ese,
and barium.[61] Of these, oxygen is by far the largest constituent,
forming, roughly speaking, about 50 per cent.

The main mass of the rocks consists of silica, and this is generally
combined with alumina, as in clay, forming aluminium silicate, and with
the commoner alkalies and alkaline earths. Another extremely abundant
compound is carbonate of lime, which, as limestone, chalk, and marl,
forms one-sixth of the earth's total rocks.

The word "mineral" means a definite chemical compound of natural
occurrence. The number of minerals is very great, and it is impossible
to go into the subject here. Reference can only be made to a few of the
more prominent ones, which are chiefly concerned in the formation of
soils.

Those formed out of silicates are, from the agricultural point of view,
the most important, as they form a very large group; and it is by their
disintegration that soils are chiefly formed. They consist of silica and
alumina, along with various other substances, chiefly alkalies and
alkaline earths. It is important to note one peculiarity about the
solubility of silicates. We have two classes of silicates: the one,
which is called "acid," and contains an excess of silica; the other,
"basic," and which contains an excess of base. Now, while the former of
these is more or less insoluble, the second is soluble. This fact has an
important signification in the process of the disintegration of the
silicate minerals we are about to consider.

The first and most important class are the _Felspars_. Felspar is not
really a definite mineral, with a definite chemical composition, but
rather the name of a class of minerals of which there are several
different kinds. The felspars are composed of silica and alumina, along
with potash, soda, and lime, with traces of iron and magnesia. Their
principal constituents, however, are silica and alumina, along with
either potash, soda, or lime. According as the base potash, soda, or
lime predominates, the felspar is known as Orthoclase, Albite, and
Oligoclase, respectively.

The following are the analyses of the three minerals (by the late Dr
Anderson):--

--------------------+-----------------+-----------------+-----------------
| Orthoclase. | Albite. | Oligoclase.
+--------+--------+--------+--------+--------+--------
| 1. | 2. | 1. | 2. | 1. | 2.
--------------------+---