Superficial Geology

On Stonecutters Island the rock is uniform, equigranular and medium-grained, with an average grain size of
3 mm. Single, fresh, black biotite crystals are common, and the rock is light grey or light pink. It has been
quarried for building stone on the western side of the island (3174 2022). Near the western tip of the island
(3159 1998), where volcanic rocks of the Shing Mun Formation have been mapped, the granite becomes finer
grained and probably represents a chilled margin. To the northeast of Stonecutters Island, towards Sham
Shui Po, granite formed a chain of small islands that are depicted on the early maps of the Territory. These
islands were later removed by blasting to provide a safe passage for shipping. Similar rocky islets were
present in Hung Horn Bay prior to its reclamation.
Northern Hong Kong Island. Medium-grained granite crops out along the northern shores of Hong Kong
Island between Kennedy Town and Shau Kei Wan. The outcrop is bounded by the overlying volcanic rocks
in the western half of the island, and by younger intrusions of fine- and fine- to medium-grained granite to
the east of Happy Valley. Small areas of medium-grained granite have been mapped in the vicinity of Mount
Nicholson and Shouson Hill. In Belcher Street, Kennedy Town (3170 1638), medium-grained granite is found
to within a few centimetres of the volcanic country rock contact, with only a very narrow chilled zone. This
contact may be traced through the disused, wartime tunnels under Belcher's Gardens (3173 1622), where the
contact dips about 25° west and strikes 010°. Thin tongues of fine-grained granite intrude several metres into
the contact metamorphosed tuffs and associated sandstones. Between Hong Kong University and Central
District the granite is light grey to light pink, uniform, equigranular and medium-grained, with an average
grain size of 3 to 4 mm. Tunnelling operations under Statue Square (3456 1583) revealed two chloritic zones
within this granite, each about 15m wide and associated with shear zones, with the biotite completely altered
to chlorite. Railway tunnel excavations under Sheung Wan (3383 1644) exposed a northeast-striking zone of
banded greisen, 7 to 10m wide, cutting the medium-grained granite, and probably formed by late-stage
hydrothermal fluids and gases infiltrating a fracture zone (Plate 14).
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Plate 14 - Banded Greisen (HK 3853) from Underground Excavations at Sheung Wan, Hong Kong
Island (3383 1644); Natural Scale
There are few exposures of fresh medium-grained granite between Central and Causeway Bay except in
temporary building site excavations. A 30 m deep section into the hillside at the Ruttonjee Sanitorium
(3611 1522) revealed completely weathered granite with fresh equigranular medium-granite occurring as
isolated corestones at the base. The granite there is pale pink, uniformly equigranular and non-megacrystic
with an average grain size of 4 mm (Plates 19 & 20). The biotite occurs as shiny, black, single flakes. White
plagioclase grains are subordinate to the larger light pink alkali feldspars. A geochemical analysis for a
sample (HK 4237) collected from this locality is given in Table 4. Some 200 m to the east, at Morrison Hill
(3635 1518), the medium-grained granite is inequigranular in appearance, perhaps a consequence of textural
modification by infiltration of fine-grained granite. Similar modification was noted at Kellett Island (3677
1618), where pegmatite lenses are also present. Uniform, equigranular medium-grained granite is exposed on
the western side of Leighton Hill (3688 1527), and southeastwards along Happy Valley to Tai Hang Road.
150 m south of the Tai Hang Drive - Tai Hang Road junction (3787 1478) molybdenite crystals up to 8 mm