12 THE SCOTTISH BOTANICAL REVIEW 
proceed further by wind, and the surface materials are again 
sorted into coarse and fine, sand and dust. 
Secular upheaval, erosion, and deposition have doubtless 
been as necessary for the continued evolution of plant life 
on the earth’s crust as the water which rains upon its 
surface. An area doomed to a perpetual plant-covering 
without erosion would in course of time be too depleted 
of mineral nutriment for the continued existence of the 
highest types of vegetation. The waste of mineral nutriment 
involved in failure of a complete return to the soil, and 
in perpetual leaching by atmospheric precipitation and drain- 
age, must in time lead to an impoverishment which is only 
remediable by partial or complete removal of the soil-cap. 
The relations between animal and plant life would in such 
a case only ensure the return of plant food to the soil 
over ever-decreasing and limited spheres. Just as the plants’ 
nitrogen, carbon, and water form only a part of the reserves 
of these substances in the hydrosphere and atmosphere, so 
the other mineral constituents of plant food are but a portion 
stolen during each recurrent epigene phase of the geological 
cycle of the lithosphere, represented by upheaval, decay, 
leaching, erosion, and sedimentation. 
It is computed that nearly five billion tons of mineral 
matter in solution (15) are annually carried seawards by the 
rivers of the globe. Of this grand total one-twenty-fifth 
consists of potash, phosphorus, and nitric acid, the more 
important mineral constituents of plant food ; while if lime, 
the substance so essential for sustained fertility, be included, 
the loss amounts to nearly one-third. How much more of 
these substances is annually put into circulation by the 
plant life of the land, or is temporarily stored in the soil- 
cap or the ground waters, there is at present no means of 
estimating ; but it is evident that, in contrast to the areas 
subject to leaching, all large alluvial areas periodically flooded 
by the river waters are constantly recuperated with the food 
materials necessary for plant life, and that river deltas, that 
continue forming over a subsiding floor throughout a whole 
geological epoch, may be densely covered by vegetation 
without cessation. 
Causes of extensive alterations in stable vegetation, other 
