METAMORPHIC ROCKS pi 
It is, in fact, an indurated argillaceous sandstone, vary- 
ing considerably in texture. For purposes of investigation, 
it is better to obtain typical Slate-rocks—z.e., those showing 
well-defined fissile structure. Observe the bluish - gray 
colour of the specimen and its dull surface. Slates have this 
dull appearance, unless highly polished or wet. Draw 
attention to the term slate colour, 
Take a piece of Slate and a piece of Shale of nearly 
equal weight : carefully weigh these, and place them in a 
vessel of water at the beginning of the lesson. 
Notice the readiness with which the slate-mass splits into 
parallel layers. Observe the thickness of these, and make 
out that the rock is composed of a great many very thin 
folia or leaves between which run the planes of cleavage 
—that in fact it is fissile. 
Break the rock across, 1.¢., at right angles to the planes 
of cleavage, and notice the difficulty of doing so, as also 
the projecting edges of the exposed folia. 
Test for hardness and determine by comparing with 
other rocks that Slate is relatively soft. Discover, by trial, 
that the hardness often varies with the specimens, some 
kinds being much softer than others. Which is harder— 
the school slate or the pencil which is used to write on 
it? Why ? 
Grind down a piece of the rock and show that, like dry 
clay, it can be reduced to a fine powder, Compare this 
with the powder obtained by a similar treatment of Shale. 
Mix the powdered Slate with a little water and notice that 
it forms a kind of clay. Both Slate and Shale are of 
sedimentary origin, and both were originally composed of 
the same material. The alteration in the composition 
and arrangement of the mineral contents, whereby Shale 
has been converted into Slate, is the result of meta- 
morphism, 
