Besides the mechanical effects of river, rain, and wind, other changes
whose wide-reaching significance cannot be over-estimated, are taking
place on and below the earth’s surface. Chemical action is slowly
at work producing effects of the first importance to man. Rain-water
has the power of absorbing important quantities of carbonic acid
gas and oxygen from the atmosphere. On the average, rain-water
contains 1·77 per cent by volume of dissolved carbonic acid gas,
and 33·76 per cent of dissolved oxygen. In passing through the soil,
rain-water also absorbs the organic acids formed by the decomposition
of plant remains. These dissolved gases and organic acids render rain
an active chemical agent in the alteration of rocks, its effects
being conveniently classified under the headings: (1) Oxidation;
(2) Solution; (3) Formation of Carbonates; and (4) Hydration.
(1) Oxidation results in the formation of thin crusts on the
surface of rocks, the compounds of manganese and iron so frequently
present in them being also rusted or hydrated by the action of the
rain-water. Nothing is more striking than the presence of the dark
films on the desert limestones in regions which are liable to a
certain amount of rainfall, and nothing more convincing as to their
origin than their absence in those portions of the south-western
desert of Egypt where rain is of great rarity. Near the Nile, the Red
Sea and the Mediterranean, dew may take the part of rain in action,
and in a sense the results of its activity may appear more intense,
as rain is liable to wash away the products of its own handiwork.
(2) The effects of Solution are of the greatest importance,
limestone being soluble to the extent of about 1 part in 1,000 in
water saturated with carbonic acid. In many limestone countries of the
world the solution effects are marked by the production of underground
caves and channels and in some parts of the north-eastern desert of
Egypt, where chalky limestones are the main constituent, this action
has produced remarkable results—large caves, cylindrical channels,
and natural bridges being of not uncommon occurrence.
(3) Formation of Carbonates. Owing to the rains in Egypt being of very
brief duration, but nevertheless extremely active while they last,
the soluble material in the condition of the unstable bicarbonate
of lime is carried only a short distance, and losing its loosely
combined carbonic acid is redeposited in the cracks of the rocks,
as veins of carbonate of lime, or as the cementing material by
which broken fragments are consolidated into compact breccias. This
action may be seen in the cliff face south of the Pyramids, near the
Sphinx, where the sandy limestones forming the top of the hill have
been attached by the rain containing carbonic acid. The calcareous
tests of the shells in the sandy limestones have been dissolved away,
leaving only the sandy internal casts of the shells behind, and the
material so removed has been redeposited in intricate interlacing
veins in a clayey band immediately below. A vein may sometimes grow
by the accretion of successive layers, which, owing to local causes,
such as the relative content of iron oxide, etc., may display slightly
different colours, one of the results being the production of so
interesting a rock as the Egyptian alabaster, which is a carbonate
of lime. As a rule, the term alabaster is applied to the sulphate
rather than to the carbonate of lime. Probably much carbonate of
lime is also carried in solution to the sea, and there forms the
source of the material which hundreds of living animals seize upon
for the production of the shells in which they dwell. I was much
struck last year, during a journey from the Pyramids to Wasta, to
note how the oyster-beds of one age (the Pliocene) formed themselves
upon oyster-beds of a long preceding period (the Eocene), probably
on account of the greater amount of carbonate of lime at those
localities, present owing to solution of the earlier shell-structures.
That veins of carbonate of lime should be present in limestone
districts is, in view of the above statements, not surprising,
but it does appear somewhat startling at first sight, to find
marked deposits of carbonate of lime lining the floors and sides of
torrent-beds in districts entirely composed of igneous or volcanic
rocks of complicated mineral structure. Experience has shown,
however, that the lime silicates, so abundant in the more basic
members of the igneous series, such as diabases and diorites, are
liable to the attack of the rain-waters containing carbonic acid,
carbonate of lime being produced by the reaction.
(4) Of the results of Hydration, the most striking examples in Egypt
are the formation of kaolin near Aswan, due to the absorption of water
by the felspars of the granitic and gneissose rocks, and the thick
zone of decomposition (kaolinic) products, which was cut through in
excavating the navigation canal in the syenite which forms the main
rock at that locality.
The total effect of all the above-mentioned meteorological influences
results in the weathering of the rock-surface, involving the softening
and crumbling of the harder materials, but sometimes leading to
the solidification of materials previously loosely aggregated by
substances left as cementing agents when the water containing them
in solution has evaporated.
In addition to the various direct results of the meteorological
activities upon the earth’s surface, there are others which
indicate more subtle changes. Perhaps amongst the most interesting
of these is the formation of concretions—bodies composed of one
material aggregated in more or less rounded or irregular form in a
rock of another composition. Among the most interesting and abundant
of these are the layers of flint, which form bands of strikingly
parallel character in the limestones of Upper Egypt. These have not
yet been submitted to the detailed study which similar concretions
have received in Europe, but there is little doubt that they,
in large measure, represent the aggregation of gelatinous silica
round decomposing organic materials, the shells of organisms and the
framework of siliceous sponges often forming their centre. In some
cases, as in the fossil trees, the replacement appears to have taken
place molecule by molecule, as the outlines of every cell of the once
woody fibre are now replaced in silica. By a well-known transition,
this once gelatinous material has now become one of the most solid
of substances.
Ferruginous concretions, composed of oxide of iron, are present in
many of the Egyptian sandy clays, some of the beautifully-tinted
purple, yellow and red ochres being found in this form; and
the natives collect them for the use of the women as ornamental
coloration.
Of greater importance to the world at large are the gradual changes
which vegetable matter (collected under specially favourable
circumstances free of all sandy and clayey admixture) has undergone
through vast periods of time, causing the slow evolution of the
oxygen, hydrogen and nitrogen, originally present, with a gradual
predominance of the carbon. This passage from vegetable matter to coal
has been noted in Egypt in connection with the Nubian sandstone,
beds of carbonaceous material deserving the name of lignite or
even bituminous coal having been found at various localities. The
deposits found up to the present time are of such tenuity that it
is not possible on the evidence available to express optimistic
opinions as to the probable occurrence of workable coal in Egypt,
but still they are of sufficient interest to be kept constantly in
mind while the Geological Survey is prosecuting its researches. From
time to time the finding of coal-seams has been reported at Edfu,
in Kharga, at Saqiet el Teir and Abu Radham[7] in the Eastern Desert,
but the efforts hitherto made have resulted in failure.
The evidence thus far available shows that great rivers were entering
the sea in Nubia during an early geological period (the Cretaceous),
typical fresh-water shells having been found south of Aswan covered
with marine worm-tubes; leaf-imprints are abundant in some of
the sandy layers, and in isolated instances they have collected
in sufficient quantity to give rise to lignite and bituminous
coal-layers of extreme thinness, showing that this interesting
and important change has taken place, at least to some extent,
in Egypt itself. The study of coal-producing regions tends to show
that the change to coal of high commercial value requires not only
conditions favourable to the loss of the more volatile gases, but
also that the beds must have been involved in great earth-movements,
which have hastened the tendency to their being enriched in carbon,
both favourable conditions of deposition and marked disturbance of
the strata being thus required to obtain the much-desired result.
Other internal chemical activities are at work, producing changes
which are still the cause of debate and earnest study. The origin of
petroleum must undoubtedly be traced to chemical transformations of a
complicated character, if we may judge by the number of experimental
methods which yield petroleum as a product. All opinions agree that
the mineral oil is derived by some form of chemical action, though
whether it arises from the decomposition of organic remains or whether
it be of inorganic origin is still matter of dispute. Geological
students have on the whole ranged themselves on the side of the
first-named view, pointing out that the petroleum fields are all
associated with sedimentary strata, whether sands or limestones. The
inorganic view has been held as tenaciously by a number of men
experienced in the search for oil, and it is capable of argument
that sulphur dioxide and sulphuretted hydrogen, if being produced
simultaneously, may result in the alteration of limestone to gypsum,
free sulphur and petroleum being also obtained in the reaction.
In this connection it is interesting to note that gypsum, sulphur
and petroleum are associated at Jemsa, on the Gulf of Suez.
One of the most interesting features in connection with petroleum is
the phenomenon presented in most oil-fields of oil-wells separated
perhaps by only thirty metres emitting oil under pressure at the
same time; also the great pressures indicated by the remarkable
fountain flows which are of constant occurrence in the principal
petroleum fields.[8]