34 Geology and Physical Geography; 
but ceased .it more abruptly defined limits, while the partial meta- 
morphism of the former did not extend very far outside the limits 
of complete transmutation. 
Further investigation would no doubt show the existence of 
various stages, intermediate between “ regional ” and “contact” 
metamorphism, among the zones of altered rocks surrounding 
grauito areas lying between the extreme eastern and western 
metamorphic areas of Victoria; but, in the present state of know- 
ledge, only the two distinctions already noted can bo indicated 
with any degree of certainty. 
The Unaltered Lower Silurian Rocks. 
The term “unaltered” is here used simply to designate those 
Silurian rocks which do not exhibit in their structure the distinct 
effects, as above described, of “ regional” or “contact” metamor- 
phisra. 
The Lower Silurian rocks prevail westward of a lino from Mel- 
bourne to Ileathcote, and form the “ bed-rock” or 41 bottom” of all 
the western gold-fields. On these rocks lie the most extensive 
and richest auriferous gravels, and traversing them arc the gold- 
bearing quarfz reefs to which Victoria so largely owes her present 
high position as a mining country. 
The leading petrographrcal features of the Lower Silurian 
strata are their frequent sharp alternate anticlinal and synclinal 
undulations; their strike, which varies in bearing from magnetic 
north to north-west, and the invariably high rate of easterly or 
westerly incline of the bands at angles of from 60° to vertical. 
Lithologically, sandstones, slates, shales, aud mudstones constitute 
the prevailing types, aud of these there are many varieties aud 
blendings of character. The sandstones vary from hard, dense 
quartzose, and gritty, line and coarse to soft and argillaceous, the 
latter being usually more or less micaceous. They occur in bands 
of from less than an inch to many feet in thickness, sometimes 
distinctly schistose and sometimes exhibiting little or nolamination. 
At the surface their prevailing colours are different shades of 
brown, grey, and sometimes yellow, reddish, or nearly white; but 
below water-level they are dark-grey or blue, sometimes nearly 
black, and occasionally of a dark-greenish colour. Bands of hard, 
dense quartzite or quartz-rock, as distinguished from vein quartz, 
are frequent in the Whipstick country, north-west from Sand- 
hurst, and also, more rarely, in other localities. Conglomerates 
and breccias aro very rare. A slate conglomerate is noted on 
one of the geological maps as occurring at Spring Plains, and a 
quartzite breccia is described by Professor Ulrich as occurring 
near Maldon. 
The slates, shales, and mudstones exhibit as many degrees of 
texture, hardness, and colour as the sandstones. At the surface 
