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. --------------------+--------+--------+--------+--------+--------+-------- Silica | 65.72 | 65.00 | 67.99 | 68.23 | 62.70 | 63.51 Alumina | 18.57 | 18.64 | 19.61 | 18.30 | 23.80 | 23.09 Peroxide of iron | traces | 0.83 | 0.70 | 1.01 | 0.62 | none Oxide of manganese | traces | 0.13 | none | none | none | none Lime | 0.34 | 1.23 | 0.66 | 1.26 | 4.60 | 2.44 Magnesia | 0.10 | 1.03 | none | 0.51 | 0.02 | 0.77 Potash | 14.02 | 9.12 | none | 2.53 | 1.05 | 2.19 Soda. | 1.25 | 3.49 | 11.12 | 7.99 | 8.00 | 9.37 --------------------+--------+--------+--------+--------+--------+-------- | 100.00 | 99.47 | 100.08 | 99.83 | 100.79 | 101.37 --------------------+--------+--------+--------+--------+--------+--------