DRILL holes make it possible to investigate blocks of ground that by any other means would be accessible only at much greater expense, if at all. In some investigations, drill holes are intended merely to secure geological information—the position of a contact, the attitude of a formation, or the sequence in a stratigraphic column. In others they are designed to determine the presence or absence of veins or other guides to ore. In still others, drill holes are used to take samples of the ore and to provide all the information that is required for an estimate of tonnage and grade.

At many mines the resident geologist is in complete charge of drilling, from designing the exploration and making the contracts, to logging and storing the cuttings and core. This arrangement has many ad-vantages, provided the geologist is amply supplied with assistance and is not kept so busy with details and executive duties that he lacks time for analysis and speculation. At some properties the geologist is relieved of the mechanical. routine, but he usually has, and always should have, the responsibility for recommending the location of drill holes and of recording the results of the work. He should therefore be familiar with the possibilities and limitations of the types of drills most commonly used and with the principles on which they operate. While the technique of drill operation is hardly an appropriate subject for this book, the handling of samples, the interpretation of results, and something of the philosophy of exploration by drilling merit discussion from the geological point of view.

Types of Drills

Diamond drills and churn drills are the types most widely used for sampling and exploration in connection with metal mining. Hammer drills are used occasionally, especially underground for testing the walls of drifts in wide orebodies. Shot drills have been used, notably in the Northern Rhodesian copper fields. Rotary rigs for core drilling, so extensively used in oil exploration and designed for speed and economy in deep holes of large diameter, have so far found very limited use in testing metal deposits.

Certain methods employing inexpensive equipment are used for boring shallow holes in soft ground. Wash-boring rigs in which the ma¬terial is removed from inside a casing by the force of a jet of water, serve to test loose material or determine the depth to bedrock. Hand augers are used to prospect unconsolidated material, and frequently to sample mill tailings. The Empire type of drill, in which a toothed casing is rotated by manpower, is used especially to test placer deposits in regions where labor is cheap and where difficult transportation demands a light machine.