Tiny crystals of calcite recently examined under extremely high-powered microscopes and found in the Carboniferous Limestone associated with algal remains have been termed algal dust. The dust is constant in its characters and easily recognizable even under moderate powers. Algal threads may be distinguished because they are more coarsely crystalline than their matrix of algal dust. The porcellanous appearance of algal nodules and fine grained "chinastones" is due to the fine frosting acquired on the surfaces of algal dust by weathering. The dust seems to resist recrystallization more than other finely divided calcite grains such as chemicaland bacterial precipitates and comminuted shells.