type, and have been built up in a different
manner, from those represented by Etna, Vesuvius, and most of the
extinct volcanoes of Auvergne, the Eifel, and of other districts
considered in these pages.

Let us now endeavour to picture to ourselves the stages through which
the moon may be supposed to have passed from the time her surface began
to consolidate owing to the radiation of her heat into space; for there
is every probability that some of the craters now visible on her disk
were formed at a very early period of her physical history.

When the surface began to consolidate, it must also have contracted; and
the interior molten matter, pressed out by the contracting crust, must
have been over and over again extruded through fissures produced over
the solidified surface, until the solid crust extended over the whole
lunar surface, and became of considerable thickness.

It is from this epoch that, in all probability, we should date the
commencement of what may be termed "the volcanic history" of the moon.
We must bear in mind that although the moon's surface had become solid,
its temperature may have remained high for a very long period. But the
continuous radiation of the surface-heat into space would produce
continuous contraction, while the convection of the interior heat would
tend to increase the thickness of the outer solid shell; and this, ever
pressing with increasing force on the interior molten mass, would result
in frequent ruptures of the shell, and the extrusion of molten lava
rising from below. Hence we may suppose the fissure-eruptions of lava
were of frequent occurrence for a lengthened period during the early
stage of consolidation of the lunar crust; but afterwards these may be
supposed to have given place to eruptions through pipes or vents,
resulting in the formation of the circular craters which form such
striking and characteristic objects in the physical aspect of our
satellite.[14]

It is not to be supposed that the various physical features on the lunar
surface have all originated in the same way. The great ranges of
mountains previously described may have originated by a process of
piling up of immense masses of molten lava extruded from the interior
through vents or fissures; while the great hollows (or "seas") are
probably due to the falling inwards of large spaces owing to the escape
of the interior lava.

But it is with the circular craters that we are most concerned. Judging
from