`~ PRIMARY OR PALÆOZOIC ROCKS. 33 

PRIMARY OR PALASOZOIC ROCKS. 
LOWER PALAOZOIC. 

SILURIAN. 
(METAMORPHIC AND UNALTERED.) 
No formation that can be identified clearly as older than Lower 
Silurian has yet been recognized in Victoria, Perhaps, however, the 
rocks of some of the larger areas, mapped as metamorphic, represent a 
Cambrian or Laurentian series. In any case, much of the Lower Silurian 
is metamorphic, especially near the boundaries of granite masses. A 
large number of fossils have been collected, by the Geological Survey, 
from all parts of the colony, many of which are generically and 
specifically identical with those found in strata of Upper and Lower 
Silurian age in other countries. Specimens of some of them are exhi- 
bited in the Palontological collection, and figures and descriptions 
of the most interesting are being prepared by Professor McCoy for 
publication in the “ Memoirs of the Museum.” The geological sketch- 
map shows that the Silurian rocks (represented by grey or slate color) 
occupy a very large surface area: they also constitute, with a few local 
exceptions, the true “bottom” or “bed rock” of all the Victorian 
goldfields, as well as the matrix or bounding walls of every known metal- 
liferous vein, dyke, or reef; and they doubtless underlie, at a lesser or 
greater depth, a large portion of those tracts, where newer formations 
are found on the surface. ‘The depth to which they are covered, and 
the age of the overlying formation, will, in each case, approximately 
determine the probable limits of any deep alluvial gold lead, because 
when it (the Tertiary deposit, or formation, in which the gold lead occurs) 
passes on to such newer and non-auriferous rocks, the supply of gold is, 
as it were, cut off; and though the gravel continues and presents no 
apparent difference, the lead at once becomes unprofitable. The Lower 
Silurian argillaceous beds are characterized by a more or less slaty > 
and schistose structure, whilst the Upper consist mostly of jointed, 
rubbly, or concretionary shales and soft mudstones. Sandstones and 
sandy beds are common in both series. Only one limestone band (in 
the Upper) has yet been found, and even calcareous beds are rarely 
met with. Bands of conglomerate are also very rare, and the greater 
part of the formation seems to have been deposited in deep water. A 
. considerable unconformity exists between the Upper and Lower series, 
and the total thickness of both is, probably, not less that 80,000 feet. 
Building Stones.—Beds of freestone, generally a greenish-brown 
sandstone, are frequently met with, and are quarried for local building 
_ purposes. The stone can, however, seldom be procured of a uniform 
color and texture, in large quantity; and on this account, as well as 
from its being very subject to decay, when exposed to atmospheric action, 
- the Silurian sandstone is not suitable for any extensive architectural or 
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