bight or southerly extension of the Gulf
of Darien. It receives the waters of the Atrato, Bacuba, and a number
of small rivers, and penetrates the land about 50 m., but has very
little commercial importance because of the unhealthy and unsettled
character of the neighbouring country, and because of the bar across
its entrance formed by silt from the Atrato. The Gulf of Morosquillo,
a broad shallow indentation of the coast south of Cartagena, receives
the waters of the Rio Sinu, at the mouth of which is the small port of
Cispata. Between the mouth of the Magdalena and Santa Marta is the
Cienaga de Santa Marta, a large marshy lagoon separated from the sea
by a narrow sand spit, having its "boca" or outlet at its eastern
side. There is some traffic in small steamers on its shallow waters,
which is increasing with the development of fruit cultivation on its
eastern and southern sides. It extends inland about 31 m., and marks a
deep indentation of the coast like the Gulf of Uraba.

_Geology._--The geology of Colombia is very imperfectly known, and it
is only by a comparison with the neighbouring regions that it is
possible to form any clear idea of the geological structure and
succession. The oldest rocks are gneisses and schists, together with
granite and other eruptive rocks. These are overlaid by sandstones,
slates and limestones, alternating with porphyries and porphyrites
sometimes in the form of sheets, sometimes as breccias and
conglomerates. Cretaceous fossils have been found abundantly in this
series, but it is still possible that earlier systems may be
represented. Coal-bearing beds, possibly of Tertiary age, occur in
Antioquia and elsewhere. Structurally, the four main chains of
Colombia differ considerably from one another in geological
constitution. The low Cordilleras of the Chocos, on the west coast,
are covered by soft Quaternary sandstones and marls containing shells
of extant species, such as still inhabit the neighbouring ocean. The
Western Cordillera is the direct continuation of the Western
Cordillera of Ecuador, and, like the latter, to judge from the
scattered observations which are all that are available, consists
chiefly of sandstones and porphyritic rocks of the Cretaceous series.
Between the Western and the Central Cordilleras is a longitudinal
depression along which the river Cauca finds its way towards the sea.
On the wester