In a recent paper on the volcanic rocks of Kilimandjaro I referred to the presence in the basaltic lavas of olivine crystals, which appeared to possess zonal structure. Although actual proof was wanting, still the symmetrical arrangement of the inclusions, as also the mode of alteration of the crystals, rendered the occurrence of such a structure highly probable. The olivines, being rich in iron, were altered into ferric oxide or ferric hydrate; and the line of division between the fresh and decomposed portions was mostly parallel to the crystallographic outlines. There was, however, no break in the optical continuity, such as the presence of zonal structure would demand.