The attentive traveller in Egypt and Nubia cannot
fail to notice many localities, generally of small
extent, where the soil is rendered infertile by an
excess of saline matter in its composition. In
many cases, perhaps in all, these barren spots lie
rather above the level usually flooded by the inundations
of the Nile, and yet they exhibit traces of former
cultivation. Observations in India suggest a possible
explanation of this fact. A saline efflorescence
called “Reh” and “Kuller”
is gradually invading many of the most fertile districts
of Northern and Western India, and changing them into
sterile deserts. It consists principally of sulphate
of soda (Glauber’s salts), with varying proportions
of common salt. These salts (which in small quantities
are favorable to fertility of soil) are said to be
the gradual result of concentration by evaporation
of river and canal waters, which contain them in very
minute quantities, and with which the lands are either
irrigated or occasionally overflowed. The river
inundations in hot countries usually take place but
once in a year, and, though the banks remain submerged
for days or even weeks, the water at that period, being
derived principally from rains and snows, must be less
highly charged with mineral matter than at lower stages,
and besides, it is always in motion. The water
of irrigation, on the other hand, is applied for many
months in succession, it is drawn from rivers and canals
at the seasons when the proportion of salts is greatest,
and it either sinks into the superficial soil, carrying
with it the saline substances it holds in solution,
or is evaporated from the surface, leaving them upon
it. Hence irrigation must impart to the soil
more salts than natural inundation. The sterilized
grounds in Egypt and Nubia lying above the reach of
the floods, as I have said, we may suppose them to
have been first cultivated in that remote antiquity
when the Nile valley received its earliest inhabitants,
and when its lower grounds were in the condition of
morasses. They must have been artificially irrigated
from the beginning; they may have been under cultivation
many centuries before the soil at a lower level was
invaded by man, and hence it is natural that they
should be more strongly impregnated with saline matter
than fields which are exposed every year, for some
weeks, to the action of running water so nearly pure
that it would be more likely to dissolve salts than
to deposit them.
SUBTERRANEAN WATERS.
I have frequently alluded to a branch of physical
geography, the importance of which is but recently
adequately recognized—the subterranean
waters of the earth considered as stationary reservoirs,
as flowing currents, and as filtrating fluids.
The earth drinks in moisture by direct absorption
from the atmosphere, by the deposition of dew, by
rain and snow, by percolation from rivers and other
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The Earth as Modified by Human Action from Project Gutenberg. Public domain.
