Seven rocks are exposed near Hong Kong in fresh and weathered facies. Dean R. W. Brock and his associates collected a series of samples and studied the chemical and mineralogical changes that result under the tropical but monsoon climate in this district.
Except for one lamprophyre, the rocks all lie in the range of granite, syenite, and granodiorite. The results of weathering are so uniform as to indicate the effects very clearly. In chemical terms, assuming alumina constant, the changes are typical of weathering. The order of losses is lime, soda, and magnesia early; silica and potassium commonly next; and iron and titania last, with some addition in exceptional cases. Carbon dioxide is removed from some and added to others; water is regularly added; and iron is oxidized. In mineral terms, the chief product has approximately the composition of kaolinite but is more soluble in dilute sulphuric acid than most kaolinite. Thin sections show both erystaline and amorphous material, but the X-ray patterns are poor, indicating a dominance of amorphous material, in which some of the potash and other minor constituents may be adsorbed.