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.  Though sedimentary rocks occur in Mexico, they are not the predominant feature of the country.  Ridges of limestone hills lie on the slopes of the great volcanic mass toward the coast; and at a still lower level, just in the rise from the flat coast-region, there are strata of sandstone.  On our road from Vera Cruz we came upon sandstone immediately after leaving the sandy plains; and a few miles further on we reached the limestone, very much as it is represented in Burkart’s profile of the country from Tampico upwards towards San Luis Potosi.  The mountain-plateaus, such as the plains of Mexico and Puebla, are hollows filled up and floored with horizontal strata of tertiary deposits, which again are covered by the constantly accumulating layers of alluvium.

Our heavy pull up the mountain-side has brought us into a new scene.  Every one knows how the snow lies in the valleys of the Alps, forming a plain which slopes gradually downward towards the outlet Imagine such a valley ten miles across, with just such a sloping plain, not of snow but of earth.  There has been no rain for months, and the surface of the ground is parched and cracked all over.  There is hardly a tree to be seen except clumps of wood on the mountain-sides miles off,—­no vegetation but tufts of coarse grass, among which herds of disconsolate-looking cattle are roaming; the vaqueros, (herdsmen) are cantering about after them on their lean horses, with their lazos hanging in coils on their left arms, and now and then calling to order some refractory beast who tries to get away from the herd, by sending the loop over his horns or letting it fall before him as he runs, and hitching it up with a jerk round his hind legs as he steps within it.  But the poor creatures are too thirsty and dispirited just now to give any sport, and the first touch of the cord is enough to bring them back to their allegiance.