SOIL FORMATION: ROCK DISINTEGRATION. 323 



488. Effect of fire. — This may also have been hastened by 

 fires which would now more often sweep over the swamp during 

 dry seasons. In partial evidence of this are many young spruce 

 trees with scars near the ground where the bark has been 

 destroyed. This gives admittance to wood-boring insects which 

 farther aid in the process of weakening and debilitating the 

 trees. The dying off of the lower limbs of these marsh spruces 

 suggests the action of fire, as well as excessive moisture at 

 times. Many of them now present only a small convex top of 

 living branches. It is interesting to observe the gradation in 

 this respect in different trees. 



489. Weird aspect of dead spruces. — The weird aspect pre- 

 sented by a clump of these dying young spruce trees is height- 

 ened also by the changes in the form of the branches as they 

 die. The living branches have a graceful sigmoid sweep with 

 their free ends curving upwards as in many conifers. As the 

 branches die, the free ends curve downward more and more, all 

 gradations being presented in a single tree. A group of such 

 dying spruce trees is shown in fig. 266. Some have been long 

 dead; only the knotted, weather-beaten trunks still remain 

 tottering to their final condition. Others with leafless, dried, 

 sprawling branches go swirling with every wind, while a few 

 struggle on in the presence of these untoward conditions. 



490. Other morainic moors. — In other basins, where the 

 hills on all sides are still forest-clad, more equable temperature 

 and moisture conditions are conserved. This permits plants to 

 flourish here which in the exposed basins are disappearing from 

 the formations or -only leading a miserable existence. . This is 

 strikingly true of some sphagnum formations. In the atoll 

 formation described tne evidence suggests that sphagnum 

 formerly played a more active part in the evolution of that type 

 of moor than has been the case since the hills were denuded of 

 their trees. So also in the spruce moor, sphagnum probably 

 was at one time a prominent factor in the formation of the early 

 vegetation. But excessive drought during certain seasons, and 



