Physical geography. By Mary Somerville ...

122 PHYSICAL GEOGRAPHY. CHAP. XI.
a state of constant eruption, which, with the peaks of Iztacihuatl
and of Toluca, form a kind of volcanic circus, in the midst of which
the city of Mexico and its lake are situated. A chain of smaller
volcanoes unites the three. On a plain on the western slope of the
table-land, and about 70 miles in a straight line from the Pacific, is
the volcanic cone of Jorullo.' It suddenly appeared, and rose 1683
feet above the plain, on the night of the 29th of September, 1759,
and is the highest of six mountains which have been thrown up on
this part of the table-land since the middle of last century. The
great cone of Colima, the last of this volcanic series, stands insulated in the plain of that name, between the western declivity of
the table-land and the Pacific.
A high range of mountains extends along the eastern margin of
the table-land to the Real de Catorce, and the surface of the plain
is divided into two parts by the Scrra Madre, which begins at 21~
of N. lat.; and, after running north about 60 miles, its continuity
is broken into the insulated ridges of the Serra Altamina, and the
group containing the celebrated silver-mines of Fresnillo and Zacatecas: it soon after resumes its character of a regular chain, and,
with a breadth of 100 niles, proceeds in parallel ridges and longitudinal valleys to New Mexico, where it skirts both banks of the
Rio Bravo del Norte, and joins the Serra Verde, the most southern
part of the Rocky Mountains, in 40~ of N. lat.
To the south, some points of the Sierra Madre are said to be
10,000 feet high and 4000 feet above their base; and between the
parallels of 36~ and 420, where the chain is the watershed between
the Rio Colorado and the Rio Bravo del Norte, they are still higher,
and perpetually covered with snow. The mountains on the left
bank of the last-mentioned river are the eastern ridges of the Serra
Madre, and contain the sources of the innumerable affluents of the
Missouri and other rivers that flow into the Mississippi and Mexican
Gulf.
Deep cavities, called Barancas, are a characteristic feature of the
table-lands of Mexico: they are long rents, two or three miles in
breadth, and many more in length, often descending 1000 feet below
the surface of the plain, with a brook or the tributary of some river
flowing through them. Their sides are precipitous and rugged, with
overhanging rocks covered with large trees. The intense heat adds
to the contrast between these hollows and the bare plains, where the
air is more cool.
Vegetation varies with the elevation; consequently the splendour
which adorns the low lands vanishes on the high plains, which,
though producing much grain and pasture, are often saline, sterile,
1 Baron Humboldt.