. Journal of the Geological Society, 1997, 154: 1039-1052

The columnars of High Island Formation are the largest rhyolitic porphyroclastic lava columns in the Sai Kung volcanic rock regions, Hong Kong global geopark. On the basis of field investigation and measurement, the authors pursue the structural features and origin of these primitive columnar joints. It shows that not all the porphyroclastic lavas of High Island Formation occurred columnar joints, the columnar joints can be divided into three types, i.e., well-developed, sub-developed (destructed by secondary joints) and undeveloped. The sections of the volcanic columns are mainly hexagonal in shape, and partly pentagon and irregular. Their diameters are 1～3 meters and the heights are generally 20～30 meters. Most of them dip steeply towards the NW-trending and SW-trending. Occurring measurements of primitive columnar joints show theyhave various directions in cylinder, which conforms to the contraction-condensation model. Secondary joints and marine abrasion destroyed morphologies of columnar joints