or Chichiquimec, in the chapter-heading)
Indians constructed nothing but huts, time had, doubtless, obliterated
all trace of their former presence.

I can hardly describe the pleasure I felt in again viewing the woods,
the verdure, the insects, the flowers, and enjoying the light of the
sun. The interior of a cave, certainly, has the effect of producing
melancholy, attributable, no doubt, to the silence and darkness; for the
beautiful hall, radiant with stalactites, was but little likely to cause
sadness. The effect on Lucien's mind was of a serious character, and he
seemed never to be weary of asking questions.

"These natural hollows," said Sumichrast, "often occur in gypsum
mountains, but still more frequently in volcanic or calcareous masses.
Some, which are as old as the world itself, date from the earliest
upheavals of the surface of the globe, when the fused matter which
composes the centre of the earth broke through the scarcely
solidified crust, and, rushing upward, formed the mountain chains we now
see."

[Illustration: Crater of Popocatepetl.]

"Then the centre of the earth has been once in a liquid state?"

"It is so still, as is shown by volcanoes; but the period of great
catastrophes is past. The molten matter solidified on the surface, as it
became cool, and then water made its appearance, and transformed and
rendered habitable the thin crust on which we live, the thickness of
which is so inconsiderable when compared with the bulk of the globe."

"What is this molten matter composed of which is burning under our
feet?"

"The same substances which we see around us--granite, porphyry, and
basalts, which are called _igneous_ or _Vulcanian rocks_, as contrasted
with the _Neptunean rocks_, such as gypsum or lime, clay and sandstone,
the agglomeration of which is attributed to water. The science which
deals with these subjects is called _geology_, a study with which, some
day, you will be delighted."

"Then all Vulcanian rocks can be melted?"

"Yes, if they were subject to as great a heat as that existing in the
centre of the earth, which reaches an intensity at which the imagination
recoils. But to return to the subject of caves. Some have been produced
by the dissolving action of water. Thus, at some future date, the spring
which we saw gushing out from the fallen mountain might dry up or alter
its direction, and leave for the curiosity of future travellers the
sight of chambers full of stalactites