34 



D. P. PENHALLOW ON 



range from 3'00 mm. to 6-25 mm., the average being 4-24 mm. Within this zone the 

 structure is always coarse-grained, and it is here that we may expect to find the most 

 prominent shaking. In zone 2 the rings range from 1-75 to 4*00 mm., with an average of 

 2.50 mm. This zone is very commonly coarse-grained, and is much subject, though in less 

 degree than zone 1, to shaking. This part of the tree often constitutes a part of timbers of 

 the "fine-grained" class. In zone 3 the rings range from 0'75 mm. to 2-50 mm., with an 

 average of 1"39 mm. ; while in zone 4 they range from 0"75 mm. to 1-30 mm., with an aver- 

 age of 0'91 mm. Both of these zones are "fine-grained," and enter chiefiy into the com- 

 position of the best quality of timber. This part of the tree is but little subject to " shakes," 

 those which do occur being usually extensions of shakes which had their origin in zone 1 or 

 2. From this it is obvious that the growth-rings, which are always Ijroadest towards 

 the centre of the tree, become constantly narrower with increasing diameter of the stem. 

 Furthermore, in the inner zone, where the rings are broadest, the widest range of variation 

 is exhibited, this range being continually reduced with each successive zone, until in zone 3, 

 and particularly in zone 4, remarkable uniformity often prevails. 



Tlie variations noted in the growth-rings is also found to extend to the zones them- 

 selves. Thus within the same tree there is no constancy of dimension, and again, comparing 

 similar zones of diflrerent trees, wide variations may often be noted. Nevertheless a general 

 law of diminishing radial volume prevails, so that, taking the average of a number of trees, 

 the inner zone will always be thickest and the outer zone thinnest, the rate of diminution 

 for the first four zones being about as 1'6, 1-02, 0-72 and 0-11. These relations may be 

 stated in the following tabular view, in which complete zones only have been taken into 

 consideration : 



No. III. 



WIDTH OF ZONES IN CM. 



From the facts thus presented it is evident that both zones and growth-rings conform to 

 a common law of diminishing thickness, and it is possible that this may be a common result 

 of the same general conditions of growth. These relations are fully exhilnted in the follow- 

 ing synopsis : 



