situ) are coated and united together by a white, friable, calcareous tuff,
like that found at Podaguel. When this matter was deposited on the summit
of S. Cristoval, the water must have stood 946 feet above the surface of
the surrounding plain. (Or 2,690 feet above the sea, as measured
barometrically by Mr. Eck. This tuff appears to the eye nearly pure; but
when placed in acid it leaves a considerable residue of sand and broken
crystals, apparently of feldspar. Dr. Meyen ("Reise" Th. 1 s. 269) says he
found a similar substance on the neighbouring hill of Dominico (and I found
it also on the Cerro Blanco), and he attributes it to the weathering of the
stone. In some places which I examined, its bulk put this view of its
origin quite out of the question; and I should much doubt whether the
decomposition of a porphyry would, in any case, leave a crust chiefly
composed of carbonate of lime. The white crust, which is commonly seen on
weathered feldspathic rocks, does not appear to contain any free carbonate
of lime.)

To the south this basin-like plain contracts, and rising scarcely
perceptibly with a smooth surface, passes through a remarkable level gap in
the mountains, forming a true land-strait, and called the Angostura. It
then immediately expands into a second basin-formed plain: this again to
the south contracts into another land-strait, and expands into a third
basin, which, however, falls suddenly in level about forty feet. This third
basin, to the south, likewise contracts into a strait, and then again opens
into the great plain of San Fernando, stretching so far south that the
snowy peaks of the distant Cordillera are seen rising above its horizon as
above the sea. These plains, near the Cordillera, are generally formed of a
thick stratified mass of shingle (The plain of San Fernando has, according
to MM. Meyen and Gay "Reise" etc. Th. 1 ss. 295 and 298, near the
Cordillera, an upper step-formed plain of clay, on the surface of which
they found numerous blocks of rocks, from two to three feet long, either