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.