The valley grew narrower as we drove on; and at last, when it seemed to end in a great ravine, we began to climb the steep hill by a zig-zag road. Soon the air grows clearer again, the sunshine appears and gets brighter and brighter, we have left the mist behind, and are among ranges of grand steep hills, covered with the peculiar vegetation of the plateau,—Cactus, Opuntia, and the Agave Americana. In the trough of the valley lies a regular opaque layer of white clouds, hiding the fields and cottages from our view. We have already passed the zone of perpetual moisture, whose incessant clouds and showers are caused by the stratum of hot air—charged with water evaporated from the gulf—striking upon the mountains, and there depositing part of the aqueous vapour it contains. You may see the same thing happening in almost every mountainous district; but seldom on so grand a scale as here, or with so little disturbance from other agents. Yesterday was passed in the “tierra caliente,” the hot country; our journey of to-day and to-morrow is through the “tierra templada” and the “tierra fría,” the temperate and the cold country. Here a change of a few hundred feet in altitude above the sea, brings with it a change of climate as great as many degrees of latitude will cause, and in one day’s travel it is possible to descend from the region of eternal snow to the utmost heat of the tropics. Our ascent is more gradual; but, though we are three days on the road, we have sometimes scarcely time to notice the different zones of vegetation we pass through, before we change again.