Harold Michell.

An introduction to the geography of Sierra Leone online

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red rock which occurs at the surface of the ground over
the greater part of the country. It is derived in most
cases from the decay of the underlying solid rocks and
takes the place of the soil formed in temperate regions.
It is generally of little use for agricultural purposes,
because it contains very little or none of the substances
essential to plant growth, such as potash, soda and
lime. These substances formed part of the original
rock, but they have been removed in the process of

A number of caves and underground water-courses
in Sierra Leone owe their origin to the hard crust of
laterite which forms on sands and clayey sands, and
also on the piles of rock debris which collect at the foot
of mountain slopes. Surface waters, after making their
way downwards through cracks in the crust, sometimes
wash away the soft sands below so as to form large
caverns or narrow underground channels. In this way
originated Hart's Cave, which opens out on to the fore-
shore near York, and also the Devil Hole near Water-
loo, as well as many other less well-known caves and
subterranean channels.

Geogra'phy of Sierra Leone. 127

4. Summary.

The oldest rocks in Sierra Leone are the metamorphic
rocks, derived partly from ancient sedimentary rocks
and partly from igneous rocks. They consist mainly
of schists and gneisses ; schists are fissile, whereas
gneisses are not. They form most of the hills in
Ronietta District. The metamorphic rocks were in-
vaded by great m^asses of granite. Granite is a coarse-
grained igneous rock, consisting of quartz, a potash
felspar, and a little mica or hornblende. Granites
occur throughout Koinadugu District and the central
and eastern parts of Railway District. Some gabbro
masses were intruded about the same time as the
granites. Gabbro is a coarse-grained igneous rock con-
sisting principally of a lime-soda felspar and augite.
The mountains of the Colony are made up of masses of
norite, a rock closely related to gabbro.

The metamorphic rocks, the granites and the norite
are pierced by numerous dykes of dolerite. A dyke is
a wall-like mass of igneous rock which cuts through
other rocks. A dolerite is an igneous rock made up of
the same minerals as a gabbro, but less coarse in
texture. A good example of a dolerite dyke occurs
near Njala.

Sedimentary rocks consist principally of fragments
of other rocks deposited in seas or lakes, or on the sur-
face of the land. Sandstones are made up of sand
grains, clays and shales of mud, and limestones of shells
and other calcareous matter. There are at least two
series of sedimentary rocks in Sierra Leone. The
older consists mainly of shales and micaceous sand-
stones with some iuterbedded volcanic ashes; it out-
crops in the Taia River at Njala, and in the Rokelle
River below Makump. The younger consists of many-
coloured clays and sands; the plain which extends over
the south-western part of the Protectorate was formerly
the bed of the sea in which these rocks were laid down.
The level-bedded rocks exposed in the cliffs of the

rJ8 Geography of Sierra Leone.

Bullom Shore and around the coast of the Colony
belonsj to this series.

Raised beaches seen around the Colony, and deep
gorges cut by rivers into the coastal plain give addi-
tional proof of a late geological uplift. At the present
time some cliffs are being rapidly worn away along the
coasts of Sierra Leone, while others are protected by
sand-ridges thrown up in front of them. by the waves.

Laterite is a rock produced by the weathering of
other rocks under tropical conditions.



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Online LibraryHarold MichellAn introduction to the geography of Sierra Leone → online text (page 10 of 10)