RATES OF ACCRETION
203
    On bare ground or ground containing algal filaments there is a liability that the first tide may wash away the coloured area. To avoid this another device is employed. A perforated sheet of metal 6 inches in diameter is (aid on the area, the perforations following a convenient pattern, e.g. a doubly-outlined Geneva cross, with one limb parallel to the line of direction. Two-inch pins with coloured glass heads are pressed through the holes into the ground, and when the sheet of metal is removed the heads of the pins are made flush with the surface. Subsequent slicing of the area will show the height of accretion above the heads of the pins.
    To fix down on the soil permanently a disk of metal, &c., would give a false result, because it would interfere with the free colonization of the area by Algse or other vegetation.