Sessional_Paper_1926 — Page 79

75

-

Lamprophyre Dykes.-Black to gray-black lamprophyre dykes cut all previous for- mations and constitute the youngest phase of igneous activity in the Colony.

Recent: Alluvium, etc.-The flat lands in the larger agricultural valleys of the New Territories (Pat Heung. Shap Pat Heung, Lam T'suen) and along the coast line, parti- cularly in the vicinity of Deep Bay, are underlain by fine sand and clay due to the silting up of shallow bays and inlets of the sea. Here are present-day sedimentary rocks in the process of formation, with flat-lying strata, overlapping indiscriminately the eroded

surfaces of all the other formations.

Economic Considerations.

The following brief notes bearing on different phases of the potential mineral resources and the application of geology to engineering problems are offered merely as a series of suggestions to further workers in this field, rather than a statement of the economic geological potentialities of the Colony.

Metalliferous deposits.-Sinall deposits containing lead, silver, zinc, copper, tungsten and molybdenum minerals occur in several places, but in no one place are they yet known to occur in sufficient quantity to warrant economic mining.

It is interesting to note that in nearly every case, the metalliferous deposits are found in the vicinity of bodies of Lan lao granite porphyry, which leads to the inference that this rock has been the chief metallic mineral provider in the Colony. Galena (lead sulphide) occurs in many localities in different kinds of rocks, distributed in such a fashion as to warrant the conclusion that its source is in the above-mentioned rock. That it does not occur in greater quantity seems to be due to the absence of such a formation as limestone, which would have forced the lead to deposit as it was leaving the Lan Tao porphyry at the time of solidification.

The general lack of a precipitating agency like limestone in contact with the Lan Tao porphyry in the basis for the belief that metalliferous deposits will not be found in important commercial quantities within the Colony. On the other hand, the enhanced value of the lead at the present time should direct attention to exploration for it almost wherever it is known to occur.

Non-Metallic Mineral Deposits.

Clays.-Deposits of clay, satisfactory for various purposes, occur in and near the shallow-water inlet extending easterly from Deep Bay towards Uen Long. The absence of good clay beds above sea level is due to the shortness of the drainage courses, and the relief of the land which prevent the complete separation of the clay particles from the sand particles until the sea is reached.

Sands. For a similar reason, the only clean deposits of sand will be found along beaches, where they are saturated with salt water.

Bricks. The brick industry of the Colony is founded on the shale members of the Tolo Channel formation and argillaceous portions of the Repulse Bay volcanics, and uses as its raw material the soft clayey materials that have resulted from the decomposition in place of these rocks. The geological map that will accompany the final report will outline the extent of the Tolo Channel and Repulse Bay formations, and may therefore point to areas worth investigation' from this point of view. It is possible that the special report on clays may also point to localities where beds of transported clay occur of a grade suitable for the manufacture of a better grade of brick.

Soils. Soils are rocks in the geological sense that they are the products of rock decay, both residual and transported. Both the flat alluvial valleys and many of the hills are covered with soils that are related in composition to the rocks with which they are in contact. Many of these rocks are known to be rich in potash, one of the basic constituents in fertilisation. Others contain phosphorus, some carry lime, while others again are almost barren of these three constituents.

It seems that here is an excellent field for soil research, in an attempt to classify and outline soil areas, to study their special fertility or deficiency, to encourage the adaptation of crops to the different types of soil; the inevitable outcome of such an investigation being a greater production of wealth from the New Territories.