Lastly, carbon may occur as a conspicuous
constituent of rock-masses in the form of _graphite_ or _black-lead_.
In this form, it occurs in the shape of detached scales, of veins
or strings, or sometimes of regular layers;[7] and there can be
little doubt that in many instances it has an organic origin,
though this is not capable of direct proof. When present, at any
rate, in quantity, and in the form of layers associated with
stratified rocks, as is often the case in the Laurentian formation,
there can be little hesitation in regarding it as of vegetable
origin, and as an altered coal.
[Footnote 7: In the Huronian formation at Steel River, on the
north shore of Lake Superior, there exists a bed of carbonaceous
matter which is regularly interstratified with the surrounding
rocks, and has a thickness of from 30 to 40 feet. This bed is
shown by chemical analysis to contain about 50 per cent of carbon,
partly in the form of graphite, partly in the form of anthracite;
and there can be little doubt but that it is really a stratum
of "metamorphic" coal.]
CHAPTER III.
CHRONOLOGICAL SUCCESSION OF THE FOSSILIFEROUS ROCKS.
The physical geologist, who deals with rocks simply as rocks,
and who does not necessarily trouble himself about what fossils
they may contain, finds that the stratified deposits which form
so large a portion of the visible part of the earth's crust are
not promiscuously heaped together, but that they have a certain
definite arrangement.
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