SLATY CLEAVAGE. 
591 
to the evapoi’ation of water, or to a change of temperature. 
It is well known that many sandstones and other rocks ex¬ 
pand by the application of moderate degrees of heat, and 
then contract again on cooling; and there can be no doubt 
that large portions of the earth’s crust have, in the course 
of past ages, been subjected again and again to very differ¬ 
ent degrees of heat and cold. These alternations of tempera¬ 
ture have probably contributed largely to the production of 
joints in rocks. 
In many countries where masses of basalt rest on sand¬ 
stone, the aqueous rock has, for the distance of several feet 
frgm the point of junction, assumed a columnar structure 
similar to that of the trap. In like manner some hearth¬ 
stones, after exposure to the heat of a furnace without being 
melted, have become prismatic. Certain crystals also acquire 
by the application of heat a new internal arrangement, so as 
to break in a new direction, their external form remaining 
unaltered. 
Crystalline Theory of Cleavage. —Professor Sedgwick, speak¬ 
ing of the planes of slaty cleavage, where they are decidedly 
distinct from those of sedimentary deposition, declared, in 
the essay before alluded to, his opinion tliat no retreat of 
parts, no contraction in the dimensions of rocks in passing to 
a solid state, can account for the phenomenon. He accord¬ 
ingly referred it to crystalline or polar forces acting simulta¬ 
neously, and somewhat uniformly, in given directions, on 
large masses having a homogeneous composition. 
Sir John Herschel, in allusion to slaty cleavage, has sug¬ 
gested that “ if rocks have been so heated as to allow a 
commencement of crystallization—that is to say, if they have 
been heated to a point at which the particles can begin to 
move among themselves, or at least on their own axes, 
some general law must then determine the position in which 
these particles will rest on cooling. Probably, that position 
will have some relation to the direction in which the heat 
escapes. IN'ow, when all, or a majority of particles of the 
same nature have a general tendency to one position, that 
must of course determine a cleavage-plane. Thus we see the 
infinitesimal crystals of fresh-precipitated sulphate of barytes, 
and some other such bodies, arrange themselves alike in the 
fluid in which they float; so as, when stirred, all to glance 
with one light, and give the appearance of silky filaments. 
Some sorts of soap, in which insoluble margarates* exist, ex- 
* Margaric acid is an oleaginous acid, formed from different animal and 
vegetable fatty substances. A margarate is a compound of this acid Avith 
soda, potash, or some other base, and is so named from its pearly lustre. 
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