THE LONGEVITY OF TREES. 85 



section, from the circumference towards the centre, where the 

 space in which the layers could not be clearly made out was 

 estimated to have comprised three hundred more. If the in- 

 jured portion was not overestimated, the tree must have been 

 a thousand years old. We have now before us a section of a 

 fine trunk of the American Cypress (Taxodium distichum'), 

 upon the radius of which, twenty-seven inches in length, six 

 hundred and seventy annual layers may be distinctly counted. 

 The wood of this tree is so durable, that probably the age of 

 trunks of more than twice that size might be ascertained by 

 direct inspection. 



When such a section cannot be obtained, we are obliged to 

 resort to other and less direct evidence, affording only ap- 

 proximate, or more or less probable, conclusions. Sometimes 

 lateral incisions, not endangering the life of the tree, furnish 

 the means of inspecting and measuring a considerable number 

 of the outer layers, and of computing the age of the trunk from 

 its diameter and actual rate of growth. But as young trees 

 grow much more rapidly than old ones, we should greatly ex- 

 aggerate the age of a large trunk, if we deduced its rate of 

 growth from the outer layers alone. We must therefore 

 ascertain, by repeated observations, the average thickness of 

 the layers of young trees of the same species ; and by the 

 judicious combination of both these data, a highly probable 

 estimate may often be formed. 



When unable to inspect any portion of the annual layers of 

 remarkable old trees, we may occasionally obtain other indi- 

 cations upon which some reliance may be placed ; such as the 

 amount of increase in circumference between stated intervals ; 

 but as, on the one hand, we can never depend upon the entire 

 accuracy of two measurements made at widely distant periods, 

 while, on the other, the growth of a small number of years, 

 however carefully ascertained, would be an unsafe criterion, 

 this method can seldom be employed with much confidence. 

 A more common mode is to employ the average rate of growth 

 of the oldest trees of which complete sections have been ex- 

 amined, for the approximate determination of the age of re- 

 markably large trunks of the same species, where the size 



