Weathering crusts on glass are caused principally by water, which dissolves the more soluble ingredients and leaves a residue of almost pure silica. They vary in color, texture and strength according to local conditions of burial or submergence in water and the chemical composition of the glass, but almost invariably have a layered structure, as can be clearly seen when a ground and polished cross-section mounted in plastic is viewed through a microscope. By examining crusts removed from objects which had been buried or submerged for known periods of time it was established that one layer formed for each year of burial. This laminar structure results from seasonal variations in temperature or precipitation. The decomposed layer formed during wet months is dehydrated and compacted during dry months, and subsequently retains its identity. In the case of submerged objects seasonal temperature changes seem sufficient to cause layering. Data are given for eleven samples dated from about the fourth to the late eighteenth centuries A. D. The weathering crust must not have been partly removed by cleaning, and the fragility of some crusts causes difficulties, while some glasses are resistant to corrosion, but the method seems capable of accurately dating (from time of burial) glasses similar to those of the American Colonial period studied, while results on ancient glasses were very encouraging. (See also <i>IIC Abstracts, 3019). -- AATA