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.