were formed as the mass gradually ceased to boil.
The later formed of these openings are the smaller, as would be the
case in such a slowing down of a boiling process.

As the diameter of the moon is only about one fourth of that of the
earth, its bulk is only about one sixteenth of that of its planet;
consequently, it must have cooled to the point of solidification ages
before the larger sphere attained that state. It is probable that the
same changeless face that we see looked down for millions of years on
an earth which was still a seething, fiery mass. In a word, all that
vast history which is traceable in the rocks beneath our feet--which
is in progress in the seas and lands and is to endure for an
inconceivable time to come--has been denied our satellite, for the
reason that it had no air with which to entrap the solar heat and no
water to apply the solar energy to evolutionary processes. The heat
which comes upon the moon as large a share for each equal area as it
comes upon the earth flies at once away from the airless surface, at
most giving it a temporary warmth, but instituting no geological work
unless it be a little movement from the expansion and contraction of
the rocks. During the ages in which the moon has remained thus
lifeless the earth, owing to its air and water, has applied a vast
amount of solar energy to geological work in the development and
redevelopment of its geological features and to the processes of
organic life. We thus see the fundamental importance of the volatile
envelopes of our sphere, how absolutely they have determined its
history.

It would be interesting to consider the causes which led to the
absence of air and water on the moon, but this matter is one of the
most debatable of all that relates to that sphere; we shall therefore
have to content ourselves with the above brief statements as to the
vast and far-acting effects which have arisen from the non-existence
of those envelopes on our nearest neighbour of the heavens.

METHODS IN STUDYING GEOLOGY.

So far as possible the preceding pages, by the method adopted in the
presentation of facts, will serve to show the student the ways in
which he may best undertake to trace the order of events exhibited in
the phenomena of the earth. Following the plan pursued, we shall now
consider certain special points which need to be noted by those who
would adopt the methods of the geologist.

At the outset of his st