VARIOUS TYPES OF CAVITIES 169 



from it if it should enter, to a minimum. When 

 too great an excision of sound wood is required to 

 secure drainage, the lower part of the cavity, up 

 to the level of the opening, can be filled with 

 asphalt, the balance of the cavity being concreted. 



Careful bracing is ordinarily very essential to 

 the successful treatment of a large trunk cavity. 

 The amount of iron to be used, and its distribu- 

 tion, of course depend upon the strength of the 

 remaining sound wood and upon the strain it is 

 subjected to. Cavities having an opening of any 

 considerable vertical dimension generally need 

 horizontal braces. If the excavation has left the 

 trunk very weak the tree as a whole should be 

 braced to a stake or to a neighboring tree, as will 

 be described in the chapter on bracing. The pos- 

 sibility of reducing the strain on the trunk by 

 pruning in some of the branches should also be 

 considered. 



The dressing of the interior of a trunk cavity 

 should be as thoroughly done as it possibly can be, 

 and the most effective materials should be used. 

 If the inner surface is easy to get at, melted as- 

 phalt is the ideal dressing; if difficult, a dissolved 

 asphalt is likely to be more satisfactory. The 

 braces must be coated, of course, along with the 

 wood. 



With the choice of a material for filling, we 

 come to the most serious problem presented by 



