Victorian Igneous Bocks. ' 287 



Tliu (li;il)ases of Heathcote have been fully (lesci-ihed l)y Dr. 

 Howitt (8), I'rofessor Gregory (3) and Professor Skeats (i). ami 

 the age of these rocks and similar types at Lancefield is now 

 generally accepted as being between Middle Cambrian and basal 

 Lower Ordovician. The Lower Ordovician rocks only occin- at th^ 

 surface to the west of the Heathcote-Lancefield line, except in the 

 Mornington Peninsula, and Professor Gregory considered that the 

 area in central Victoria now occupied by Silurian sediments was 

 land surface during the deposition of the Ordovician Rocks, and 

 that the Heathcote-Lancefield line represented the eastern limit of 

 the Lower Ordovician. 



Professor Skeats has shown that the cherts and dia))ases are 

 interbedded, and that the former pass out conformably into normal, 

 sediments. The cherts have been shown to be due, at least in part, 

 to the silification of stratified ash, and the inference is that this asln 

 was either derived from submarine volcanoes or from a series of 

 shore line volcanoes. 



In the former case marine conditions must have extended east- 

 ward of this Heathcote-Lancefield line, and Lower Ordovician 

 rocks should underlie the Silurian. If the latter supposition be 

 correct, then this line may well represent the eastern limit of 

 deposition. At present there is no positive evidence in favour of 

 either of these views, but some negative evidence in favour of the 

 latter, viz., the absence in this locality of known occurrences of 

 Lower Ordovician sediments to the east of Heathcote, but the thick 

 covering of Silurian rocks would completely mask any possible 

 outcrop, so that this evidence is of very little value. 



In either case, however, these eruptions of basic lava are more- 

 easily explained as accompanying fault action rather than fold 

 action. The linear arrangement of the diabase in a north and 

 south direction, giving rise to the Colbinabbin Range, suggests dis- 

 tribution along a fault line. Another possible explanation is that 

 the linear arrangement is entirely due to the diabases and cherts 

 occurring along the upper portion of an anticline. 



Along the western limit of the outcrops in Victoria of Lower 

 Ordovician sediments similar associations of diabases and cherts 

 are found at Mount Stavely and at the Hummocks to the north of 

 Casterton. The simplest explanation is that the Lower Ordovician 

 marine transgression was caused by subsidence between two fault 

 lines, i.e., the earth movements were of the Atlantic coast type. 

 Movements of the Pacific coast type can give rise to a linear 

 arrangement of the associated volcanoes, as evidenced in the great 



