82 
SLATE. 
and dry it in the sun, we find that it will split into 
similar shapes. In like manner, sandstones and 
other rocks expand by the application of heat, and 
contract again on coohng; and it is supposed by 
geologists that large portions of the earth's crust 
have, in the course of past ages, been subjected 
again and again to very different degrees of heat and 
cold, which alterations of temperature have prob- 
ably caused the phenomenon in question. As such 
a structure, however, is most conspicuous among 
rocks of the finest texture, it is supposed by some 
that it is owing to the influence of crystallization. 
In proof of this, Scoresby states that the icebergs 
of Spitzbergen are full of rents, extending perpen- 
dicularly downward, dividing them into innumerable 
columns. On the other side, however, it is stated, 
that in Saxony, where masses of basalt rest on 
sandstone, the aqueous rock has, for the distance of 
several feet from the point of junction, assumed a 
columnar structure similar to that of the trap ; also, 
that hearthstones, after exposure to the heat of a 
furnace, without being melted, have become pris- 
matic. The most probable conclusion is, that these 
fissures are the result of different causes, such as 
some modification of crystalline action, or simple 
contraction during consolidation or during a change 
of temperature. Nearly all the metallic ores have 
been found in slate, particularly lead and copper. 
Carbonaceous matter first begins to appear in the 
slate rocks, there being none in the primary. Draw- 
ing-slate has about 11 per cent, of carbon. This 
rock is also the oldest that contains any organic re- 
mains. 
2. Transition Limestone, 
Transition limestone differs from the primitive in 
being less crystallized in its structure, more com- 
pact and fine grained, more variegated as to colour 
being white, red, brown, gray, black, or streaked. It 
