this area 

 had j>articix)ated for a time in the uplift of the late Permo- 

 Carboniferous. The Hawkesbury sandstone system consists 

 of coarse-grained sandstones, which, evidently, must have 

 been deposited in a storm-tossed brackish sea, for the sedi- 

 ments carry no fossils except a few Macrotaeniopteiis leaves 

 and fragments of fossil wood. Brackish water affords the 

 most suitable habitat for thinshelled molluscs, and a shallow 

 storm-tossed sea gives the best conditions for the complete 

 destruction of shell remains by the grinding action of the 

 sands. The frequency in the Ha\^kesbury system of false 

 bedding is also indicative of changing currents and shallow 

 water. 



The Hawkesbury period was followed by an uplift 

 which inaugurated a lake period in the Sydney area. In 

 the lake then formed the Wianamatta shales were laid 

 down. Mr. F. Chapman, F.L.S., concludes from his study 

 of the fossil microzoa of the series that the Wianamatta 

 shales were deposited in fresh or brackish water. [7] 

 (Records Geol. Surv. of N.S.W., Vol. VIII., Part IV.) 

 The other fossils of the formation, comprising thin- shelled 

 mollusca and labyrinthodont remains, support this con. 

 elusion. 



The Hawkesbury sandstone sea was at the start 

 considerably larger than the area now covered by its forma- 

 tion. 



Mr. W. S. Dun [8] has described fossil leaves in Lower 

 Trias rocks from Benolong, in the Dubbo district. The 

 sea gradually dwindled in size as elevation proceeded, and 

 at last only the Wianamatta lake remained. 



The elevation of the whole of the eastern belt of the 

 present Australian Continent led to a relative depression 

 of the interior, and in later Triassic times (the Trias-Jura 



