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.
To make the account of the journey from the coast to Mexico somewhat clearer, a few words must be said about the formation of the country, as shown in a profile-map or section.

The interior of Mexico consists of a mass of volcanic rocks, thrust up to a great height above the sea-level.

The plateau of Mexico is 8,000 feet high, and that of Puebla 9,000 feet.

This central mass consists principally of a greyish trachytic porphyry, in some places rich in veins of silver-ore.

The tops of the hills are often crowned with basaltic columns, and a soft porous amygdaloid abounds on the outskirts of the Mexican valley.
Besides this, traces of more recent volcanic action abound, in the shape of numerous extinct craters in the high plateaus, and immense "pedrigals" or fields of lava not yet old enough for their surface to have been disintegrated into soil.