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
----------------------+-------------+-------------+-------------+----------

Clays are formed by the disintegration of any of the crystalline rocks;
the purest clays being formed from felspar. A pure clay consists simply
of silica and alumina, all the other constituents having been washed
out. Disintegration, however, seldom reaches such an extent; otherwise
clay soils would be completely barren, which they are notably not. The
impurities present in clay, which consist of alkalies, especially potash
and other mineral ingredients of the plant, are what confer on clay
soils their fertility. Clays differ,