130 PRELIMINARY WORKS. 



(4.) Treatment of excessive laj-ers of vegetable matter. 



(5.) Fixation of shifting sand. 



(6.) Fixation of unstable soil on slopes. 



It would require a volume to itself to deal exliaustivelj- 

 with these matters; hence, in this place, only a few short 

 remarks can be made on the more important points. 

 Those who require more detailed information will find 

 it in Volume IV. of this Manual and in special works on 

 the several subjects. 



1. Treatment of an Impermeable Suhstratiim. 



Impermeable strata in the soil consist in some cases of an 

 accumulation of cla}', which has bj^ degrees been washed out 

 of the surface layers and deposited at a certain depth ; in 

 other cases sand or gravel has been converted into a hard 

 rock-like mass by the addition of organic matter, clay, or 

 oxide of iron. Such a stratum is frequently called a jmn. It 

 may be found at varying depths below the surface ; if at a 

 depth of 4 feet or more, it does not, as a rule, interfere with 

 the growth of forest trees, except perhaps on arid unirrigated 

 ground, but if it occurs nearer the surface it may produce the 

 following injurious effects : — 



(a.) Interference with the development of a proper root 

 system, followed by small height growth and liability 

 of the trees to be thrown by strong winds. 

 {h.) Interference with the movement of water in the soil, 

 preventing its penetration into the subsoil during wet 

 weather, and its ascent during dry weather ; in other 

 words, it may render the soil too wet at one time and 

 too dry at another. 

 The remedy is to break through the impermeable stratum, 

 so as to connect the upper and lower permeable layers of the 

 soil. According to the nature of the pan and its depth below 

 the surface one of the following methods of treatment must 

 be adopted : — 



(a.) Deep ploughing. 



