periods, this name would be objectionable upon
theoretical grounds. If a general name be needed, that of "Eozoic"
(Gr. _eos_, dawn; _zoe_, life), proposed by Principal Dawson, is the
most appropriate. Owing to their metamorphic condition, geologists
long despaired of ever detecting any traces of life in the vast pile
of strata which constitute the Laurentian System. Even before any
direct traces were discovered, it was, however, pointed out that
there were good reasons for believing that the Laurentian seas had
been tenanted by an abundance of living beings. These reasons are
briefly as follows:--(1) Firstly, the Laurentian series consists,
beyond question, of marine sediments which originally differed
in no essential respect from those which were subsequently laid
down in the Cambrian or Silurian periods. (2) In all formations
later than the Laurentian, any limestones which are present can
be shown, with few exceptions, to be _organic_ rocks, and to be
more or less largely made up of the comminuted debris of marine
or fresh-water animals. The Laurentian limestones, in consequence
of the metamorphism to which they have been subjected, are so
highly crystalline (fig. 21) that the microscope fails to detect
any organic structure in the rock, and no fossils beyond those
which will be spoken of immediately have as yet been discovered in
them. We know, however, of numerous cases in which limestones,
of later age, and undoubtedly organic to begin with, have been
rendered so intensely crystalline by metamorphic action that
all traces of organic structure have been obliterated. We have
therefore, by analogy, the strongest possible ground for believing
that the vast beds of Laurentian limestone have been originally
organic in their origin, and primitively composed, in the main,
of the calcareous skeletons of marine animals. It would, in fact,
be a matter of great difficulty to account for the formation
of these great calcareous masses on any other hypothesis. (3)
The occurrence of phosphate of lime in the Laurentian Rocks in
great abundance, and sometimes in the form of irregular beds,
may very possibly be connected with the former existence in the
strata of the remains of marine animals of whose skeleton this
mineral is a constituent. (4) The Laurentian Rocks contain a
vast amount of carbon in the form of black-lead or _graphite_.
This mineral is especially abundant in the limestones, occurring
in regular beds, in veins or str