NOTES AND QUERIES
159
rocks that are the product of rapid cooling at close to atmospheric pressure.
The minerals composing the rocks have a high content of silica and are said, therefore, to be acid (as opposed to basic rocks that contain comparatively small amounts of silica). Acid rocks are inherently more viscous in the molten state than are basic rocks, and so volcanoes containing acid material are particularly liable to explosions. The peak of Tai Mo Shan, and the high ridges that fan out from it, are composed of coarse tuff - material that was blown from a volcano in solid particles and then cemented together. By contrast, the lower slopes on the southern and eastern sides are formed from material that was blown from the volcano in a viscous condition; this material is also cemented together but its texture is fine because it solidified at atmospheric pressure. On the northern slopes, running down to the Lam Tsuen valley, the rocks are essentially the same except where they have been altered by intrusions of more coarse-grained materials.
Probably the most important practical aspect is that the rocks, like most others in Hong Kong, are high in silica. Consequently they contain only low concentrations of the important plant nutrients and so yield soils of low fertility.
According to Grant (1960) the two main types of soil in Hong Kong have fairly well-defined distributions:
(i) red-yellow podzol: is formed from granitic rocks at all altitudes, and on other rocks above 450 metres;
(ii) krasnozem: is never formed from granite, but is formed from other rock types below about 400-450 metres.
The best way to study a soil is by means of a pit which reveals a profile of the soil from the surface downward. Road cuttings and the like are convenient for this purpose. On this basis, the two main types of soil may be described briefly as follows:
(i) Red-yellow podzol. The layer of soil proper is usually quite shallow, about 30-45 cm. above the parent rock. Three or four layers are usually visible: a greyish-yellow or greyish-red top layer, then a paler greyish layer, and then a red, yellowish-red or yellow layer above the parent material.
(ii) Krasnozem. The layer of decomposed rock is usually very thick, from 2 to 12 metres. The color is a shade of reddish
NOTES AND QUERIES
159
rocks that are the product of rapid cooling at close to atmospheric pressure.
The minerals composing the rocks have a high content of silica and are said, therefore, to be acid (as opposed to basic rocks that contain comparatively small amounts of silica). Acid rocks are inherently more viscous in the molten state than are basic rocks, and so volcanoes containing acid material are particularly liable to explosions. The peak of Tai Mo Shan, and the high ridges that fan out from it, are composed of coarse tuff - material that was blown from a volcano in solid particles and then cemented together. By contrast, the lower slopes on the southern and eastern sides are formed from material that was blown from the volcano in a viscous condition; this material is also cemented together but its texture is fine because it solidified at atmospheric pressure. On the northern slopes, running down to the Lam Tsuen valley, the rocks are essentially the same except where they have been altered by intrusions of more coarse-grained materials.
Probably the most important practical aspect is that the rocks, like most others in Hong Kong, are high in silica. Consequently they contain only low concentrations of the important plant nutrients and so yield soils of low fertility.
According to Grant (1960) the two main types of soil in Hong Kong have fairly well-defined distributions:
(i) red-yellow podzol: is formed from granitic rocks at all
altitudes, and on other rocks above 450 metres;
(ii) krasnozem: is never formed from granite, but is formed
from other rock types below about 400-450 metres.
The best way to study a soil is by means of a pit which reveals a profile of the soil from the surface downward. Road cuttings and the like are convenient for this purpose. On this basis, the two main types of soil may be described briefly as follows:
(i) Red-yellow podzol. The layer of soil proper is usually quite shallow, about 30-45 cm. above the parent rock. Three or four layers are usually visible: a greyish-yellow or greyish- red top layer, then a paler greyish layer, and then a red, yellowish-red or yellow layer above the parent material. (ii) Krasnozem. The layer of decomposed rock is usually very thick, from 2 to 12 metres. The color is a shade of reddish
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