The decomposition of mica is very slow, however, as it is a peculiarly hard mineral. Other important minerals are _Hornblende_ and _Augite_. These are composed of silica, alumina, iron oxide, manganese oxide, lime and magnesia. These are the chief minerals out of which soils are formed. It is scarcely necessary to say that few soils are made up out of any of these three minerals alone. Nearly all rocks are formed out of a mixture of these minerals. Where, however, any one mineral predominates over the rest, the nature of the soil will be thereby affected. In order to illustrate this, it may be well to mention the composition of one or two of the commoner rocks. 1. _Granite_, which is so abundant in certain parts of the north of Scotland, and which gives rise to the soils in the neighbourhood of Aberdeen, is made up of a mixture of quartz, felspar, and mica. It depends on the felspar present--_i.e._, whether it is orthoclase, oligoclase, or albite--whether the soil will be rich in potash or not. Granite containing orthoclase felspar produces a fairly fertile soil. An important consideration, which is apt to complicate this question, is the situation of such soils. They are generally so high above sea-level, that their fertility is seriously impaired on these grounds. 2. _Gneiss_, another common rock, is similar in composition, only that it contains very little felspar, and a correspondingly greater amount of mica. 3. _Syenite_ contains quartz, felspar, and hornblende. The rocks of which greenstone and trap are types, are found very largely scattered over the country. They are of two kinds, diorite and dolorite. 4. _Limestone_ is of two great classes. We have (1) Common, (2) Magnesian. The following are the analyses of these two classes by Dr Anderson:-- ----------------------+---------------------------+------------------------ | Common. | Magnesian. +-------------+-------------+-------------+---------- | Mid-Lothian | Sutherland. | Sutherland. | Dumfries. ----------------------+-------------+-------------+-------------+---------- Silica | 2.00 | 7.43 | 6.00 | 2.31 Iron oxide and alumina| 0.45 | 0.76 | 1.57 | 2.00 Carbonate of lime | 93.61 | 84.11 | 50.21 | 58.81 Carbonate of magnesia | 1.62 | 7.45 | 41.22 | 36.41 Phosphate of lime | 0.56 | - | - | - Sulphate of lime | 0.92 | - | - | - Organic matter | 0.20 | - | - | - Water | 0.50 | - | - | - | ----- | ----- | ----- | ----- | 99.86 | 99.75 | 99.00 | 99.53 ----------------------+-------------+-------------+-------------+----------