The geological aspect of the district, the shapes of the rocks of Keri and Oco, which have so much the character of islands, the water-worn hollows in the first named of these rocks, situated at exactly the same height as the cavities in the opposite island of Uivitari, all testify that the Orinoco once filled the whole of this now dry gulf or bay. Probably the waters formed a wide lake as long as the northern dike was able to withstand their pressure. When it gave way, the prairie now inhabited by the Guareke Indians must have been the first part which appeared above the waters; which may subsequently, perhaps, have long continued to surround the rocks of Keri and Oco, which rising like mountain fortresses from the ancient bed of the river, present a picturesque aspect. As the waters gradually diminished they withdrew altogether to the foot of the eastern hills, where the river now flows. This conjecture is confirmed by several circumstances. The Orinoco, like the Nile near Philæ and Syene, has the property of imparting a black colour to the reddish white masses of granite which it has bathed for thousands of years. As far as the waters reach, one may remark on the rocky shore the leaden-coloured coating described in page 189: its presence, and the hollows before mentioned, mark the ancient height of the waters of the Orinoco. In the rock of Keri, in the islands of the Cataracts, in the gneiss hills of Cumadaminari above the island of Tomo, and lastly at the mouth of the Jao, we trace these black-coloured hollows at elevations of 150 to 180 (160 to 192 English) feet above the present height of the river. Their existence teaches us a fact of which we may also observe indications in the river beds of Europe; viz. that the streams whose magnitude now excites our astonishment are only the feeble remains of the immense masses of water belonging to an earlier age of the world. These simple remarks and inferences have not escaped even the rude natives of Guiana. The Indians everywhere called our attention to the traces of the former height of the waters. There is in a grassy plain near Uruana an isolated granite rock, on which, according to the report of trustworthy witnesses, there are at a height of more than eighty feet drawings of the sun and moon, and of many animals, particularly crocodiles and boas, engraven or arranged almost in rows or lines. Without artificial aid it would now be impossible to ascend this perpendicular precipice, which deserves to be carefully examined by future travellers. The hieroglyphical rock engravings on the mountains of Uruana and Encaramada are equally remarkable in respect to situation.