The Heathcotian. 169 



Central Victoria from south to north. The deposition of the 

 ordovician beds began by the sinking of the ground to the west 

 of the line from Heathcote to Melbourne, and by a second 

 subsidence in eastern Victoria from Bogong to Dargo. The 

 oldest deposits were naturally deposited on the sinking shore 

 lines, so that the Lancefield series was laid down on the 

 eastern side of the land made up of Heathcotian and perhaps 

 also still older rocks ; as the subsidences continued later sedi- 

 ments accuniulated further and further to the west (PI. XXV., 

 Fig. 5). During the time represented by the unconformity 

 between the ordovician and silurian rocks, the country to 

 the west of the Heathcote-Mel bourne line was probably 

 upraised ; subsidence or denudation in the counties of Evelyn, 

 Anglesey and Rodney led to a silurian sea running northward 

 from Port Phillip to the Murray basin. This silurian sea was 

 bounded both to the west and east by highlands of ordovician 

 rocks (PI. XXIII). It is possible that gulfs from the 

 ordovician sea ran into the old Heathcotian land area, Sandy 

 Creek and Wombat Creek ; and in these places the ordovician 

 deposits may have been covered by silurian sediments. The 

 silurian rocks may also have extended westward from Mount Ida 

 over the eastern border of the ordovician series ; hut in all 

 probability, the main part of the ordovician and silurian rocks 

 were deposited in independent basins; and these basins were 

 separated by a range of old rocks, which once extended from 

 Geelong to Mount Camel, and of whicli the Colbinabbin and 

 Mount William Ranges are the best preserved remnants. 



The range of the Heathcotian rocks does not appear to have 

 been continuous from Lancefield to Geelong even in upper 

 ordovician times. No trace of it is known where the 

 silurian and ordovician rocks occur near together, between 

 Keilor and Sunbury. Rocks of the Melbournian division of the 

 silurian system are exposed near rocks belonging to the upper 

 ordovician. Thus, near Diggers' Rest, there are slates with 

 Dicranograptus ramosus and Coenograptus gracilis, which are 

 regarded as upper ordovician. The slates along the Coimaidai 

 Creek, near Bacchus Marsh, contain Didymograptus caduceus, 

 Didymograptus extensus and Tetragraptus quadribrachiatus ; 

 they are typical of the Castlemaine series, the uppermost of Mr. 



