SOME PECULIAR FEATURES OF AFRICAN SOIL.
In making some of his early explorations Livingstone passed over a
country, the firm, compact soil of which was perfectly level. Here he
found a little soil covering a limelike tufa deposit. This extended over
a tract of several hundred miles. It supported a vegetation of fine,
sweet, short grass, and baobab and other trees.
In several parts of this tract he found large salt pans, as they are
termed. One was fifteen miles broad and one hundred miles long.
Although these curious spots seemed perfectly level, yet they have a
slight slope to the northeast, towards which the rain water, which
sometimes covers them, gently trickles. The salt dissolved in the water
has in this way been gradually transferred to one large pan lying to the
northeast. On this may be seen a cake of salt and lime an inch and a
half thick. Some of these pans have merely a covering of salty lime;
others are covered with a thick deposit of shells.
These shells are identical with those of the mollusk species of Lake
N'gami. There appear to be three varieties, spiral, univalve, and
bivalve.
Livingstone found that in every salt pan of the country there is a
spring of water on one side. The water of the spring is brackish. In one
instance he found two springs, one salter than the other.
He was of the opinion that the supply could not come from beds of rock
salt; for, where the natives have removed the salt from the pans, no new
deposit has ever formed.
He thought it probable that these salt deposits are the remains of the
slightly brackish lakes of antiquity. Large portions of these lakes
must, in the course of ages, have evaporated to leave a deposit where
the waters have receded.
An instance of this kind is Lake N'gami; for the waters of the lake when
low become brackish.
The largest quantities of salt have been found in the deepest hollows or
in the lowest valleys, where there is no outlet. Livingstone cites an
instance of a fountain, the temperature of which was upwards of 100 deg.
This fountain, though strongly impregnated with salt, had no deposit,
because situated on a flat portion of the country.
When these salt deposits occur in a flat country with a tufa layer
covering the soil, a chemical change takes place in the soil; the tufa
is dissolved and the ground kept in a state unfavorable to the growth of
plants.
Livingstone discovered one large salt pan with a deposit of salt an inch
and a half in thickness. This deposit contained bitter salt in addition,
probably the nitrate of lime. In order to make the deposit wholesome and
palatable, the natives mix the salt with the juice of a gummy plant.
They then place the mixture in the sand and build a fire over it to bake
it. The action of the fire renders the lime insoluble and tasteless.
The natives in the vicinity of this salt pan keep large flocks of sheep
and goats at various points on the outskirts of the desert. These flocks
thrive wonderfully wherever salt and bushes are to be found.
The milk of goats does not coagulate readily, like that of cows, on
account of its rich quality. The natives have discovered that by mixing
a tea made from the fruit of a special plant with the milk of goats they
can cause it to coagulate quickly.
It is the custom among some of the natives to put the milk into sacks of
untanned hide from which the hair has been removed. When these sacks are
hung in the sun the milk soon thickens. The whey is then drawn off by a
plug at the bottom of the sack. Fresh milk is then added, until a thick
sour curd fills the sack. When one becomes accustomed to this, its taste
is delicious.
The richer natives mix this milk with the porridge they make from their
meal. It is considered very nutritious and strengthening, and takes the
place of our roast beef for nutriment.
The natives, in speaking scornfully of those who are poor or weak, use
the expression, "They are water-porridge men;" since they cannot afford
to mix their porridge in the approved way, and hence cannot expect to
gain strength.
Speaking of other sections of the country, Livingstone tells us that he
found some portions capped by a conglomerate rock mixed with iron. In
many places the iron looked as if it had been melted; for the rounded
masses resembled slag in an iron foundry, and the under surface was
smooth and even.
Probably this deposit was of an aqueous origin; for it contained
water-worn pebbles of various kinds. These were generally small in size.
Below the conglomerate lay a mass of pale red, hardened sandstone, and
beneath that a layer of what are called trap rocks. Lowest of all lay a
coarse-grained sandstone, which contained a few pebbles. Occasionally a
white rock of lime formation was found, and also banks of loose, round
pebbles of quartz.
The land slopes contained bogs surrounded by clumps of straight, lofty
evergreen trees, which looked extremely graceful on a ground of
yellowish grass. Many of these bogs pour forth a solution of iron. These
exhibit the prismatic colors upon their surface.
It would be of interest to note the curious conditions and formations of
the soil in other sections, but we must pass on to other views.
Those of you who are interested in the life of Livingstone will find
many features of the soil described in the journals this explorer
bequeathed to the world.