32 STRATIFIED OR SEDIMENTARY ROCKS. 
conglomerate, breccia, limestone, &c.—is exclusively confined to, or 
characteristic of, any geological age or formation; and therefore the 
geological age of a deposit will in no case afford information respecting 
the nature of the rock or its petrographic character. ‘These characters 
are almost entirely dependent on circumstances, varying in each locality 
or country, and on the nature of the formation, from which the material, 
forming the rock, has been supplied, as well as on the physical influences, 
to which it may have been subjected since its original deposition. Thus, 
in the stratigraphical arrangement, adopted in this collection, specimens 
of sandstone, shale, conglomerate, breccia, limestone, ironstone, &c. 
occur in each geological period; and it may be observed, that some of 
the most recent Tertiary rock specimens are quite as hard, solid and 
compact as those, that belong to Secondary or Primary formations ; and 
others, differing widely in geological age, are almost identical in their 
composition, texture and appearance. ‘ 
Sandstone.—A typical sandstone may be described as consisting of 
small grains of some solid mineral, usually quartz, bound together into 
a solid rock, either by some cementing medium or by simple pressure. 
If the grains are fine, it is a fine-grained sandstone ; if coarse, a coarse- 
grained sandstone, &e. Tf the cementing medium is lime, it becomes a 
calcareous sandstone; if iron, a ferruginous sandstone, &c. As the 
grains increase in size, it becomes either a grit or a conglomerate. If 
the particles are angular, a breccia. 
Argillaceous Shale and Clay- Slate are laminated clay-rocks, formed 
from sediments of clayey mud, and have assumed their present form by 
a slow process of transmutation and mechanical consolidation. Admix- 
tures of other matter, either during decomposition or subsequently, 
frequently render them either arenaceous, carbonaceous, micaceous, 
ferruginous, calcareous, or bituminous, as the case may be. They may 
both be described as laminated fissile rocks; in the one instance this 
texture is due to original stratification, and in the other to slaty 
cleavage. Clay-slate, though also distinctly stratified, does not readily 
separate, except along the cleavage-planes, which are quite independent 
of its original bedding. It is not confined to any particular geological 
period, although the genuine clay-slates (roofing-slate, &c.) usually occur 
only in the older formations. There are, however, exceptions to this rule, 
and in the Swiss Alps genuine roofing-slates and also common arena- 
ceous and micaceous clay-slates are found belonging to the chalk and 
even to the Tertiary periods. The origin of slaty cleavage is still 
unsettled, but it is now generally supposed to be due to great lateral 
pressure, induced by forces in connection with the upheaval, disturbance 
and contortion of the rocks, in which it occurs. It is more or less a 
characteristic feature of all the Lower Silurian and older rocks of 
Victoria, but is seldom, if ever, observed in any formation in Victoria 
newer than Upper Silurian. 
Sedimentary rocks are frequently much changed by infiltration of 
mineral matter or other metamorphic action, that tends to obliterate their 
original mechanical, granular structure, and they assume a semi-crystal- 
line appearance, not unlike some rocks of undoubted igneous origin, and 
are then classed as Metamorphic Rocks, 

