THE earth's upper crust in the continents appears from seismology to consist of three layers, an upper one of granitic constitution, about 10 km. thick; an intermediate one about 20 km. thick, the properties of which fit tachylyte; and a lower one probably of dunite, extending half-way to the centre of the earth. Above the granitic layer is the sedimentary layer, with an average thickness of probably about 2 km., but considerably thicker in special regions. The outflow or inflow involved in maintaining isostatic compensation is in the lower layer, but at a smaller depth than 50 km.