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 such as we have inspected." Our geological chat was interrupted by an exclamation from l'Encuerado, who had just discovered a tree which the Mexicans call "the Tree of St. Ignatius." Its fruit is of a brown color, with a woody husk, something like small melons, which, as they hang on the tree, strike against one another with a sharp sound. L'Encuerado informed Lucien that this fruit is in the habit of bursting suddenly with a loud explosion, and that the flat beans which they contain are much used as medicine. [Illustration]