Fiber mats formed by attenuating molten streams of attenuable mineral material by subjecting the streams to the action of a hot attenuating gas blast. The gas blast induces gas from the surrounding atmosphere, and the combined blast and induced gas forming a fiber-carrying current. The current is directed toward a preforated fiber-collecting conveyor on which the fibers are deposited in the form of a mat and the gas of the current passes through the conveyor. Provision is made for withdrawing a peripheral portion of the fiber-carrying current at a point intermediate the zone of attenuation and the perforated fiber-collecting conveyor.