llisation, and have had placed before you the
facts which are found associated with the cleavage of slate rocks.
Such facts, as expressed by Helmholtz, are so many telescopes to our
spiritual vision, by which we can see backward through the night of
antiquity, and discern the forces which have been in operation upon
the earth's surface

Ere the lion roared,
Or the eagle soared.

From evidence of the most independent and trustworthy character, we
come to the conclusion that these slaty masses have been subjected to
enormous pressure, and by the sure method of experiment we have
shown--and this is the only really new point which has been brought
before you--how the pressure is sufficient to produce the cleavage.
Expanding our field of view, we find the self-same law, whose
footsteps we trace amid the crags of Wales and Cumberland, extending
into the domain of the pastrycook and ironfounder; nay, a wheel cannot
roll over the half-dried mud of our streets without revealing to us
more or less of the features of this law. Let me say, in conclusion,
that the spirit in which this problem has been attacked by geologists,
indicates the dawning of a new day for their science. The great
intellects who have laboured at geology, and who have raised it to its
present pitch of grandeur, were compelled to deal with the subject in
mass; they had no time to look after details. But the desire for more
exact knowledge is increasing; facts are flowing in which, while they
leave untouched the intrinsic wonders of geology, are gradually
supplanting by solid truths the uncertain speculations which beset the
subject in its infancy. Geologists now aim to imitate, as far as
possible, the conditions of nature, and to produce her results; they
are approaching more and more to the domain of physics, and I trust
the day will soon come when we shall interlace our friendly arms
across the common boundary of our sciences, and pursue our respective
tasks in a spirit of mutual helpfulness, encouragement and goodwill.

[I would now lay more stress on the lateral yielding, referred to in
the footnote concerning Mr. Warren De la Rue's attempt to produce
finely granular white-lead, accompanied as it is by tangential
sliding, than I was prepared to do when this lecture was given. This
sliding is, I think, the principal cause of the planes of weakness,
both in pressed wax and slate rock. J. T. 1871.]

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XIII. ON