Chap. 11. UNHEALTHINESS OF BRANCH RIYERS. 95' 
doubt, that gold is occasionally found within two or 
three days' journey of Aveyros ; but all lengthened 
search is made impossible by the scarcity of food and 
the impatience of the Indians, who see no value in the 
precious metal, and abhor the tediousness of the gold-' 
searcher's occupation. It is impossible to do without 
them, as they are required to paddle the canoes. 
The weather, during the month of July, was uninter- 
ruptedly fine ; not a drop of rain fell, and the river sank 
rapidly. The mornings, for two hours after sunrise, 
were very cold ; we were glad to wrap ourselves in 
blankets on turning out of our hammocks, and walk 
about at a quick pace in the early sunshine. But in the 
afternoons the heat was sickening ; for the glowing sun 
then shone full on the front of the row of whitewashed 
houses, and there was seldom any wind to moderate 
its effects. I began now to understand why the branch 
rivers of the Amazons were so unhealthy, whilst the 
main stream was pretty nearly free from diseases aris- 
ing from malaria. The cause lies, without doubt, in the 
slack currents of the tributaries in the dry season, and 
the absence of the cooling Amazonian trade-wind, which 
purifies the air along the banks of the main river. The 
trade-wind does not deviate from its nearly straight 
westerly course, so that the branch streams, which run 
generally at right angles to the Amazons, and have a 
slack current for a long distance from their mouths, are 
left to the horrors of nearly stagnant air and water. 
Aveyros may be called the head-quarters of the 
fire-ant, which might be fittingly termed the scourge 
of this fine river. The Tapajos is nearly free from 
