THAT the use of percussive rock drill machines in treating hard -crystalline and homogeneous rocks is cheaper than any other method of modern mining is beyond doubt, provided, strictly, that the machines are worked skilfully and under favourable circumstances. The cause of high mining costs, and in many instances the failure of the use of rock drills, is want of attention to the facts herein dealt with, which unquestionably demand careful consideration if economic mining is to be accomplished.