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.