THE GEOLOGICAL HISTORY OF VENEZUELA
Ancient land of Guayana—Comparison with Scottish
Highlands—Gneisses, schists, and granite—Dykes—Roraima
Series—Strange peaks—The Caribbean Series—“All that
glitters is not gold”—La Galera—Segovia Group—Natural
castles—Capacho Limestone—The Golden Hill—Cerro
de Oro Series—Formation of mountains—Early outlet
of Orinoco—Cumaná Series—Shoals and islands—Llano
gravels—Cubagua beds—Igneous rocks—Earthquakes—Hot springs—A
natural kettle—Coal—Iron—Gold—Copper—Lead—Petroleum and
asphalt—Sulphur—Salt—Urao—Ornamental stones—Wealth in minerals.
A certain interest, often described as spurious but none the less common,
generally attaches to the ancient for the sake of its antiquity, and
we may, hope, therefore, that the story of the rocks of Venezuela will
appeal even to non-geological readers.
The Guayana Highlands appear not only to be formed of the oldest rocks in
Venezuela, but from such scanty study as has been made of parts of the
region, to represent one of the most ancient land-surfaces in the world.
Different as they are in appearance, they offer many analogies with the
north-western highlands of Scotland from a geological point of view; and
here it is known that the processes of building-up and breaking-down have
preserved for us glimpses of a land which stood, as it now stands, long
ages ago when living organisms, if there were any, had not reached such a
stage in their development as to leave their traces in the deposits of
the time.
The great elevated platform from which rise the peaks and mountain chains
of Guayana appears everywhere to be composed of similar rocks, gneisses,
hornblende schists, and granites, all containing evidence of great
antiquity in geological time. This Guayana complex, as it may be called,
has been considered by geologists as more or less equivalent in age to
the Lewisian gneiss of Scotland, and therefore one of the oldest members
of the Archæan system.
What may have been the original condition of the rocks it is
impossible now to say, but one of the agencies which has brought them
to their present form has left its traces in the form of “dykes” of
quartz-porphyries and felsite, which were once forced in a molten
condition into crevices and joints of the then less solid deposits.
After the cooling of these intrusions and wearing down of the whole
mass by atmospheric influences, the movements of the earth’s crust
produced a shallow sea or series of lakes over what is now Guayana, and
in these waters a series of beds of red and white sandstones, coarse
conglomerate, and red shale were laid down to a thickness of about 2,000
feet. Later the area was again elevated into dry land, the sediments
were consolidated, and again veins or dykes of basalt, dolerite, and
similar dark, heavy rocks in a molten condition forced themselves into
the fractures of gneisses and sandstones alike. These sandstones are here
named the Roraima Series, from their occurrence in that mountain, and
they now remain as isolated peaks or chains of hills all over Guayana,
which, since that far-off period when the series was first consolidated,
seems to have been always dry land.
The points at which the Roraima beds have been left as upstanding masses
of horizontally stratified material, in place of being completely denuded
from the ancient foundation of gneiss, appear to have been determined in
many cases by the presence of exceptional accumulations of molten igneous
rock, which has hardened and remained as a cap to protect the softer
sandstones below from the effects of atmospheric weathering. Where this
has been the case the strange vertical-sided, flat-topped mountains of
Guayana are the result.
While in all probability northern Venezuela has no rocks quite so ancient
as those of Guayana, the geological history of this part of the country
has been much more eventful, and the number of earthquakes suggest that
even yet the form of the earth’s crust in this region is undergoing
comparatively violent changes.
As is commonly the case, to find the oldest rocks we have to search the
hills, and because the masses of gneiss, silvery mica-schist, marble,
and so on, which form the highest parts of much of the mountain region,
were first studied by Mr. G. P. Wall in the Caribbean Hills in 1860,
he named the whole the Caribbean Series. The beds which make up the
series may have been deposited at a period corresponding to that of the
ancient Silurian rocks of Wales, but it seems very possible that they are
older in some parts than in others. They form the central region of the
Venezuelan Andes, where there is a core of granite which probably cooled
at a date subsequent to the consolidation of the gneiss and schists.
The silvery mica-flakes of the latter are very often mistaken by the
inhabitants for gold or silver ores, and the author had more than one
pretty but valueless specimen offered for sale, with its locality to be
revealed as a secret of great worth! The same rocks extend all the way
along the coastal range across the Boca del Draco into Trinidad, and
northward in Margarita the mountains are formed of similar gneisses and
schists. In the Llanos north of El Baúl there is a peculiar elevated
plateau known as La Galera, from which rise many hills of gneiss and
granitic rock, but these may perhaps be an outlying island of the Guayana
complex.
After the deposits of the Caribbean Series had been consolidated and
elevated into dry land, but before they had been thrown into high
mountains such as they form to-day, perhaps at the same time that the
granite of the Sierra Nevada of Mérida was being pushed up under them in
a molten condition, the seas around were receiving deposits of quartz
sand, mud, and lime, which were later consolidated to form a series
of red and yellow sandstones, shales, slates, and black bituminous
limestones, which now outcrop along the Sierras, and chiefly in the
Segovia Highlands, suggesting for the whole the name Segovia Group. The
animals which inhabited the seas of those times left their shells and
remains in the rocks, and the forms (Ammonites, _Inoceramus_, &c.) which
have been found by various travellers show that these deposits were
formed at a period approximately corresponding to that of the lowest
parts of our chalk or of the Cretaceous System generally.
Here and there throughout the mountains of northern Venezuela, the
traveller is sure to be struck by the sight of great cliffs and
castle-like masses of limestone rock, which add greatly to the effect
of the scenery where they occur. From their position it is clear that
these were originally parts of a more or less continuous accumulation of
lime in a deep, still sea, after more turbulent waters had deposited the
Segovia Group. The German traveller Dr. Sievers called this limestone the
Capacho Limestone, from Capacho in Táchira. The fossils are similar to
those of the period of the higher parts of the chalk.
After the deposition of the Capacho Limestone the earth’s crust, which
had in this region remained tranquil for a considerable period, again
underwent some changes, and in the new shallower sea thus formed sand
and mud, with some lime, were alternately laid down. The resulting
sandstones, shales, and limestones were named by Dr. Sievers the Cerro
de Oro Series, from a hill formed of these rocks in Táchira, and called
Cerro de Oro, or Golden Hill, because the very abundant iron pyrites in
it were mistaken for the precious metal. Many fossils have been found
in the group, and from these it seems that in Venezuela, instead of the
break between Cretaceous and Tertiary which we have in England, there
was a continuous series of deposits, so that at the base we have chalk
fossils, and higher up Eocene forms, the general character of the animal
life changing gradually from one to the other.
With the consolidation of the Cerro de Oro beds we have a new period of
disturbance, in which the mountain chains of northern Venezuela began to
be formed as we know them to-day, and the waters of the Orinoco began
to flow into the Atlantic more or less by the present mouth of the
river. Alongside the newly formed hills, or islands as they would then
be, sandstones and shales were deposited to a considerable thickness.
They are found outcropping now along the coast and under the Llanos,
as well as round the Lake of Maracaibo. The first fossils from them
were collected by Mr. G. P. Wall from Cumaná, and it seems fitting to
distinguish the whole as the Cumaná Series.
After the deposition of the Cumaná Series, and the crust movements
which led to the consolidation and folding of these rocks, the physical
features of Venezuela must have been very much what they are to-day,
save that many of the smaller islands and parts of the coast were still
submerged as shoals, whilst the Llanos seem to have been a great swampy
or submerged plain, with deep water in parts, over which the Orinoco
sediments gradually accumulated in the form of current-bedded sands
and clays surmounted by gravels, which we may term the Llano deposits.
At the same time, along the coast and on the shoals shell-beds were
being formed, and can now be seen at Cabo Blanco, west of La Guaira,
and similar places, while practically the whole surface of the Island
of Cubagua is formed of them, suggesting the name Cubagua Beds. About
this period some volcanic rocks were thrust up and cooled both in the
Peninsula of Paraguana and near San Casimiro, south of Carácas. In the
mountains great masses of gravels containing huge boulders and some
_Megatherium_ and other bones were being piled up by the rivers. Last of
all we have the still-accumulating recent alluvium of the modern streams,
attaining its widest extent in the Delta and round the Lake of Maracaibo.
No volcanoes, active or recently extinct, are known in Venezuela, but
the country has, like most of South America, been continually subject
to earthquake shocks of greater or less intensity. Some of these are
historic, but of the many others recorded not a few had far-reaching
effects on the population. The first important tremor noticed after
the discovery and settlement of the shores of the Caribbean was that
of 1530, which shook the city of Nueva Cadiz on the Island of Cubagua
and destroyed the fortress of Cumaná, thus checking for some time the
colonisation of the mainland in this region. Thirteen years later New
Cadiz was visited again by earthquake and hurricane, and so disastrous
were the results that from that day to the present Cubagua has been,
what it was before the arrival of the Spaniards, a desert island. The
many shocks experienced in the course of the seventeenth and eighteenth
centuries seem to have been generally unaccompanied by much loss of life
or property, but this period of comparative quiescence was followed
by one of the historical examples of severe earthquake early in the
nineteenth century. In March, 1812, a shock destroyed great parts of
Carácas, La Guaira, Barquisimeto, Mérida, and other towns, and in the
capital alone ten thousand people were killed. The great earthquake
which, on August 13, 1868, made itself felt all over South America, so
much affected some of the Venezuelan rivers that their waters over-flowed
the banks, and even remained for a short time in new channels. In
1894 Mérida and other towns in the Andes suffered much damage, houses
and churches being shaken down; the destruction was in some cases
extraordinarily complete. Since that time only slight tremors have been
felt.
The internal heat in the north-eastern spur of the Andes, which traverses
Venezuela, manifests itself at many points in the form of hot springs.
One, containing much sulphur, is found at Las Trincheras, between
Valencia and Puerto Cabello. The temperature of this spring varies,
but in 1852 it was found by Karsten to be only a few degrees below the
boiling point of water; in general, however, it does not exceed 195° F.
or 17° below boiling point. Wall found one south of Carúpano actually
boiling. All along the flanks of the coastal Cordillera there are
mineral springs, generally at fairly high temperatures, and many more
are known throughout the Andes. Nearly all these springs have been used
in the treatment of various diseases, though none has achieved especial
popularity.
While hot springs may be interesting to the visitor, they are hardly
valuable assets to a country such as Venezuela, but this part of the
world has always, and with much justice, held the reputation of being
rich in minerals. There is coal of fairly good quality in more than one
of the Cretaceous and Tertiary groups of strata, near Barcelona, Tocuyo,
Coro, and Maracaibo, as well as in the Andes, but the often-associated
iron is only found in large quantities in the Guayana gneiss south of the
Orinoco Delta.
Gold, that great lure for the early European ventures to the west, may
be said to occur in almost every State of Venezuela, but it has only
been worked with profit in Guayana, even though samples of a quartz-reef
near Carúpano are said to have assayed 7 oz. to the ton. When Sir Robert
Dudley visited the coast of the Gulf of Paria in 1595, he heard of a
goldmine near Orocoa (Uracoa), on the eastern side of the Llanos, which
may mean that the gravels are occasionally auriferous, but unfortunately
he failed to reach the place. Placer workings are the chief source of
the precious metal in the Guasipati goldfields in Guayana, but the reefs
from which it is derived have been discovered and worked at odd times.
In British Guiana, where the conditions are similar, Mr. Harrison says
that the gold is generally found along the later intrusive dykes, the
smallest dykes being the richest, while most gold is found where a basalt
intrusion crosses one of the older ones.
The ores of copper are fairly common in the northern Cordillera, and
the mines of Aroa in Yaracuy have been worked for many years. Here the
pyrites veins occur in the Capacho Limestone, not far from where it has
been invaded by a mass of granite. In the Andes it seems to occur in
the more ancient rocks, as near Seboruco in Táchira, and Bailadores in
Mérida. A mine near Pao seems to be in Cretaceous rocks.
Many other metallic ores occur at various points, notably galena in the
Andes, but one of the most common minerals of northern Venezuela is
petroleum, known in its desiccated form as “Bermudez asphalt” over half
the world. Boring for the original mineral oil has only recently been
undertaken. Sulphur is one of the less valuable minerals which occur in
considerable quantities, but the so-called salt-mines are not strictly
mines at all, and are described in Chapter XVI. Humboldt had heard of the
strange mineral lake near Lagunillas, which contains a large proportion
of _urao_ or sesqui-carbonate of soda, a mineral not usually found in
nature, and here apparently supplied from springs rising in the Segovia
rocks. It is used locally for mixture with tobacco juice to make a
chewing mixture called _chimo_, but there have been projects to obtain
the salt in large quantities for the manufacture of caustic soda.
In addition to those already mentioned, all the following minerals or
ornamental stones have been found in one part or another of Venezuela,
viz., marble, kaolin, gypsum, calcium phosphate, opal, onyx, jasper,
quartz, felspar, talc, mica, staurolite, asbestos, and ores of antimony,
silver, and tin.
The minerals of Venezuela have merely been mentioned casually in this
place, an account of the extent to which they have been exploited being
deferred to the chapter on the general development of the country. It
is evident, however, when one considers their number and the extent of
their distribution, that the geological changes which have played their
part in the building up of the physical features of the country have left
Venezuela in possession of splendid assets in this respect.