oad from Vera Cruz, overhanging the summit of the Sierra Madre at the
limit of the lowlands. Other high peaks are the Nevedo de Toluca, often
snow-crowned, 14,950 feet; and Tancitaro, 12,660 feet.

The Mexican mountains are mainly of underlying granite formation. The
Jurassic, Cretaceous, and Tertiary Ages rocks are much in evidence
throughout the country, whilst the highest ranges, as we have seen, are
of volcanic origin. The singular plains of Yucatan are largely of
calcareous formation, probably a Tertiary limestone. One of the most
plentiful rocks over vast areas of Mexico, and that which forms the
striking variation of scenery, is the mountain limestone, the
excessively hard stratified crystalline rock of the Lower Cretaceous
period. This rock formation extends right across Mexico--although
isolated in places--from sea to sea, and its existence possibly goes to
show that the Pacific and the Gulf were one, in earlier geological
times. The predominating shades of these extensive strata are blue and
grey; occasionally there are black bands alternating, and they lie
upheaved at such angles as remind the traveller of the still more
pronounced strata of the high summits of the Peruvian Andes.[25] The
mountain limestone is of very hard texture; white and crystalline, it
wears away but slowly under the action of the elements, although on the
steep mountain tracks over which we are journeying we shall observe it
broken into cubes like sugar, beneath the incessant trampling of hoofs,
or worn away to silver-sand and borne down by the streamlets into the
river valleys.

[Footnote 25: See my book, "The Andes and the Amazon."]

The rock-formations of the tablelands are those to which Mexico owes
her fame as a silver-producing country, and it is in the high region,
from 5,000 to 9,500 feet above sea-level, that her historical mines are
encountered; and the zone of territory embraced by these well-known
centres, from Pachuca to Guanajuato and onwards to Chihuahua, may be
described without exaggeration as the richest argentiferous region on
the surface of the globe. It is to the metamorphic formation that the
abundance of mineral ores is due, and the igneous rocks which have
given rise thereto--the granites, basalts, diorites, porphyries, and
others. This metalliferous zone is more than 1,500 miles long,
extending from the State of Chihuahua and Sonora in the north to Oaxaca
and Chiapas in the south.

As we cross the coast-zone