360 TRANSACTIONS OF SECTION C, 
5. On the Tertiary Alkali Rocks of Victoria. 
By Professor Ernest W. Sxeats, D.Se. 
From Mount Leinster in Benambra, Frenchman’s Hill near Omeo, and Noyang 
in Dargo, three areas in Eastern Victoria, the late Dr. Howitt (1) described 
igneous rocks which helong to the alkali series. They were all regarded by 
Howitt as of Paleozoic age. The age of the rocks of Noyang, which consist 
mainly of intrusions and lava-flows of quartz-ceratophyre, has not been closely 
investigated and may be Paleozoic. Recent work (2), however, has shown, 
especially in the case of the Omeo rocks, that they are probably of mid- or even 
of late-Tertiary age. The alkali rocks of Frenchman’s Hill, described by 
Howitt as intrusive orthophyres, consist really in the main of lava-flows of 
anorthoclase trachyte which has a very scoriaceous margin to the flows. There 
is a central plug of a coarser quart7-bearing rock allied to sdlvsbergite and a 
more or less radial system of dykes which are principally trachytic in character. 
Some, however, contain quartz, one at least is a bostonite, and six or seven prove 
to be dykes of nepheline-phonolite. The district is one which has been affected 
by a succession of elevatory movements of the plateau type since the mid-Tertiary 
period, and, according to Griffith Taylor (3), a more or less meridional Senkungs- 
feld runs through the Omeo district a few miles east of Frenchman’s Hill. The 
rocks of Mount Leinster in Benambra consist principally of sélvsbergites, bos- 
tonites, and pyroclastic rocks of alkali trachyte. Petrologically and chemically 
many of the rocks of Mount Leinster and of Frenchman’s Hill closely resemble 
some of the alkali rocks of Mount Macedon, and, Jike them, are probably of mid- 
Tertiary age. The district has been elevated at intervals during the Tertiary 
period, but physiographically has not been closely studied. 
About fourteen miles north-east from Mansfield in north-central Victoria and 
about three miles from Tolmie, in the Tolmie Highlands, there occurs a volcanic 
hill, known locally as Gallows Hill, which has recently been shown to consist of a 
voleanic centre of probably late Tertiary age and to consist of lava-flows of 
nepheline-phonolite. From a locality near Barwite, east of Mansfield, another 
nepheline-phonolite has been found, but its field relations are at present uncer- 
tain and no account of either of these rocks has yet been published. Fenner (4) 
has recently shown that block elevation and depression have affected the Mans- 
field area in recent geological times, and that Gallows Hill lies near one of the 
fault scarps. 
The best-known area of alkali rocks in Victoria is the Mount Macedon district, 
about forty miles north-west of Melbourne (5). The series is of mid-Tertiary 
to late-Tertiary age, and the rock sequence from below upwards, while not always 
demonstrable, appears to be as follows :—Anorthoclase trachyte, sdlvsbergite, 
anorthoclase basalt, macedonite, woodendite, anorthoclase-olivine-trachyte, 
olivine-anorthoclase-trachyte, limburgite. Immediately succeeding these alkali 
rocks come lava-flows of normal basalt and of andesitic basalt. The new types 
macedonite and woodendite contain over 1 per cent. of P,O., and are related to 
the orthoclase-basalts and to the mugearites. 
While this part of Victoria shows evidence by the existence of more than one 
elevated peneplain of successive movements of the plateau type, no definite 
evidence of faulting or differential movement has heen recognised in the district. 
In the western district of Victoria more or less extensive lava-flows of anortho- 
clase-trachyte occur near Coleraine, Carapook, &c. (6). Generally the trachytes 
appear to be older than the newer basalts, but near Coleraine a dyke of trachyte 
penetrates a small hill composed of a basic rock resembling olivine-basalt, while 
at the Hummocks north of Casterton another trachyte dyke similarly penetrates 
a vent or small flow of olivine-basalt. Among the ejected blocks from the 
earlier members of the Pleistocene newer basalts of Lake Bullenmerri, near 
Camperdown, are some consisting of essexite and containing analcite. In the 
western district of Victoria clear evidence of comparatively recent elevatory 
movements is noticeable. No definite faults have yet been proved, however, 
and the normal basalts are much more widely spread than the alkali rocks. In 
view of Harker’s generalisation as to the close correspondence between the occur- 
rence of alkali rocks and elevatory movements of the plateau type, generally 
accompanied by faulting, the above reference to earth movements is pertinent. 
