18J PEESIDENT's address SECTION c. 



Gregory accepted Howitt's determinations with the exception of 

 that of the aphte, which he describes as a fine-grained granodiorite. 



Undoubtedly, intrusives, lavas and pyi'oclastic rocks are all repre- 

 sented, and I have agreed with Dunn that the bulk of the rocks 

 are tuffs, agglomerates, and lavas. I have suggested that Howitt's 

 aplite may be termed a microgranite in a broad sense, and that it 

 may represent the final most acid intrusive rock of the diabase series. 

 The term diabase is here used in a broad way to cover a number of 

 laore or less altered basic rocks, some of which are intrusive, while 

 naany are lavas and consolidated tuffs. Tlie minei*als of the diabase 

 include augite, enstatite, secondary hornblende, andesine to labra- 

 dorite among the felspars, quartz, sphene, ilmenite, and magnetite. 

 Secondary minerals besides hornblende ai-e calcite, chlorite, epidote, 

 albite, quartz, zeolites, chalcedony, &c. The bedded cherts, hitherto 

 regarded as altered sedimentaiy rocks, I have referred to as probably 

 in great part silicified submarine tuff. Some of the minerals of the 

 diabase series, volcanic fragments and Radiolaria have been recog- 

 nised in them. The foliated diabase of Red Hill (Howitt's diabase 

 schist or schalstein) is also fragmental, and maybe an altered subaerial 

 tuff. The alterations undergone by the diabase rocks are remarkable 

 and interesting. The tuffs have been foliated, and the bulk of the 

 rocks have been recrystailized. In many cases metasomatic changes 

 have been superimposed upon the purely structural and mineralogical 

 rearrangement of the rocks. Secondary silicification of the diabase is 

 in places complete. In one locality the foliated diabase is altered 

 almost completely to a mass of minute quartz crystals ; in other 

 places silicificat'on has replaced the diabase and produced red and 

 green and black jasperoids, and all stages of the process can be seen 

 in the field and in rock sections. There is little doubt that the 

 silicification was due to solutions passing through the rocks after the 

 consolidation of the diabase, and that the same solutions altered the" 

 bedded tuffs to black bedded cherts. 



In other places the diabase has been more or less completely 

 replaced by carbonates, while in another place it occurs as a mixed 

 carbonate and chert rock. 



A peculiar alteration at the margin of the diabase north of 

 Heathcote has given rise to a green substance named " Selwynite," 

 formerly thought to be a mineral, but now known to be a rock con- 

 sisting of a green chrome-bearing micaceous mineral, chromite, 

 pyroxene, and another highly birefringent micaceous mineral. 

 Corundum was found by me in association with the selwynite, and also 

 as pseudomorphs in one of the jasperised diabases. Much less is known 

 of the petrology of the other districts referred to. The Hummocks 

 and Moiuit Wellington areas consist of diabasic rocks more or less 

 completely altered and recrystallised as serpentine. At Mount Stavely 

 diabases and porphyritic rocks of a rather less basic type occur. The 

 rocks of Dookie, Tatong, Limestone Creek, and Nowa Nowa have not 

 yet been subjected to microscopic examination. 



There remain the Lancefield and Geelong areas. The rocks of the 

 aboriginal quarries of Moimt William, N. of Lancefield, are described 

 by Gregory (24) as amphibolites and impure nephrites. They repre- 

 sent diabases which have been completely recrystallised. The rocks 



