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SLATES. 8 91

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 or SLATE.
Dar/c Slate, two Analyses.

1. Percentage of iron ...................... 5.85
2. “ “ ............... . ...... 6.13
Mean. . . . 5.99

Whitislz-green Slate.
1. Percentage of iron. . . . .................. 3.24
2. “ “ . . . . .................. 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 experi-
ments 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 unhappy trilobites squeezed,
beds contorted, nodules of greenish marl flattened ; and all
these sources of independent testimony point to one and
the same conclusion, namely, that slate-rocks have been

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