and 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 fria," 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.

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 f