93 



Not attackL'il. l-icsistance is due- to silicious crystals in \hv cells, rather 

 than to an alkaloidal poison as in the case of greenheart. This doubtless 

 makes the resistance of manharklak more permanent. The wood is, 

 however, excessively heavy and hard, being impossible to work witli 

 ordinary steel tools. 

 *roledo wood (See manbarklak). 



Dl'R.XBILITV OF UXTRE.ATKT) .\N'D UnPROTI'XTED WoOD 



The value of an\- nu-lhod of protecting piling or of any substitute for wooden 

 piling depends upon the life which can be obtained from the protection in question as 

 compared with that of untreated, unprotected wooden piling. In a region as large as 

 San Francisco Bay and its tributaries, where the water conditions vary from the 

 freshness of river water to ocean salinit>-, and from the extreme contamination by 

 sewage and factory wastes from great cities to the relative purity of ocean water, it is 

 to be expected that great differences will be found in the durability of untreated piling 

 in various locations. The life of nati\e species may \ary from as little as six months 

 at the point of worst borer inft'statiDn, to as much as 4 or 5 years in specially isolated 



Fig. 35. Green Douglas fir pile driven at Mooring Z, Mare Island, on August 14, 1920, and pulled 

 December, 1920. Teredo mivalis had penetrated four inches in this four-month interval. 



(U. S. Navy Photo.) 



or protected localities. This assumes ample salinit>' of the water. Going up the Sacra- 

 mento River or elsewhere into increasingly fresher water, one can, of course, find all 

 degrees of diminishing intensity of attack. In the biological part of this report, other 

 factors are discussed which influence the distribution of the several borers and the 

 severity of their attack. 



It is significant that of the se\eral thousand untreated wooden piles taken luider 

 observation by this Committee in 1920 (after the teredo had been working in the 

 Carquinez Strait region for three years or more) every one had been torn out or re- 

 placed by the end of the following year, except some which in the meantime were 

 salvaged by the concrete jacket method. 



llntreated piles, unprotected, still ha\'e their use in temporary marine structures, 



