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coarse layer is found bent by the pressure into sinuosities
like a contorted ribbon. Mr. Sorby has described a striking
case of this kind. This crumpling can be experimentally
imitated; the amount of compression might, moreover, be
roughtly estimated by supposing the contorted bed to be
stretched out, its length measured and compared with the
shorter distance into which it has been squeezed. We find
in this way that the yielding of the mass has been consider-

Let me now direct your attention to another proof of
pressure; you see the varying colors 'which indicate the
bedding on this mass of slate. The dark portion is gritty,
being composed of comparatively coarse particles, which,
owing to their size, shape, and gravity, sink first and con-
stitute the bottom of each layer. Gradually, from bottom
to top the coarseness diminishes, and near the upper sur-
face we have a layer of exceedingly fine mud. It is the
mud thus consolidated from which are derived the German
razor-stones, so much prized for the sharpening of surgical
instruments. When a bed is thin, the fine white mud is
permitted to rest upon a slab of the coarser slate in contact
with it: when the bed is thick it is cut into slices, which
are cemented to pieces of ordinary slate, and thus'rendered
stronger. The mud thus depOsited is, as might be ex-
pected, often rolled up into nodular masses, carried for:
ward, and deposited among coarser material by the rivers
from Which the slate-mud has subsided. Here are such
nodules enclosed in sandstone. Everybody, moreover, who
has ciphered upon a school-slate must remember the whitish-
green spots which sometimes dotted the surface of the
slate, and over which the pencil usually slid as if the spots
were greasy. N ow these spots are composed of the finer
mud, and theycould not, on account of their fineness, bite
the pencil like the surrounding gritty portions of the slate.
Here is a beautiful example of these spots: you observe

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