"Malarial Fevers. 597 



In the interior, too, about the low grounds bordering the 

 Piedmont districts of the Cordilleras, or on the water- 

 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. Tlie trade-wind does not deviate 

 from its nearly straight course west, so that the branch-streams, which are 

 generally at right angles to the Amazons, and have a slack current for a long 

 distance from their mouth, are left to the horror of nearly stagnant air and 

 water." This may apply, possibly, to the Lower Amazons, but on the tribu- 

 taries of the Alto-Amazonas, where the currents are stronger all the year 

 round than those lower down are at the rainy season, you find the intermit- 

 tents to prevail ; nor is the trade-wind either as certain or as strong as below. 

 And it may probably be true that the borders of tlie Amazons are constantly 

 kept swept by the rising floods, which carry off the decaying material which 

 might engender the malaria. It seems to me that with the impenetrable 

 mass of vegetation as ramparts, as it were, on the banks of the Amazons, 

 and for leagues to the interior, the trade-wind could make little success in 

 getting through this to ventilate the shores at sufficiently low an elevation 

 from the river-margin to prevent malarial emanation from affecting those 

 who would necessarily sleep or work a very few feet above the level of the 

 stream itself It is a fact noted both by Darwin and Humboldt, in their 

 travels in Peru and other tropical places, that there was to be encountered a 

 greater prevalence of malarial emanation in those districts where there was 

 a dry soil, generally sandy, short grass growing thereabouts, and stagnant 

 pools, as about Arica, Callao, Vera Cruz, Carthagena, and such-like localities. 

 It may, therefore, be, that, toward the head-waters, or the higher inland re- 

 gions of the Amazons tributaries — the dense foliage of the main river not be- 

 ing so great, the sun being able to peneti-ate the forest-growth, while at the 

 same time there are present, from the immense rain-fall in these situations, 

 as on the river-margins of the main stream, lagoons which never are washed 

 out or dried up entirely- — the same conditions would obtain as in the places 

 referred to above, the actual temperature also being raised by the arid and 

 often sandy nature of the soil about these localities. 



The island of Marajo, at the mouth of the Amazons River, is about the 

 size of the State of Rhode Island, or larger. The northwest, north, and west 

 sides of it are low savanna lands. On the south, southwest, and southeast, 

 the country is densely wooded with tropical growth. Some years ago, a town 

 existed on the north side, but the unhealthfulness of the place was so fatal 

 that it Avas broken up as a commercial port altogether, while Para, to the 

 south and west, on the opposite or south side of tlie river, is free from local 

 fevers (excepting occasional epidemics of yellow fever). It may be possible 

 that the northeast trades blowing to the southwest toward Para, may be pu- 

 rified by having to pass through the dense rampart on the southwest side of 

 Marajo. Para itself is situated on the south arm of the Amazons, or the 

 Para, river, as it is known locally, and is surrounded by the virgin growth of 

 the tropical forest all about the place. The prevailing winds there are north- 

 east. [Keller, in his Exploration of the River Madeira, makes the follow- 



