a a oe 
. » 
or, Fh ; 
Parr il. § iii.] MACROSCOPIC CHARACTERS OF ROCKS. 91 
different minerals in varying proportions, they exhibit many inter- 
mediate varieties. Transitions of this kind are denoted by such 
phrases as “granitic gneiss,” that is, a gneiss in which the normal 
foliated structure is nearly merged into the massive structure 
of granite; “argillaceous limestone”—a rock in which the lme- 
stone is mixed with clay; “calcareous shale”—a fissile rock con- 
sisting of clay with a proportion of lime. It is evident that such 
rocks may graduate so insensibly into each other, that no sharp 
line can be drawn between them either in the field or in their 
terminology. 
3. State of Aggregation.—The hardness or softness of a rock, 
in other words, its induration and friability, or the degree of aggrega- 
tion of its particles, may be either original or acquired. Some rocks 
(sinters for example) are soft at first and harden by degrees; the 
general effect of exposure, however, is to loosen the cohesion of the 
particles of rocks. A rock which can easily be scratched with the 
nail is almost always much decomposed, though some chloritic and 
talcose schists are soft enough to be thus affected. Compact rocks 
which can easily be scratched with the knife, and are apparently 
not decomposed, may be fine grained limestones, dolomites, iron- 
stones, mudstones, or some other simple rocks. Crystalline rocks, as 
a rule, cannot be scratched with the knife unless considerable force 
be used. They are chiefly composed of hard silicates, so that when - 
an instance occurs where a fresh specimen can be easily scratched, 
it will generally be found to be a limestone (see § vii. p. 179). The 
ease with which a rock may be broken is the measure of its 
frangibility. Most rocks break most easily in one direction; atten- 
tion to this point will sometimes throw light upon their internal 
structure. 
Hracture is the surface produced when a rock is split or broken, 
and depends for its character upon the texture of the mass. Finely 
granular compact rocks are apt to break with a splintery fracture 
where wedge-shaped plates adhere by their thicker ends to, and lie 
parallel with, the general surface. When the rock breaks off into 
concave and convex rounded shell-like surfaces, the fracture is said 
to be conchoidal, as may be seen in obsidian and other vitreous rocks, 
and in exceedingly compact limestones. ‘The fracture may also be 
_ foliated, slaty, or shaly, according to the structure of the rock. Many 
opaque, compact rocks are translucent on the thin edges of fracture, 
and afford there, with the aid of a lens, a glimpse of their internal 
composition. A rock is said to be flinty, when it is hard, close- 
grained, and breaks with a smooth or conchoidal fracture like flint; 
friable, when it crumbles down like dried clay or chalk; plastic, when 
like moist clay it can be worked into shapes; pulverulent, when it 
falls readily to powder; earthy, when it is decomposed into loam or 
earth; zncoherent or loose, when its particles are quite separate, as 
in dry blown sand. 
4, Colour and Lustre.—These characters vary so much even 
