In the Tables III and IV are given in condensed form various generalities and facts regarding outcrops which may be of value for reference. It should be remembered, however, that no hard and fast limits can be given in tables of this kind and that they are valuable chiefly as summaries. (Refer.: W. H. Emmons, as cited above. Also R. A. F. Penrose, Jr., "The Superficial Alteration of Ore Deposits"-Journal of Geology, Vol. II, page 288.)

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CHAPTER XV

GEOLOGICAL PROSPECTING BY DRILLING

A field in which geological knowledge is of great importance is that of prospecting or exploring as it is often called, with drilling machines. The work of the geologists consists in the location of drill holes to the best advantage, the keeping of records of the drilling, and the interpretation of the data so obtained. Where the surface is not entirely covered by drift or vegetation, a geological map is the best basis upon which to plan drilling operations.

The drills used in mining exploration or development work are of two types. The core drills, of which the diamond drill is the commonest, use bits which are rotated by means of rods and the cutting is essentially an abrasive action. The bit is an annular ring upon which are mounted black diamonds or borts to make the cutting points, and the core passes back through the bit into the core barrel. One of the great advantages of the diamond drill is that it furnishes good specimens of the formation through which it passes. On the other hand, relatively little material is cut, so that samples for assaying are small, which is bad, unless the formation is extremely regular in its metallic content, as is the case with deposits of iron ore, for instance. Where the ground is much broken and fractured, diamond drill work is slow and also the hole is very apt to follow along some fracture plane.

The churn drill uses a heavy bit which is raised and lowered usually by means of a walking beam. The cutting is done by impact and crushing and it is unusual to obtain specimens suitable for ordinary rock determination. When specimens are found in the cuttings the chances are that they have been knocked

from the upper portions of the hole in raising or lowering the tools. The ordinary churn drill hole, however, varies from 6 in. to a foot in diameter, depending upon the size of the bit and consequently gives a much larger sample than the diamond drill. In addition to assaying, much information as to the nature of the rock and ore may be obtained by panning the cuttings.

Drilling is a quick and relatively economical method of prospecting and is coming into very general use. Its chief application is to the exploration of bedded deposits, the large regular bodies of iron ore, and vein deposits where the vein material is uniform and the values are regularly distributed. It is not a good method to use with ore bodies that do not have a general tabular form, or those in which the values are "spotty" or scattered. The importance of keeping good records of drilling can hardly be exaggerated. The holes should be accurately located and careful records kept of the formations which are cut. In doing this, specimens must be saved and marked with the hole number and the depth or distance from the mouth of the hole. Daily reports are made by the drillers upon blanks provided for the purpose and samples are kept for assay and rock determinations. If a core drill is used, the specimens of the core are kept in proper order in core boxes or trays with grooves or compartments to keep the core from getting mixed. With a churn drill, before the sample is ground, representative material should be selected and put in a small envelope or bottle for record. The character of the country rock may be determined with more or less accuracy from the examination of this material with a hand lens or small microscope.

The geologist examines the core or cuttings, which may be panned at the drill or at the laboratory. He determines as completely as he can the nature of the country rock and the ore minerals. It is desirable to have a man especially trained in rock and mineral determination for this work, and if there are several drills on the job, it is better to have one man to examine the samples from all of them so that uniformity of