174 GNATS OR MOSQUITOES — CHAPTER VIII 



subsoil water may lie, puddles will necessarily form, provided 

 only the surface layer be sufficiently retentive. Hence sites 

 having a clayey surface are necessarily favourable to the 

 development of malaria, and even if the actual surface be 

 not of this character, it is quite possible for a thin inter- 

 mediate stratum of such material to so hold up the surface 

 water as to admit of the surface being practically water- 

 logged, in spite of the subsoil being quite pervious and the 

 ground water deep. An impervious stratum of this sort 

 is to be found in many parts of the Gangetic alluvium some 

 four or five feet below the surface, as is shown by the fact 

 that in such districts any attempt to increase water storage 

 by the deepening of existing tanks merely results in their 

 drying up. 



In the Sitapur district (Oudh) for example, several tracts 

 of country of this description are to be found, and though 

 the subsoil water lies some thirty or forty feet from the 

 surface they are notoriously malarious. 



I am not aware that such an experiment has ever been 

 tried, but in the case of limited areas, such as inhabited 

 sites, it seems possible that their sanitary condition might 

 in such cases be improved by the construction of a number 

 of blind wells carried through the impervious layer so as to 

 open up the porous subsoil beneath. 



It would be impossible within any moderate limits to 

 give any account of the various kinds of soil that are to be 

 met with in India, as it is needless to say that in so vast a 

 country almost every possible combination of soil and 

 contour formation is to be met with, and malaria is rife 

 more or less throughout ; but a few words with respect to 

 the commoner formations may not be out of place. 



Between the foot of the Himalayas and the old island 

 India of past geological times stretches an immense level 

 plain reclaimed from the sea by the silt deposited by the 

 mountain streams that now fall into the Ganges and Indus. 

 After this old narrow sea had completely silted up, the rivers 

 continued to raise their beds, wandering from side to side 

 whenever the detritus deposited in their shoals accumulated 

 sufficiently to raise them above the level of the land hard by. 



