172 Causes of Fertilitij or Barrenness of Soils. 
chains and lofty hills, bare, rushed, and incultivable, is the 
parent of the rich soils found in the surrounding valleys ; indeed 
all day soils have resulted from the decomposition of granites 
containing felspar. Most soils are formed by the decomposition 
of the beds beneath, assisted by the deposit of animals and the 
growth and decay of vegetable matter. The principal agent is 
moisture, which swells out the particles, diminishing the cohesive 
attraction, and preparing the way for the chemical and mechanical 
action of the atmosphere, which gradually separates the various 
substances from previous affinities, and prepares, by new com- 
binations, food for the vegetables which speedily appear. How 
the plant first originates, is often mysterious ; whether a seed may 
be transported by birds, or whether nature has the power of 
spontaneous production, is a question which might form subject 
for curious and interesting investigation, but which is quite 
foreign to the present paper. Yet I cannot forbear mentioning a 
fact, which I have from good authority, as it goes some way to 
prove the latter hypothesis. The shepherds of the moors in 
Scotland are sometimes in the habit of firing the heather ; the 
ashes remain on the surface ; without any seed being sown, a rich 
herbage of Dutch clover springs up. 
The lower forms of vegetable life first appear in new land, 
or more properly, in disintegrated rock, and by their roots 
clinging to and intersecting the mass, as well as by the pro- 
tection afforded to the surface against the destructive effect 
of heavy rain, &c., materially assist in the process of forming a 
soil. 
The action of frost is also very imj)ortant, the expansion of 
the moisture in the soil or rock breaking up the particles, and 
preparing the way for the chemical forces to act : these are 
chiefly the oxygen of the air and carbonic acid gas, dissolved in 
rain water ; both possess powerful affinities for many mineral 
substances — oxygen forming oxides, generally more soluble and 
looser in nature than the original minerals ; carbonic acid acting 
upon lime, magnesia, and the alkalies, destroying previous com- 
binations, and forming soluble carbonates. 
The character of a soil will depend upon the nature of the 
parent bed ; thus soils from granite consist principally of silicate 
of alumina, with soluble alkalies and iron, all more or less clayey 
in nature and destitute of lime. The soils from the chalk (espe- 
cially the upper beds) contain large quantities oi carbonate of 
lime with alkalies and iron, but very little clay, and form light 
free-working soils. In the former case, the felspar of the granite 
is the substance from which the clay soil is derived. Felspar 
■consists of : — 
