On Malaria. 313 



gage and concentrate the miasmata, and these by the cooler air of 

 the fall produce endemic and epidemic diseases. 



Salt marsh, although less fruitful of malaria than fresh water bogs 

 and marshes, has erroneously obtained a reputation for salubrity. 

 The grass of a salt marsh is not pulpy and rank like fresh water weeds ; 

 consequently it is decom.posed less rapidly than that which grows in 

 stagnant fresh water, and the effluvia are diluted by the flow of the 

 tide below high water mark ; but with these qualifications, salt 

 marshes are proportionably insalubrious with fresh water marshes, 

 provided both are exposed to great degrees of heat. This is seen 

 extensively in the Mediterranean, and near the outlets of rivers within 

 the tropics. The rule holds in every climate, the pestilential effects 

 being in the direct ratio of the degree of heat, commencing with the 

 Oronoco and other tropical rivers. 



" The soil on the tops or sides of hills may also contain materials 

 suited to the formation of malaria, if there is clay sufficient in quality 

 and proportions to retain the necessary moisture."* So on plains 

 not marshy, a surface or subsoil of hard clay may so far hold the wa- 

 ter after great rains, as to keep the surface nearly marshy ; and in 

 those peculiar seasons when great heat and drought ensue, the re- 

 sults upon the foregoing principles are obvious. In addition to these, 

 the following local causes may be enumerated — wet meadows and pas- 

 tures, coppices grown up with tangled underwood, plashy grounds, 

 mill ponds, flax ponds, and agricultural ditches and drains. 



A tract of undulating country which came under my own observa- 

 tion, exhibited these pernicious influences, as they affect mild climates, 

 in the experience of 1828. I forbear to mention names, as in giv- 

 ing this or any other local illustration, I wish not to excite alarm, but 

 to suggest the mode of correction ; and the following particulars are 

 stated with a view to aid our judgment as to other situations, and the 

 best modes of reform. This tract, for forty or fifty miles margined on 

 three sides by the sea, is a rolling country, the soil generally a stiff 

 loam upon a substratum of clay, overlying sand at no great depths. 

 The formation is fundamentally primitive, but contains many tracts of 

 superficial alluvion. The hills are of no great elevation but rather ab- 

 rupt, succeeding each other rapidly, sometimes forming wet, basin 

 shaped valleys, and at others, ponds of similar outline, seldom ad- 

 mitting any outlet for the superfluous waters or the wash of the hills. 



* Bancroft on Fevers. 



Vol. XVII.— No. 2. 13 



