Glacier surges may result (1) from the warming of a cold glacier, which follows an unusual snow cover, with the formation of a temperate basal layer, or (2) from the fact that two values of the sliding velocity correspond to a single value of the friction.The last results about the friction law obtained by the present author are summarized. As a rule, for increasing sliding velocities the friction goes through a maximum and next through a minimum. Thus for one value of the friction three values of the velocity, two stable ones and an unstable one, can exist.Since the derivative of the friction with respect to the sliding velocity may become negative, the coefficients c0 and D0 in the perturbation equation may become infinite (their ratio remaining finite), and next negative. By this way, when a flood wave runs down a glacier an area with a negative diffusion coefficient may appear. Then the profile of the wave is altered: it vanishes at the upper part of this area and swells beyond measure at the lower one. Factors which may limit the amplitude of this surge are pointed out.