ta.' [Footnote: Transactions of the Geological Society, ser. ii,
vol. iii. p. 477.]

The utterance of such a man struck deep, as it ought to do, into the
minds of geologists, and at the present day there are few who do not
entertain this view either in whole or in part. [Footnote: In a letter
to Sir Charles Lyell, dated from the Cape of Good Hope February 20,
1836, Sir John Herschel writes as follows: 'If rocks have been so
heated as to allow of a commencement of crystallisation, that is to
say, if they have been heated to a point at which the particles can
begin to move amongst 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. Now 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.'] The
boldness of the theory, indeed, has, in some cases, caused speculation
to run riot, and we have books published on the action of polar forces
and geologic magnetism, which rather astonish those who know something
about the subject. According to this theory whole districts of North
Wales and Cumberland, mountains included, are neither more nor less
than the parts of a gigantic crystal. These masses of slate were
originally fine mud, composed of the broken and abraded particles of
older rocks. They contain silica, alumina, potash, soda, and mica
mixed mechanically together. In the course of ages the mixture became
consolidated, and the theory before us assumes that a process of
crystallisation afterwards rearranged the particles and developed in
it a single plane of cleavage. Though a bold, and I think
inadmissible, stretch of analogies, this hypothesis has done good
service. Right or wrong, a thoughtfully uttered theory has a dynamic
power which operates against intellectual stagnation; and even by
provoking opposition is eventually of service to the cause of truth.
It would, however, have been remarkable if, among the ranks of
geologists themselves, men were not found to seek an explanation of
slate-cleavage involving a less hardy assumption.

The first step in an enquiry of this kind is to seek facts. This has
been done, and the labours of Daniel Sharpe (the late President of the
Geological Society, who, to the loss of science and the sorrow of all
who kn