106 BPPECTS OF FORESTS ON MOtSTUEB. 



as their decay, one might be led to conclude that they sprung up 

 immediately after the Flood, if not before it. The natives have also 

 the yearly custom of burning the dry grass, which on some occasions 

 destroys shrubs and trees even on the very summit of the mountains. 

 To this system of extermination maybe attributed the long succession 

 of dry seasons.'' " To the same cause," says he, " may be traced the 

 diminution of fountains, and the entire failure of some which formerly 

 afforded a copious supply, such as Griqua Town, Campbell, and a 

 great number of others which might be mentioned. It has been 

 remarked that since the accidental destruction of whole plains of the 

 Olea similis (wild olive) by fire, near Griqua Town, as well as the 

 diminishing of large shrubs on the neighbouring heights, a gradual 

 decrease of rain has succeeded in that region." 



In treating of these, the secondary causes of the desiccation of 

 South Africa, I had occasion to quote, from Marsh's treatise on " The 

 Earth as Modified by Human Action," the following statement, which 

 I followed up with the remarks and quotations from the same and 

 other authors, which follow : — 



" ' Whenever a tract of country, once inhabited and cultivated by 

 man, is abandoned by him and domestic animals, and surrendered to 

 the undisturbed influences of spontaneous nature, its soil sooner or 

 later clothes itself with herbaceous and arborescent plants, and at no 

 long interval with a dense forest growth. Indeed, upon surfaces of a 

 certain stability, and not absolutely precipitous inclination, the 

 special conditions required for the spontaneous propagation of trees 

 may all be negatively expressed, and reduced to these three : — 

 exemption from defect or excess of moisture, from perpetual frost, 

 and from the depredations of men and browsing quadrupeds. When 

 these requisites are secured, the hardest rock is as certain to be over- 

 grown with wood as the most fertile plain, though for drier seasons 

 the process is slower in the former than in the latter case. Lichens 

 and mosses first prepare the way for a more highly organized vege- 

 tation. They retain the moisture of rains and dews, and bring it to 

 act, in combination with the gases evolved by their organic processes, 

 in decomposing the surface of the rock they cover ; they arrest and 

 confine the dust which the wind scatters over them, and their final 

 decay adds new material to the soil already half-formed beneath and 

 upon them. A very thin stratum of mould is sufficient for the 

 germination of seeds of the hardy evergreens and the birches, the 

 roots of which are often found in immediate contact with the rock, 



