ings, or disseminated through
the body of the limestone in the shape of crystals, scales, or
irregular masses. The amount of graphite in some parts of the
Lower Laurentian is so great that it has been calculated as equal
to the quantity of carbon present in an equal thickness of the
Coal-measures. The general source of solid carbon in the crust
of the earth is, however, plant-life; and it seems impossible to
account for the Laurentian graphite, except upon the supposition
that it is metamorphosed vegetable matter. (5) Lastly, the great
beds of iron-ore (peroxide and magnetic oxide) which occur in the
Laurentian series interstratified with the other rocks, point
with great probability to the action of vegetable life; since
similar deposits in later formations can commonly be shown to
have been formed by the deoxidising power of vegetable matter
in a state of decay.

In the words of Principal Dawson, "anyone of these reasons might,
in itself, be held insufficient to prove so great and, at first
sight, unlikely a conclusion as that of the existence of abundant
animal and vegetable life in the Laurentian; but the concurrence
of the whole in a series of deposits unquestionably marine, forms
a chain of evidence so powerful that it might command belief
even if no fragment of any organic or living form or structure
had ever been recognised in these ancient rocks." Of late years,
however, there have been discovered in the Laurentian Rocks certain
bodies which are believed to be truly the remains of animals,
and of which by far the most important is the structure known
under the now celebrated name of _Eozooen_. If truly organic, a
very special and exceptional interest attaches itself to _Eozooen_,
as being the most ancient fossil animal of which we have any
knowledge; but there are some who regard it really a peculiar
form of mineral structure, and a severe, protracted, and still
unfinished controversy has been carried on as to its nature. Into
this controversy it is wholly unnecessary to enter here; and it
will be sufficient to briefly explain the structure of _Eozooen_,
as elucidated by the elaborate and masterly investigations of
Carpenter and Dawson, from the standpoint that it is a genuine
organism--the balance of evidence up to this moment inclining
decisively to this view.

[Illustration: Fig. 22.--Fragment of _Eozooen_, of the natural
size, showing alternate laminae of loganite and dolomite. (After
Dawson.)]

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