It is perhaps premature to say much of the use of the airplane in the study of geology until it has been thoroughly tested. But it should be possible from the air to locate and map ore bodies, metalliferous veins, and outcrops of rock: for it is well known that rocks at the outcrop differ in color, in the forms of erosion developed in them, and in the kind of plants which they support. It is of interest that Colonel Alfred H. Brooks, who was Chief Geologist of the American Expeditionary Forces in France during the war, found that geologic boundaries could be recognized on air photographs and that by means of these photographs he could correct existing geologic maps and identify [Illustration: FIG. 53--Canyon in sedimentary rocks near the mouth of the Pecos River, Texas. The rocks consist of flat-lying strata, and the tortuous lines resembling the grain in wood denote the outcrops of hard layers and the benches formed on these layers by erosion. This photograph illustrates the use of air photography in geological reconnaissance. Scale not known.] formations in inaccessible areas within the enemy lines. His method was to use air photographs in the study of the geologic formations of areas accessible to him. Then, having familiarized himself with the appearance of the different rock formations and structures on the photographs, he was able to recognize the same features on photographs of areas held by the enemy and so project his mapping over into inaccessible territory.[4] The prospector should effect a great saving of time by using air photographs to guide him to places where he can find exposures of rock and to help him to avoid places where it would be useless to look for exposures. Particularly in wooded regions air photographs are valuable in indicating localities where exposures can be found in areas so covered with forest that examination on the ground would not be worthy of consideration. Prospectors for oil are planning to use airplanes for this purpose in northern Canada, in South America, and in other places where much of the country is so densely wooded that much time is usually spent in looking for clear space. USE IN EXPLORATION Exploratory work should benefit in many ways. General reconnaissance has been carried on to a considerable extent in foreign lands with airplanes and to some extent also in America. Wide areas along the Mexican border have been photographed for the making of new maps and for the correction of existing maps. The same photographs would be useful in geologic reconnaissance. The new photographs of southern Arizona are said to show mountain ranges many miles away from their location on existing maps. Such corrections are of importance to the geologist as well as to the geographer and the map-maker. Amundsen intends to employ several small planes in his Arctic work now under way. Mjöberg[5] has projected an expedition to New Guinea in which the use of airplanes is a fundamental condition. CHAPTER XII