which they have evidently acquired long after.

Nay, more. These parallel planes of cleavage, at each of which the
slate splits freely, will run through a whole mountain at the same
angle, though the beds through which they run may be tilted at
different angles, and twisted into curves.

Now what has made this change in the rook? We do not exactly know.
One thing is clear, that the particles of the now solid rock have
actually moved on themselves. And this is proved by a very curious
fact--which the reader, if he geologises about slate quarries much,
may see with his own eyes. The fossils in the slate are often
distorted into quaint shapes, pulled out long if they lie along the
plane of cleavage, or squeezed together, or doubled down on both
sides, if they lie across the plane. So that some force has been at
work which could actually change the shape of hard shells, very
slowly, no doubt, else it would have snapped and crumbled them.

If I am asked what that force was, I do not know. I should advise
young geologists to read what Sir Henry de la Beche has said on it in
his admirable "Geological Observer," pp. 706-725. He will find
there, too, some remarks on that equally mysterious phenomena of
jointing, which you may see in almost all the older rocks; it is
common in limestones. All we can say is, that some force has gone
on, or may be even now going on, in the more ancient rocks, which is
similar to that which produces single crystals; and similar, too, to
that which produced the jointed crystals of basalt, i.e. lava, at the
Giant's Causeway, in Ireland, and Staffa, in the Hebrides. Two
philosophers--Mr. Robert Were Fox and Mr. Robert Hunt--are of opinion
that the force which has determined the cleavage of slates may be
that of the electric currents, which (as is well known) run through
the crust of the earth. Mr. Sharpe, I believe, attributes the
cleavage to the mere mechanical pressure of enormous weights of rock,
especially where crushed by earthquakes. Professor Rogers, again,
points out that as these slates may have been highly heated, thermal
electricity (i.e. electricity brought out by heat) may have acted on
them.

One thing at least is clear. That the best slates are found among
ancient lavas, and also in rocks which are faulted and tilted
enormously, all which could not have happened without a
proportionately enormous pressure, and therefore heat; and next, th