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 strings, 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.
Pages:
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133