Trappean Bocks. 
23 
^‘trap” and the adjacent granites, while in some places a gradual 
passage from one to the other, as regards mineral composition, is 
observable, and the different forms appear to blend with one another 
as though they were simply rocks of varying mineralogical struc- 
ture belonging to the same general mass. The trappean area of 
Mount Macedon, on the Main Divide, as shown upon the geological 
map of the locality, is partly bounded on the east by granite, ap- 
parently not presenting any abnormal mineralogical character, 
judging from the notes upon the map, which indicate coarse and 
fine-graiued varieties, with large and small plates of mica, and, 
occasionally, schorl. Between this granite and the <( trap” only 
an approximate boundary line is indicated. The rock of Mount 
Macedon itself, and of several other places in the same area, is 
described in the notes upon the map as “ dark-grey felspar trap, 
granular, and not at all porphyrifcic.” Mount Diogenes, another 
point on the Main Divide, north-eastward from Mount Macedon, 
is described as a dome-shaped mass of rock of a porphyritic 
character, i.e., containing disseminated isolated crystals. A speci- 
men from here is described, in a catalogue published by Mr. 
Selwyn in 1868, as consisting of a light-coloured granular base, 
with black specks, probably hornblende, and enclosing crystals of 
orthoclaso felspar and quartz. A general note describes the “ trap” 
of the northern portion of the area as displaying “a soft ash- 
coloured earthy and amygdaloidal character, whilst that of the 
southern (near the granite) is dark -grey, granular, and crystal- 
line.” Other varieties of “felspar trap,” “felspar porphyry,” and 
“ granitic porphyry,” are described in Mr. Selwyn’s catalogue as 
having been obtained from the Mount Macedon Range, and from 
neighbouring trappean areas. Specimens lately examined show a 
mineralogical composition, justifying the application of the term 
“syenite porphyry” to some of the Mount Macedon rocks. 
The “trap” of the Dandenong Ranges is shown on the Geo- 
logical Sketch-map as being bounded on the south by granite, 
and on the other sides by Upper Silurian rocks. Specimens of 
“ternary granite” aud syenitic porphyry, from near Dandenong, 
and of “ micaceous felspar trap,” “felspar porphyry,” and “syenitic 
felspar porphyry,” from the Dandenong Ranges, are described in 
Mr. Selwyn’s catalogue. 
The Upper Silurian rocks near Lillydale show considerable 
metamorphism near their contact with the “ trap” in the northern 
part of the Dandenong Range, indicating a heated condition of the 
trappean mass subsequent to the deposition of the Silurian strata. 
The great trappean area, of which Mount Juliet is the eeutre, 
and which also includes portion of the Main Divide, between 
Mount St. Leonard and Mount Arnold, is more or less bordered by 
granite, which forms intervening belts between it and the Silurian 
•rocks. 
