344 Ernest W. Skcats: 



which lie on the eastern tlanks of the diabase and srranitic rocks 

 of Heathcote as altered Ordovician rocks, and think that they 

 probably continue for s^nne distance unknown underr.eath the 

 Silurian sediments. 



South of Lancefield, and again north of Keilor, the igneous 

 rocks are not represented at the surface, and the Ordovician 

 and Silurian series come into direct relatinn with .each other. 

 Although no precise contact of the two series has been observed, 

 it is probable thait the- Ordovicians pass underneath the Silurians, 

 and that the latter are laid down unconformably upon them. 

 I think the lineal development of the diabases at the surface may 

 be due to their being brought up in or near the axis of a big 

 fold of the Ordovician series, and if so, the Ordovicians may 

 continue eastwards for some distance beneath the Silurian rocks. 

 Of course a pre-Ordovioian series must ha,ve provided the bulk 

 of the detrital material from which the Ordovician sediments 

 have been furmed, and such a series may be represented under- 

 neath the Silurian series, but of its positicm we have, I think, 

 no positive evidence. No Middle or Upper Ordovician rocks are 

 known near the Heathcote district. It would appear that move- 

 ments of elevation took place, and probably during the Middle 

 and Upper Ordovician periods this was a land area. The ex- 

 posure of the harder igneous rocks at the surface would lead to 

 their forming a ridge which, where developed, probably defined 

 the western shore line of the Silurian sea, along which coarse 

 conglomerates containing cherts and diabase fragments were 

 hiid down. 



(f) Correlation of the Heatlicote Rocks with Otlicr Areas. — 

 Professor ( iregory and Mr. Dunn have correlated with the rocks of 

 Heathcote somewhat similar diabasic and cherty rocks from a num- 

 ber of other localities. References to these papers is given at tlie 

 commencement of this paper. In most of the cases cited the 

 rocks in question are either surrounded by rocks of much newer 

 age or by rocks whose age is unknown. In such cases the 

 lithological resemblance of basic rocks, and sometinu s cherts, to 

 the Heathcote ssries is considerable, and quite pissibly may 

 imply a similarity in age, but I think such a correlation, in the 

 absence of stratigraphical rehitions, ^hould l)e cautiously applied, 

 especially in .view of the recent work of Mr. Thick',- in which 



