Dyke Stones . 
115 
nearly pure hornblende rocks in places — which forms ‘ shoot-like’ 
the northern termination of the dyke, whilst southward in strike 
it changes within a very short distance through finely granular to 
a nearly dense highly felspathic aplianito, closely resembling the 
rock of the Cohen's Reef and Gaffney’s Creek dykes just spoken 
of.'’ 
Besides their ordinary constituent minerals these dyke stones are 
frequently more or less densely impregnated with copper, iron, 
and arsenical pyrites, evidently the products of natural chemical 
action subsequent to the injection of the dykes. 
All the dykes of the first group comprising the orthoclase and 
plagioclase felspar divisions are pretty clearly of Palaeozoic ago, 
and the portions we now see at the present surface may have been 
perhaps thousands of feet below the surface at the time the dykes 
were injected, owing to the vast denudation that has subsequently 
taken place. 
2nd group. — The dykes of the second group are all more or 
less of a basaltic character, resembling the Older and Newer 
Volcanic basalts. They are found iuterso iting the Mesozoic rocks 
of Bacchus Marsh and’ Western Port, frequently accompanying 
faults in the last-named locality. The “lava-streaks ” of Sandhurst 
are simply basalt dykes, the rock of which differs little, if any- 
thing in composition frem the Newer Volcanic basalt of the 
plains, except in being generally decomposed to a soft soapy clay. 
These dykes are to all appearance of Tertiary age, and do not 
appear to be in any way connected with the auriferous character of 
the lodes which they intersect. With respect to origin, many of the 
dykeB of the various classes referred to, especially the non-auri- 
ferous of the first group and the lava dykes of the second group, 
are clearly of Plutonic origin, i.e. y they were injected in a molten 
or plastic condition, as evidenced by the alteration of the rocks 
which they traverse at the planes of contact ; but in the cases of 
most of the auriferous dykes examined by me, and notably the 
typical one of Cohen’s Reef, at Walhalla, all signs of contact 
alteration are absent, the planes of demarcation between the dyke 
stones and the containing slates are clearly defined ; and yet there 
is no visible sign of alteration in the latter, such as would naturally 
have been effected had the dykes been injected in a molten state. 
I am led, therefore, to the belief that such dykes were in great 
measure the products of hydrothermal action, and that when 
injected into the fissures of the Silurian rocks they were rather in 
the condition of super-heated mud than of actual fusion ; that 
their development was due to mixed mechanical and chemical 
action, differing from the distinctly mechanical intrusion of the 
molten dyke masses on the one hand, and the chiefly chemical 
deposition of mineral vein-stones on the other. 
