The most natural idea, and the one to which we
should be led by analogy, is to suppose these regions to represent huge
swamps, in which the variation in depth of the water produces the
diversity of colors. Yellow would predominate in those parts where the
depth of the liquid layer was reduced to little or nothing, and brown,
more or less dark, in those places where the water was sufficiently deep
to absorb more light and to render the bottom more or less invisible.
That the water of the sea, or any other deep and transparent water, seen
from above, appears more dark the greater the depth of the liquid
stratum, and that the land in comparison with it appears bright under
the solar illumination, is known and confirmed by certain physical
reasons. The traveler in the Alps often has occasion to convince himself
of it, seeing from the summits the deep lakes with which the region is
strewn extending under his feet as black as ink, whilst in contrast with
them even the blackest rocks illumined by the sunlight appeared
brilliant.[B]
Not without reason, then, have we hitherto attributed to the dark spots
of Mars the part of seas, and that of continents to the reddish areas
which occupy nearly two-thirds of all the planet, and we shall find
later other reasons which confirm this method of reasoning. The
continents form in the northern hemisphere a nearly continuous mass, the
only important exception being the great lake called the Mare Acidalium,
of which the extent may vary according to the time, and which is
connected in some way with the inundations which we have said were
produced by the melting of the snow surrounding the north pole.
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