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This formation weathers usually to a dark brown sandy clay, and underlies the lowest hills in the Colony.
Repulse Bay Formation.--The weathering process, above mentioned, was interrupted at a considerably later point of time, by a sinking of the land surface and an extensive and long-continued period of volcanic extrusion and consequent sedimentation. Upwards of 2,000 feet of volcanic sediments, agglomerates, breccias, ash, tuffaceous sandstone, with lavas, accumulated both on the land surface and in the sea-bottom, constituting what is here called the Repulse Bay formation. It is now well exposed in the southern half of Hongkong Island, on the western part of the Lan Tao Island, and in the vicinity of Junk Bay. The series is generally nearly flat-lying.
It is of considerably later date than the Tolo Chanuel series, and is separated from it by a structural unconformity.
The ashy portions of this formation weather usually to a whitish soil, while other parts produce a brown earth.
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Tai Mo Shan Formation. The extrusion and accumulation of the Repulse Bay volcanic rocks were accompanied by a great deal of intrusion in the form of necks, plugs, sills, sheets and dykes, which invaded both the Tolo Channel and Repulse Bay formations. These intruded masses of lava (or magma) did not reach the surface, but came to rest at positions not far below the surface as flat sheets or walls of rock lying parallel to the bedding planes of the volcanic series, or as dykes cutting across them. The channels through which this magma travelled upwards constitute necks, or lenticular-shaped plugs.
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This formation consists typically of quartz porphyry and quartz-feldspar-hornblende porphyry (rhyolite porphyry) and is now observed on the surface on account of the removal by erosion of the covering rocks under which it solidified. It is usually dark gray in colour, and weathers to brown clayey earth with hard boulders (or nigger heads") with a dull maroon quartz-studded surface, that is very characteristic. It is best exposed in a large area surrounding Tai Mo Shan, Southeast of Tung Chung on Lan Tao Island, in the Nine Pin Islands and hills around Rocky Harbour.
This formation, and the Repulse Bay volcanics, are the most resistant to weathering, and form the high peaks of the Colony.
Hongkong Formation.-The next event of major importance in the sequence is the intrusion of the well-known Hongkong granite, a medium to coarse grained, pink to gray rock, consisting of orthoclase feldspar, quartz and mica. It intrudes all the previous formations, but its coarseness of grain indicates that it came to rest and solidified under a great thickness of cover, and is only exposed at the surface today because of the removal of this thick cover by erosion.
It weathers to a light buff-coloured grit and is extensively exposed in the New Territories underlying the golden hills north of Kowloon and in the Central portion of Hongkong Island. Due to its coarseness and uniformity of grain it breaks down easily, and is more susceptible to decay than all other solid forinations except the Tolo Channel formation.
Lan Tao Formation.---The latest important intrusion is that of the Lan Tao granite porphyry, which intersects all the previous formations. It varies in visible character from a tine-grained rock with small crystals of pink orthoclase (Carlsbad twins) and quartz, to a medium grained grey rock with Carlsbad twins up to two inches in length, large crystals of quartz, and bundles of black hornblende. The great variation of phase which this formation exhibits is accounted for by the fact that it came much closer to the surface than the Hongkong granite before solidifying, and consequently did not have rock walls of uniform composition and temperature against which to cool.
The largest exposures are on the northwest half of Lan Tao Island from which as a centre the magma seems to have followed northeasterly-trending shear zones through the other rocks, extending across the New Territories to the vicinity of Tai Po Market.
The rock is recognised by the large lath-shaped or rectangular feldspar crystals which it contains and by its weathering to a dark brown lateritic quartz-studded clay.