170 Proceedings of the Royal Society of Victoria. 



Hall's three subdivisions of the lower ordovician. At Matlock 

 we have a<,fain upper ordovician graptolites such as Dicellograptus 

 morrisi, and Diplograptus foliaceus. It is therefore probable 

 that while the ordovician series was being deposited, denudation 

 was destroying the land area of Heathcotian rocks. The trans- 

 gressions of the ordovician sea carried the higher part of the ordo- 

 vician rocks round the southern flank of the Mount William and 

 Lancefield Ranges across the line that formerly connected them 

 to the Heathcotian rocks near Geelong. In the neighborhood of 

 Melbourne the rocks of the silurian and ordovician systems are 

 not, so far as I know, exposed in actual contact ; for the junction 

 is covered by the basalt sheets. Yet it is possible that the 

 Melbouinian rocks rest unconformably upon the upper ordo- 

 vician. 



Again, east of Melbourne, in the basin of the Upper Yarra, 

 and on the main divide near Mount Matlock, there are some 

 upper ordovician i-ocks ; they show that the ordovician trans- 

 gression had carried the sea into the area occupied by land 

 throughout the lower ordovician. 



VIII. — The Subdivision of the Silurian System. 



The relation of the ordovician and silurian rocks in Victoria 

 raises the question of the silurian succession in Victoria. Surprise 

 has often been expressed at the absence of clear junctions 

 between the rocks of those two systems or of the superposition 

 of the silurian upon the ordovician. But, as we have seen, this 

 ditficulty is explained by the geology of Heathcote. 



The silurian rocks of Victoria occur in three distinct types. 

 There is a series of coarse grained shore deposits which are best 

 shown at Mount Ida, near Heathcote. They consist of coarse- 

 grained conglomerates, grits and sandstones. The second type 

 consists of alternations of shale and sandstone. It is well 

 developed around Melbourne, near which the beds are often 

 intensely contorted. The third type consists of a series of 

 lenticular masses of limestone, associated with sandstone and 

 shales ; they occur at Lillydale, Loyola, Cape Liptrap and 

 the Thomson Kiver. The relation of these three lithological 

 divisions of the silurian is obscured by the intense folding of 

 certain bands. These contortions are especially well shown near 



