WOODY LAYERS. 
9 
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they are not nearly so distinct; indeed, in some it is 
impossible to trace them. 
The woody layers, when first formed, are full of sap, 
but they change and gradually become solidified by the 
compression of each subsequent layer; and it seems 
obvious, that as each zone is moulded upon the one of 
the previous year’s growth, it would, by cohesion, 
become amalgamated with it. The perfecting of the 
concentric layer is, however, a very gradual process, 
FIG. 4. FIG. 5. 
and the time necessary to convert a new layer of sap- 
wood into heart-wood (which alone represents the 
serviceable timber in most trees) varies from about one 
year, as in Hornbeam, Ash, Beech, and in some other 
species, to thirty years or even more, as in Oak, &c. &c. 
It seems, as a rule, from evidence to be shown later on 
in Table II., that Oak trees which form their wood most 
rapidly under ordinary conditions of growth are the 
best in quality. In the Firs it is the same, and we see 
it also in the Pines (Kauri excepted). I incline, there¬ 
fore, to the opinion that it must be the case in every 
