A study has been made of the two types of marks designated by F. W. preston as rib-marks and hackle-marks, respectively, which appear on fracture surfaces. A rib-mark is considered to represent a halt of the fracture, and a series of closely-spaced rib-marks indicates a fracture which moved with an extremely low velocity, less than one centimeter per second. Experiments are described by which these low fracture-velocities were obtained. Hackle-marks are considered to be caused by a large shearing force, causing simultaneous failure of the glass at several internal points. The knowledge gained about the conditions of formation of the two types of surface marks can be used to determine the conditions existing at the time of failure of a piece of glass. The origin, the direction of movement, and the relative strength of the force causing fracture can frequently be deduced, and from them the cause of failure may be ascertained. -- AATA