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Geology and Physical Geography : 
On t-lie other hand, the dykes of diorite and allied rocks are 
described by Professor Ulrich among “rocks in which the pre- 
dominating felspar is tricliiiic (plagioclase),” as being “ essentially 
compounds of oligoclase and hornblende, occasionally associated 
with quartz and mica/ 9 and “also known under the general term, 
‘Greenstone’ — Hornblendic Greenstone, in contradistinction to 
the Augitic Greenstone (diabase), not yet observed in Victoria.” 
These dykes, which will be further referred to in the chapter on 
auriferous reefs, &c., constitute marked and highly important 
mining features in the great central mass of Upper Silurian 
country between the meridian of Melbourne and that of the 
Macalister River, as with them are associated many of the auri- 
ferous quartz veins of that tract of country. They vary from a 
few inches to several hundred feet in thickness, and sometimes this 
variation takes place suddenly, as in the case of the Morning Star 
Dyke, at Wood’s Point, which in one place suddenly expands from 
a few feet to nearly 300 feet in width. These dykes run some- 
times with and sometimes across the strike of the Silurian rocks; 
thus, for instance, in the Walhalla and Wood’s Point district they 
run north-westerly with the strike of the Silurian strata, while in 
the Alexandra district they have a direction often nearly east and 
west at right angles to, or obliquely across, the strike of the 
Silurian rocks. They are sometimes found risiug in pinnacles or 
shoots at intervals along some general course, with only the 
“ track ” of a line of fissure between. Near the surface they are 
usually soft and decomposed into a brownish earthy rock; but, as 
a general rule, become hard and undecornposed at any considerable 
depths. In mineral composition and texture great variations are 
frequently observable at short distances apart in the same dyke. 
Some are coarsely granular and highly hornblendic, or with 
hornblende and felspar distinctly recognisable ; others are almost 
structureless (aphauitic), or felstone-like in their character. 
Quoting from Professor Ulrich’s notes in the Technological 
Museum Catalogue — “The extensive dyke of Gaffney’s Creek, and, 
in a less degree, the dyke of Cohen’s Reef, are in some parts dis- 
tinctly crystalline granular, i.e., show a granitic texture, with 
triclinic felspar and hornblende plainly perceptible, whilst for the 
greater part opened they are micro-crystalline granular, apparently 
quite dense, light greenish grey — felstone-like — with neither 
hornblende nor felspar recognisable. In fact, if it was not for 
the occasional changes in texture and composition just mentioned, 
the rock would sooner be taken to bo a true felstonc than as 
belonging to diorite, representing the variety felspathic diorite- 
aphanite. A fine illustration of both a textural and mineralogical 
change combined is exhibited by the dyke on which the Thomson 
River copper mine has been opened. The copper-ore-bearing part 
is hero a coarse granular highly hornblendic diorite — in fact, 
