Sessional_Paper_1926 — Page 80

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Quartz. Many large quartz veins, with well-formed rock crystals occur throughout the western half of the Colony. While these are of no immediate value (since they seem barren of metallic minerals), they afford a resource of high-grade silica that might be of value in the glass-making or crystal bead industries.

Building Stones. The Colony is well supplied with three types of hard resistant building stone, all of the granite type; but is devoid of a supply of the softer and more easily worked stone, such as limestone, sandstone, marble and serpentine.

(1) Hongkong Granite.—This is an excellent stone for building purposes, of high crushing and tensile strength where fresh and unaltered, and of a pleasing delicate pink and white mottled colour. It is not sufficiently tinted to be of value as an ornamental stone. Most of the quarrying is done in this rock and large blocks and lintels are derived from it. It is the most easily decomposed of the three stones, and care should always be exercised by builders in the selection of pieces that are to undergo considerable stress, in order to make sure that the pink or white feldspar constituent of the rock has not lost its lustre. This loss of lustre means rock decomposition and a great decrease in strength.

(2) Tai Mo Shan Porphyry.-This dark-coloured close grained rock has not been used extensively in building in Hongkong, except for blocks in retaining walls and foundations. It is very resistant and hard, of high crushing and tensile strength, and less subject to decomposition than the Hongkong granite. It is more difficult to work and it is not pleasing in appearance, due to its dark colour and lack of uniformity.

Lan Tao Granite Porphyry.--This rock does not seem to be used in any type of building in the Colony, as far as the writer has observed; yet it is one that has many features that highly commend it for certain purposes.

It occurs in many beautiful varieties, with several colours, and always shows white or pink rectangular Crystals set in a dark gray or mottled ground mass. It is not usually suited to the extraction of large blocks, like the Hongkong granite because its joint system is on a smaller scale, but small slabs and blocks up to three or four feet in longest dimensions can be quarried in quantity. It is the only stone in the Colony that has any merit as an ornamental stone, and when polished would produce a very pleasing effect."

It is hard, resistant, and in most cases more difficult to work than the Hongkong granite. One of its varieties is being quarried at Tai Po Market for local building pur- poses; but the ornamental varieties are best seen on the northeast part of Lan Tao Island and adjoining islands, and on the large peninsula projecting southerly from Lan Tao Island towards the Soko group.

Water Supply.--Surface water supplies, on which the Colony is dependent, are very variable from season to season, necessitating the construction of retaining dams and reservoirs. The rapid variation in the stream flow is due to several factors, among which are the following: (1) a dry season extending from October to March, followed by a wet season during which a great deal of rain falls in intermittent downpours; (2) short drainage courses, small catcliment basins and marked topographical relief, all of which mean a rapid run-off; (3) general absence of forests in the drainage basins (forests provide shade, thus reducing evaporation, slaken the velocity of flow, permitting greater seepage into the ground); (4) large areas of granite and similar igneous rocks, which are highly impervious to underground water, and therefore do not permit natural under- ground storage of water.

In connection with underground storage, the question has on several occasions been asked as to whether a supply of artesian water might be secured on the island of Hong- kong or in the New Territories. An artesian flow is dependent mainly upon three con- ditions, viz. porous strata like sandstone or volcanic ash in monoclinal or synclinal attitude, an impervious cover, usually of shale, on top of the porous beds, and the outcrop of such dipping beds at a higher elevation than that at which the water is required. Generally speaking these three conditions do not prevail within the Colony, except on a very small scale. The Island of Hongkong is devoid of all of them. In the New Territories, there are only two localities in which an artesian flow of any conse-