On the Food of Plants. 
499 
process has been goinnr on. Such is the general principle; it is 
necessary, however, to be more explicit. 
Soils are usually described as either purely argillaceous, cal- 
careous, or sandy, or else as having a mixed character : thus, 
when clay and sand are both present in considerable quantity, the 
term " loam " is applied ; a mixture of clay and carbonate of lime 
is called a "marl,'' tScc, These words are exceedingly indefinite, 
and their uncertain use frequently leads to error ; indeed a good 
system of classification of soils, founded principally on their 
chemical constitution, would be a great boon to scientific 
agriculture. 
1. Clay is derived primarily from the decomposition of an un- 
stratified rock, and as this generally contains a very large propor- 
tion of felspar, that substance may be looked upon as the principal 
source of the body in question. It is no argument against this 
view that many beds of ancient clay exist among the secondary 
strata, and occasionally form the surface of the ground, and con- 
sequently the soil itself; there is every reason to believe that 
these beds owe their origin to the destruction of very far more 
ancient masses of granite, or trachyte, or basalt, the ruins of 
which were transported by water to the bed of some then existing 
sea and there deposited. 
Beside these clay strata belonging to the secondary or middle 
series, must be placed the still more ancient formations of slate, 
frequently of immense thickness, and so altered by the gradual 
but ceaseless effects of ever-acting causes, that their origin is 
sometimes lost sight of: still there is no difficulty in tracing the 
change from the softest clay to the most compact slate. 
The disintegration of granite has been studied with considerable 
success. The felspar which it so largely contains is so con- 
stituted that it may be looked upon as a double salt of alumina 
and potash, analogous to common alum, having silicic acid, or 
silica, in the place of sulphuric. 
Alum, anhydrous . . KO, S O3 + Al^ O3, 3 S O^. 
Felspar K 0, Si + Al, Oa, 3 Si O3. 
When this substance is exposed to the action of water charged 
with carbonic acid, it slowly suffers decomposition in such a 
manner that the greater part of its alkali is removed together with 
some silica, in a soluble condition, while all the alumina with the 
rest of the silica and a little potash remain behind under the form 
of clay. This decomposition is sometimes accelerated by the 
formation of sulphuric acid from the iron pyrites so frequently 
present.* 
In certain districts of Cornwall and Devonshire, as well as in 
* Mitscherlicli, Lehrbuch, vol. ii. p. 157. 
