Cristobalite taken from the hot face of a used silica brick from an open-hearth furnace roof is found to be well crystalline with tetragonal symmetry. The line pattern with cobalt Kα radiation has been indexed and the lattice dimensions of the unit cell are a 4·9615, c 6·9054 kX units.

The author wishes to thank Dr. T. Swinden, Director of Research, The United Steel Companies Limited, for permission to publish this paper.