GEOLOGICAL PHOTOGRAPHY Photography as applied to geological inves tigation may be roughly classed under the following headings : (I) Photographs showing plains, valleys, escarpments, base levels, moun tains, lakes, rivers, glaciers, etc., taken for the purpose of illustrating the origin of landscape, and the action of atmospheric and other pro cesses of denudation. (z) Photographs of the faces of cliffs, quarries, railway cuttings, and other exposed surfaces, to show the way in which strata have been laid down, and changes which have taken place producing unconformity, overfolding, faults, dip and strike, anticline and syncline, crushing, cleavage, and joints. (3) Photographs of fossil remains, both in situ and after cleaning, to show the types of animal and plant life existing during the formation of the strata in which their fossil remains are embedded. (4) Photo-micrographs of thin sections of rocks, showing their structure, composition, and any minute animal and plant remains that may be present.
For most geological photography, a good stand camera, having a long extension of bellows, swing back, and rising front, will be found most serviceable. A good lens is all-important, as the value of a geological photograph depends uron its perfect clearness and sharpness of detail. The lens should be provided with a deep
hood for use in the field, which will prevent flare, • and flat, foggy-looking negatives, caused by reflections and rays of light falling obliquely on the front surface of the lens. The tripod should be substantial and rigid, so that there will be no fear of vibration during exposure, for it is always best to stop down the lens and give a full-time exposure, so that a crisp, sharp negative, full of detail and gradation, may be obtained. It is often desirable when photo graphing a portion of the face of a cliff, or part of the strata laid bare in a quarry or railway cutting, to include in the photograph some familiar object of which the size is well known, to act as a kind of rough scale by which one may judge the relative thickness of a deposit, or the size of a fossil seen in situ. For this pur pose, some workers use their geological hammer, but as hammers vary a good deal in size and shape, it is much better to include a twelve inch or two-foot rule. Orthochromatic plates should always be used. F. M-D.
(See " Dusting-on (Powder) Process.")