40 INFLUENCE OF ROCKS UPON THE FERTILITY OF SOIL, &C. 
stance. If they were to lose this power, or the air and earth, the 
capacity of furnishing these elements, all living things would die ; 
first the brute creation, but our own race would not evade the gene- 
ral ruin. The inhabitants of the ocean as well as of the land, the 
lower as well as the higher orders of living beings are nourished 
either directly or indirectly by vegetable matter. That shell fish 
do not thrive upon a bed of pure siliceous sand, is known to ac- 
curate observers, who frequent the borders of the sea, nor would 
they be able to exist at all but for the supply of vegetable or 
animal matter that is brought to them dissolved or suspended in 
the water of the ocean that flows over them. 
A very few vegetables are proper air plants, that is, require 
only some solid body to which to apply themselves, or a fibrous 
substance, such as moss, cotton, or amianthus, amongst which to 
insinuate their roots, and they will thrive and grow without be- 
ing brought into contact with either earth or water. The Tilland- 
sia Usneoides or long moss of the Low-Country, is an example. 
But in general, we know that trees and plants require the presence 
of soil into which to strike their roots, as also that to those which 
are most valuable by reason of their affording food or clothing, or 
otherwise subserving the convenience or necessity of man or other 
animals, a soil of a peculiar kind is necesary, or they fail of at- 
taining the highest perfection of size and quality of which they 
are capable. 
Under the action of the elements the rocky strata of the globe 
are undergoing disintegration and decay. Some merely crumble 
clown into a coarse gravel, whilst others are resolved at once into 
an impalpable powder. It is by the disintegration of the rocks 
that arable land has been produced in all quarters of the world. 
Is there any connexion between the structure and composition of 
the rocks and the fertility of the soils they form ? There is a 
very evident connexion of this kind. Why the relations should 
be such as are observed in nature we are unable to tell ; why for 
instance the valley of Egypt should be more productive than the 
neighbouring sands of Lybia, or the low-ground alluvions of our 
large rivers, than the pine barrens by which they are skirted on 
either hand. We can only generalize those facts which expe- 
rience and observation have well ascertained. 
The fertility of different soils so far as it depends upon their 
own constitution and the character of the rocks from which they 
have been formed, is determined by four principal circumstances. 
1. Their composition ; the kind of earth of which the rocks 
are made up. 
2. Their susceptibility of disintegration by the action of the 
elements. 
3. The nature of their upper surface, whether level, or rugged, 
and broken. 
4. The amount of decayed vegetable or animal matter the soil 
in question may contain. 
