The llanos are not, however, absolutely level; there are flat banks of slight elevation (a few feet) called _bancos_, only to be observed at their edges, and extending for miles, and also _mesas_, convexities gently, almost imperceptibly, rising to a very moderate height, and yet sufficiently important to form water divides. A glance at the map shows that in the llanos south of Carácas the streams flow right across Venezuela, from the hills to the Orinoco or its tributaries, and, in fact, the whole of the western llanos are very gently inclined to the south-east, but in the eastern part of the llanos south of Cumaná we see a water divide, the Rivers Tigre and Guanipa flowing eastwards to the Delta, and farther west a number of rivers flowing into the Orinoco, while north of the divide various streams, the Aragua, the Unare, and others, empty themselves into the sea west of Barcelona, this being due, not to a range of hills but to the convexity of the plains, the mesas of Tigre, Guanipa, &c. It is hard to understand that lack of water is one of the difficulties besetting the traveller on the llanos when one sees the network of streams indicated on the maps. In the wet season there is of course plenty of water, but in the dry season most of these tributary streams cease to flow. Those which take their rise in the hilly country have, indeed, some water in their upper courses all the time, while in their lower part water from the main streams, the Orinoco, the Apure, the Portuguesa, and others, is able to back a good way up, on account of their very gentle fall, but their middle courses run quite dry, pools remaining here and there in the hollows. The water in these gets somewhat foul, but by digging in the sand in their neighbourhood sweeter water may be obtained. The River Guárico, which partly dries up in this way, is said formerly to have flowed all the year round. It takes its rise near Lake Tacarigua (Lake of Valencia), and perhaps the diminution of its water supply is due to the same cause as the shrinking of that lake, the cutting down of forest and the cultivation of land near its source. In the wet season many of the streams overflow and flood wide areas, the cattle having to take refuge on slightly higher tracts. When the waters again retire many alligators and water-snakes bury themselves in the mud, and one hears of people being startled by sudden upheavals of the ground, followed by the emergence of some disturbed monster. In one case a traveller settled for a night in a hut which had been flooded the previous season, and the barking of the dogs awoke a huge alligator, who heaved up the floor, made a dash at the dogs, and then fled into the open.