ii8 The Control of Mosquitoes 



this nature when the heavy rocks roll down the 

 slope faster and go farther than the accompany- 

 ing earth, it generally happens that a layer of 

 rocks and botilders lies under the heavy fills thus 

 made. 



Much of the rock used in filling taken from Cule- 

 bra Cut decomposes rapidly when exposed to the 

 air, and turns into soil in a few months time. The 

 freshly made dumps are at first sufficiently porous 

 to absorb the rain-water that collects in the low 

 parts of the uneven surface of the dump. However, 

 in a year's time settlement takes place and also the 

 impervious particles of the decomposed rock as 

 well as clay are washed by storm water into the 

 depressions. When the depressions cannot be 

 economically drained it is frequently possible to 

 get rid of the water by leading it down to the 

 spaces in the layer of loose stones at the bottom 

 of the dump. This is done by drilling a hole 

 near the pond and using a little dynamite. 



Within a year's time the new dumps on the 

 Isthmus were covered with dense vegetation and 

 had to be frequently inspected as the older they 

 became the more numerous were the small surface 

 pools. 



The vegetation may make travel difficult. 

 Sometimes it is btumed in the dry season, to facili- 



