352 EFFECTS OF BUENING FOEEST. 



Aside from tlie destruction of the trees and the laying bare of 

 the soil, and consequently the freer admission of sun, rain and 

 air to the ground, the fire of itseK exerts an important influence 

 on its texture and condition. It cracks and sometimes even pul- 

 verizes the rocks and stones upon and near the surface ; * it con- 

 sumes a portion of the half-decayed vegetable mould which served 

 to hold its mineral particles together and to retain the water of 

 precipitation, and thus loosens, pulverizes and dries the earth ; 

 it destroys reptiles, insects and worms, with their eggs, and the 

 seeds of trees and of smaller plants; it supplies, in the ashes 

 which it deposits on the surface, important elements for the 

 growth of a new forest clothing, as well as of the usual objects 

 of agricultural industry ; and by the changes thus produced, it 

 fits the ground for the reception of a vegetation different in char- 

 acter from that which had s]3ontaneously covered it. These new 

 conditions help to explain the natural succession of forest crops, 

 so generally observed in all woods cleared by fire and then 

 abandoned. There is no doubt, however, that other influences 

 contribute to the same result, because effects more or less analo- 

 gous follow when the trees are destroyed by other causes, as by 

 high winds, by the woodman's axe, and even by natural decay.f 



* In the burning over of a hill-forest in the Lower Engadine, in September, 

 1865, the fire was so intense as to shatter and calcine the rocks on the slope, 

 and their fragments were precipitated into the valley below. — Eivista Forestale 

 del Regno d' Italia, Ottobre, 1865, p. 474. 



f The remarkable mounds and other earthworks constructed in the valley 

 of the Ohio and elsewhere in the territory of the United States, by a people 

 apparently more advanced in culture than the modern Indian, were overgrown 

 with a dense clothing of forest when first discovered by the whites. But 

 though the ground where they were erected must have been occupied by a 

 large population for a considerable length of time, and therefore entirely 

 cleared, the trees which grew upon the ancient fortresses and the adjacent 

 lands were not distinguishable in species, or even in dimensions and character 

 of growth, from the neighboring forests, where the soil seemed never to have 

 been disturbed. This apparent exception to the law of change of crop in 

 natural forest growth was ingeniously explained by General Harrison's sug- 

 gestion, that the lapse of time since the era of the mound-builders was so 

 great as to have embraced several successive generations of trees, and occa- 

 sioned, by their rotation, a return to the original vegetation. 



The successive changes in the spontaneous growth of the forest, as proved 

 by the character of the wood found in bogs, are such as to have suggested the 

 theory of a considerable change of climate during the human period. But 



