permitted to rest upon a
slab of the coarse slate in contact with it; when the fine 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 expected, often rolled up into nodular masses, carried
forward, 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. Now these spots are composed of the finer mud, and
they could 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 them, on the cleavage surface, in broad
round patches. But turn the slate edgeways and the section of each
nodule is seen to be a sharp oval with its longer axis parallel to the
cleavage. This instructive fact has been adduced by Mr. Sorby. I
have made excursions to the quarries of Wales and Cumberland, and to
many of the slate yards of London, and found the fact general. Thus
we elevate a common experience of our boyhood into evidence of the
highest significance as regards a most important geological problem.
From the magnetic deportment of these slates, I was led to infer that
these spots contain a less amount of iron than the surrounding dark
slate. An analysis was made for me by Mr. Hambly in the laboratory of
Dr. Percy at the School of Mines with the following result:

ANALYSIS OF SLATE.

Dark Slate, two analyses.

1. Percentage of iron 5.85

2. Percentage of iron 6.13

Mean 5.99

Whitish Green Slate.

1. Percentage of iron 3.24

2. Percentage of iron 3.12

Mean 3.18

According to these analyses the quantity of iron in the dark slate
immediately adjacent to the greenish spot is nearly double the
quantity contained in the spot itself. This is about the proportion
which the magnetic experiments suggested.

Let me now remind you that the facts brought before you are
typical--each is the representative of a class. We have seen shells
crushed; the trilobites squeezed, beds contorted, nodules of greenish
marl flattened; and all these sources of independent testimony point
to one a