and watered by the two brooks Cuitimba and San Pedro. In the month of
June, 1759, hollow sounds of an alarming nature were heard, and
earthquakes succeeded each other for two months, until, at the end of
September, flames issued from the ground, and fragments of burning rocks
were thrown to prodigious heights. Six volcanic cones, composed of
scoriae and fragmentary lava, were formed on the line of a chasm which
ran in the direction from N. N. E. to S. S. W. The least of these cones
was 300 feet in height; and Jorullo, the central volcano, was elevated
1600 feet above the level of the plain. It sent forth great streams of
basaltic lava, containing included fragments of granitic rocks, and its
ejections did not cease till the month of February, 1760.[588]

[Illustration: Fig. 57.

_a_, Summit of Jorullo. _b_, _c_, Inclined plane sloping at an angle of
6 degrees from the base of the cones.]

Humboldt visited the country more than forty years after this
occurrence, and was informed by the Indians, that when they returned,
long after the catastrophe, to the plain, they found the ground
uninhabitable from the excessive heat. When he himself visited the
place, there appeared, around the base of the cones, and spreading from
them, as from a centre, over an extent of four square miles, a mass of
matter of a convex form, about 550 feet high at its junction with the
cones, and gradually sloping from them in all directions towards the
plain. This mass was still in a heated state, the temperature in the
fissures being on the decrease from year to year, but in 1780 it was
still sufficient to light a cigar at the depth of a few inches. On this
slightly convex protuberance, the slope of which must form an angle of
about 6 degrees with the horizon, were thousands of flattish conical
mounds, from six to nine feet high, which, as well as large fissures
traversing the plain, acted as fumeroles, giving out clouds of
sulphurous acid and hot aqueous vapor. The two small rivers before
mentioned disappeared during the eruption, losing themselves below the
eastern extremity of the plain, and reappearing as hot springs at its
western limit.

_Cause of the convexity of the plain of Malpais._--Humboldt attributed
the convexity of the plain to inflation from below; supposing the
ground, for four square miles in extent, to have risen up in the shape
of a bladder to the elevation of 550 feet above the plain in the highest
part. But Mr. Scrope has