The writer's attention was directed to Dr. Omori's description of the 1910 eruption of Usu-san through a review which appeared in Nature (1911, p. 221) from the pen of Professor Milne, F.R.S. The following pages constitute a résumé, rather than an additional review, and are justified on account of the interest of Omori's observations to many geologists who may not find it convenient to consult his original paper. All three figures here published have been redrawn for the purpose. Plate XII is based on a coloured map, and Plate XIII on photographs which have already been reproduced by Professor Milne in his review.