Abstract：Wonderful Hexagonal columns (columnar joints) of Cretaceous High Island Formation (Kkh) volcanic rocks make up the key landscape of the Sai Kung Volcanic Rock Region in southern Hong Kong National Geopark, but whether they belong to lava or pyroclastic rock has been disputing. On the basis of field geological investigation and rock thin section observation, these volcanic rocks are determined to be a specific lava—rhyolitic porphyroclastic lava which is characterized by common columnar joints, porphyroclastic & pearl-rim textures of phenocrysts, and felsitic texture & flow structure in groundmass. They are not only the latest Mesozoic products in Hong Kong but also constitute a large central extrusive facies dome within the Sai Kung Caldera. This porphyroclastic lava dome is inferred to be the largest one (～150 km2) so far known in the world, and its residual parts after long-term erosion are current landscape which can be seen in the Sai Kung Volcanic Rock Region.