Here too, we
may say that it is not only the mineralogical character of rocks, whether
they are crystalline, granular, or densely fossiliferous, but the
geometrical form of the Earth itself, which indicates the mode of its
origin, and is, in fact, its history. An elliptical spheroid of revolution
gives evidence of having once been a soft or fluid mass. Thus the Earth's
compression constitutes one of the most ancient geognostic events, as every
attentive reader of the book of nature can easily discern; and an analogous
fact is presented in the case of the Moon, the perpetual direction of whose
axes toward the Earth, that is to say, the increased accumulation of matter
on that half of the Moon which is turned toward us, determines the relations
of the periods of rotation and revolution, and is probably contemporaneous
with the earliest epoch in the formative history of this satellite. The
mathematical figure of the Earth is that which it would have were its
surface covered entirely by water in a state of rest; and it is this assumed
form to which all geodesical measurements of degrees refer. This
mathematical surface is different from that true physical surface which is
affected by all the accidents and inequalities of the solid parts.
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