An auxiliary cartridge for use in rock breaking is disclosed which comprises an elongate body, an end closure at each end of the body and a charge of combustible material in the space defined by the body and the end closures. One of the end closures includes a frangible membrane which closes off a charge of combustible chemicals. On being subjected to the pressure of an external ignition of fast burning combustible material, the membrane fails and permits flame to flash through it from externally of the auxiliary cartridge to internally thereof thereby to ignite said charge.