ii6 The Control of Mosquitoes 



larvae can mature. Lowlands that cannot be 

 drained and those parts of extensive swampy 

 land that produce Anopheles can often be filled 

 to advantage. 



It is also important to enact laws preventing 

 excavation or filling from becoming the source of 

 new broods of mosquitoes. In borrowing material 

 for fills, relatively porous material should be se- 

 lected, if available, because depressions in a fill 

 made of clay will retain rain-water. If the ma- 

 terial is taken from a hillside, the bottom of the 

 borrow pit shotild be left properly graded. 



If seepage planes are suspected, the proposed 

 site of the borrow pit should be examined with 

 an earth augur to determine the depth of any 

 seepage plane that may be present, and the 

 borrow-pit floor must be kept well above the damp 

 soil overlying the plane of seepage. If this is not 

 done it may become necessary, later, to install 

 seepage intercepting ditches to take care of the 

 borrow pit. The finished surface of fills should be 

 properly graded and allowance made for subse- 

 quent settlement, which always takes place after 

 filling. 



Attempts to cover wet places where subsurface 

 water under pressure is the difficulty will often 

 fail when a fill is shallow, and the entire new fill 



