山口県徳山市黒髪島花崗岩体の構造

The Kurokami Island in the Tokuyama Bay is composed of Cretaceous granite, which forms a composite body extensively exposed in the Sanyo region. The Kurokami granite is homogeneous and slightly porphyritic.
In order to elucidate tectonic problems of the granite, a statistical survey of the joints was undertaken. Many joints, developed within the granite mass, have been measured at several stations, and the data have been plotted on the equal-area net. Geometrically, two types of joint can be recognized in the granite from the whole area diagram. The one type is the horizontal joint with low angle dip. The horizontal joint is considered to have been originated in the granite batholith, consolidated deep within the earth's crust, then release of pressure by removal of thick cover of rocks caused the rock to swell upwards into a dome structure with joints parallel to the surface. The other type is the vertical joints, showing girdle with some maxima in the diagram. The origin of the vertical joints is not certain. A few of them, however, show some correlation with aplites in the granite. Latent joints, which is important in quarrying favorably building stones, are developed. The trend of latent joints is generally parallel to the exfoliation surface.