It has been suggested that petrologists may be interested in a brief account of the Cambridge University collection of rock-slices, comprising at present some forty thousand slides. It had its small beginning sixty years ago, when microscopical petrography was still something of a novelty. Among the first in this country to use the new method of research was Dr. T. G. Bonney, then in residence at Cambridge; but the large collection which he gradually amassed came to us only at his death, and the earliest slides in our cabinets were made for E. B. Tawney, at that time Assistant to the Woodwardian Professor. The preparation of thin slices of rocks was not the comparatively easy and rapid process. that it is to-day (emery was the only abrasive used), and it was not unusual to give credit to the practitioner in published papers. The best known, if not the only one, in England was F. G. Cuttell; later it was customary to get the slicing done in Germany.