CONDITIONS INFLUENCING PREVALENCE 173 



Always assuming tbat there be no water-logging of the 

 soil, it is obvious that the more porous it be, the less the 

 probability of the formation of puddles of sufficient per- 

 manence to admit of the rearing of a brood of Mosquitoes. 

 Sand for example, holds water so badly that short of com- 

 plete water-logging the formation of puddles is an absolute 

 impossibility ; but the deposition of a very thin layer of 

 fine silt by flood water will suffice to render such soils 

 sufficiently retentive. In the same way, rocky soils are 

 usually healthy, because they are generally found in hilly 

 regions, and are therefore associated with good surface 

 drainage ; but if the peculiarities of the rock are such as to 

 lead to the formation of basin-like depressions during the 

 process of weathering, such a soil may be highly favourable 

 to the development of malaria, always provided that the 

 rock be of uniform and impervious texture, so that the 

 water cannot soak away. For this reason the malariousness 

 of Hong Kong was, in our older books on hygiene, ascribed 

 to some mysterious influence of a soil consisting of de- 

 composed granite. 



In a certain sense, of course, it really is the decomposi- 

 tion of granite that is at the root of the mischief, but this 

 is not because granite, whether intact or decomposed, can 

 have anything to do with the effect ascribed to it, but 

 because the denudation of such rocks leaves the undecom- 

 posed portion hollowed out into basins as watertight as the 

 best porcelain, and moreover, where such rocks form the 

 beds of water courses, they are particularly liable to be worn 

 into " pot-holes" by the action of pebbles retained in some 

 chance depression. 



According to Drs. Stephens and Christophers, (B.S.M.C, 

 July, 1900, p. 43), the persistence of Anopheles during 

 the drier portions of the year in Freetown, Sierra Leone, 

 is due to puddles retained m rock basins of this description, 

 and similar conditions have been noted in Southern India, 

 in addition to which I have myself met with Anopheles 

 pools of this description at Jhansi. 



Speaking generally, it is the character of the surface soil 

 that is of the greatest importance, as however deep the 



