26 
Geology and Physical Geography : 
in some places undecom posed, and in others decomposed to a 
condition resembling the decomposed Older Volcanic rocks of the 
Tertiary period. On the slopes of the hills above, the rock appears 
slightly decomposed, and exhibits a curiously-blended mixture 
of portions of a dense aphanitic structure, with others of a porous 
vesicular character, almost resembling pumiccstone. 
North of Coleraine, one variety is a dark, douse, flinty, and 
exceedingly fine greenstoue or Labradorite dioritc. Another 
description is wackenitic in character, resembling an altered 
volcanic mud, rather soft, yellowish-brown, and highly felspathic, 
with porphyritically-inclnded crystals of felspar ; sometimes 
vesicular, and containing in the vesicular cavities a soft, greenish, 
clayey material (lithomarge). In its composition, this rock also 
approaches phonolite. 
At the Nigretta falls, on the Wannon, the rock is a typical 
quartz and felspar porphyry of a brownish-pink felsitic base, with 
embedded quartz and felspar crystals. At Cavendish, further 
eastward, is a dark greenish-grey, dense, and much-jointed rock, 
apparently aphanite, not showing porphyritic structure. From an 
outcrop in the Grampians, coloured as granite on the Geological 
Sketch-map, specimens were obtained and described, in Mr. A. 
E. C. Selwyn’s catalogue of 1868, as granite felspar-porphyry, 
syenitic porphyry, and felstone porphyry. So little is known of 
the geology of this portion of the colony, that a mere reference to 
the lithological character of the rocks is all that can at present he 
given. 
On the northern slopes of the Barrabool hills, west of Geelong, 
occurs a small area of what Professor Ulrich describes as “diallage 
rock 11 — a coarsely-crystalline granular mixture of light-green 
Labradorite, and of a dark-green augitic mineral, which, according 
to its lustre and cleavage, is diallage. This Plutonic mass is sur- 
rounded by Mesozoic rocks, hut is evidently older than the latter, 
from the fact of beds of Mesozoic conglomerate in the neighbour- 
hood containing pebbles of the diallage rock. On accounts of its 
colour and susceptibility to fine polish, this rock is well adapted 
for ornamental purposes, though very hard to work. 
The general conformity of the ordinary Lower Plutonic granites 
to ouo type, in all parts of the world, indicates a general similarity 
if not a synchronism of the conditions under which they attained 
their present structure. Their varieties in character and arrange- 
ment of mineral constituents may be justly attributed to— (1) local 
differences as regards the presence of those constituents in greater 
or less quantity, the entire absence of some of them, or the 
addition of otl/er than the common minerals ; (2) differences 
in the character of the sedimeutary rocks, which, in a trans- 
muted form, have entered into the composition of the granites ; 
(3) differences of heat, rates of cooling, intensity of chemical 
