In May, 1917, in the course of the writer’s visit to the South Dakota School of Mines at Rapid City, Professor J. J. Runner called his attention to a peculiar form of calcium carbonate occurring in the Columbia gold mine at Keystone. The fissures in the chloritic schist country rock of this deposit are filled in part with gold-bearing translucent quartz, and in part with the carbonate mineral. Many specimens can be picked up on the mine dump. Some of the carbonate shows the typical rhombo-hedral cleavage of calcite, but most of it is made up of lamellas with a pearly luster. In convergent polarized light these lamellas yield interference figures which are biaxial with a small axial angle, like aragonite, and Professor Runner regarded them as representing the latter mineral. In other respects, however, the material is not essentially different from the lamellar calcite which has been described in two previous articles in this magazine.1 The indices of refraction and the specific gravity are essentially those of calcite, and the transition from the normal to the lamellar form is too gradual for distinct crystalline substances to be represented. The unexpected optical behavior suggests, however, a possible explanation of the origin of the lamellar structure: it may be due to the partial alteration of the calcite into aragonite. Further study of this possibility is to be desired.