Sessional_Paper_1926 — Page 77

73

HONGKONG.

No. 1926

GEOLOGY AND MINERAL RESOURCES OF THE COLONY OF HONGKONG BY W. L. UGLOW, M.A., M.S., PH.D.

The following resumé of the chief facts and conclusions concerning the geology and potential mineral resources of the Colony of Hongkong is submitted by the writer as a popular summary, based on the field work of the winter of 1925-1926, and must be con- sidered as subject to much revision after fulfilment of laboratory work and collaboratiou with previous workers in this field.

Rock Formations.

Recent. Alluvium, Mud Flats, Deltas, Stream Flood Plains.

Tertiary or Late Mesozoic.

Late Mesozoic (Lower Cretaceous).

LAMPROPHYRE DYKES.

Lan Tao Formation:-Granite and Syenite porphyries. Hongkong Formation :-Granite, with associated acidic dykes.

Tai Mo Shan Formation :-Quartz porphyry, rhyolite porphyry (including

Rocky Harbour Volcanics.)

Repulse Bay Formation :-Volcanic ash and breccia (including "Junk Bay formation of thinly-bedded volcanic sediments.)

STRUCTURAL UNCONFORMITY,

Tolo Channel Formation :-Sandstone, Shale, Conglomerate.

These formations are arranged in chronological sequence from the oldest at the bottom to the youngest at the top; and the table shows that no rock formations have been discovered of older date than the middle of Mesozoic time. This means that the rocks of the Colony are relatively very young with reference to geological time, and that. with the exception of a period of time in the Lower Cretaceous when the Tolo Channel series was being deposited beneath the sea, the area comprised in the Colony has been (as far as any record is preserved) continuously part of a land mass.

A geologically recent depression or sinking of this part of the South China Coast accounts for the highly indented coast line, the large number of islands, the deltas and shallow-water borders to many of the shores, and the formation of the alluvial flats.

Lithology and Structure.

Tolo Channel Formation. This is the oldest formation known in the Colony, and only remnants of it are now exposed, particularly along Tolo Channel and in the valley northeast of Castle Peak Bay. It is probably the same formation that underlies the low hills and flats of the Canton River Valley and is so well exposed in the Bocas Tigris. It was deposited in Lower Cretaceous time beneath the sea as a flat-lying series of gravel, sand, silt and mud with streaks of carbonaceous material, derived from infalling vegeta- tion. Fossils (ammonites) were found in this formation by Dr. Heanley and were deter- inined as of lower Cretaceous age.

Following the deposition of this formation came a period of mountain building, in which the rocks were buckled and uplifted into a series of northeasterly trending hills, which no doubt extended over the entire colony.

A lorg period of weathering and erosion followed, which resulted in the wearing down and levelling of the hill tops.