About forty miles above the mouth of the Guaviare, where the Orinoco again changes the direction of its course from parallel to meridian, the Delta and mouth of the great Ventuari tributary is encountered and beyond this the course is in a south-easterly direction and so continues as far as the source. The famous Casiquiare bifurcation is about 150 miles above the mouth of the Ventuari; and Esmeralda, the highest point of attempted permanent civilisation on the Orinoco, is some twenty miles beyond. Esmeralda was, even in Humboldt’s time, a flourishing mission, but has been now for many years little more than a name, the houses being reduced to two or three huts. The name is derived from the quantity of fragments of chloritic and colourless quartz which lie scattered about the grass plains on which the mission was established. All travellers bear witness to the beauty of the position, with the peak of Duida visible beyond the forest to the north-east; but they also agree in considering Esmeralda the worst place for the tiny sandflies (mosquitoes of the Spaniards), which are the great plague of the Upper Orinoco. Duida appears to be one of the peaks of the vertical-sided tableland type usual in Guayana, belonging to a range of mountains extending into the Piaroa territory, though broken at many points by river valleys, including that of the Ventuari. Though similarity of form cannot always be regarded as indicating identity of composition, the fact that such observations as have been made point to the floor of the whole elevated plateau of Guayana, here as elsewhere, being made of the same granite as that of the Roraima Hills and the Callao goldfields. It seems justifiable to suppose that all these peculiar mountains are of the same probably pre-Cambrian sediments, and it may be that Mount Duida and the range of which it is part will be found to be pierced by the dykes and sills which elsewhere in the Guayanas are often found to be accompanied by gold and other ores. Beyond Esmeralda several travellers, from the time of Don Francisco de Bobadilla downwards, have reached the Raudal de los Guaharibos, but all have been compelled to turn back from those rapids, on account of the ferocity of the Indians of that name. Only one traveller claimed to have been more fortunate than the rest and to have reached the source of the great river. The Guaharibo Rapids are situated some 120 miles up-stream from Esmeralda.