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REPORT OF COMMISSION VII 49 
ment. Brief reference to igneous and metamorphic rocks is made in the papers 
by Wilson and by Jolliffe, referred to above, however, and Hack’s mapping 
(1942) of volcanic necks in the Hopi Country of Arizona was based to a consider- 
able extent on photos. Certainly many workers in the fields of igneous and meta- 
morphic petrology and volcanology use photos as a matter of course. 
Areal or general geologic mapping involves the systematic study of the various 
phases of bedrock geology in specified areas, and in some instances surficial geol- 
ogy is involved also. It is here that photo interpretation finds its widest ap- 
plication. Basic procedure was outlined back as far as 1936 by Farrington. 
Examples of mapping based to a large extent on photo-interpretation are given 
by Fitch (1942), Wengerd (1947), and Cameron (1949). Other examples, though 
unannounced, are to be found in virtually all geologic mapping projects now 
being carried on in areas for which photos are available. The part actually played 
by interpretation varies widely with conditions. In some instances, interpreta- 
tion is largely limited to finding outcrops and planning traverses to them, in 
preparation for field study. Even this modest application may save much lost 
motion and greatly increase the efficiency of mapping operations. Where con- 
ditions are favorable, interpretation can be carried much further, providing a 
basis for extending structures and formation boundaries long distances beyond 
points at which they are examined on the ground. In fact, it is found that in 
many situations geological features which are obscure or difficult to follow on 
the ground are sufficiently distinctive on photos to be traced with ease. Even in 
tropical jungle areas it is found that rock bodies may have distinctive expression 
on photos. Thus the special value of photo-interpretation as applied to geologic 
mapping lies both in the economies of time, effort, and resources which it offers, 
and, under favorable circumstances, in the greatly increased accuracy of the 
product. In short, the result is better maps in far less time at much lower cost. 
The application of photo-interpretation to soil science, in its historical de- 
velopment, has roughly paralleled that in geology (Baldwin et al., 1947; Bush- 
nell, 1951). From early and somewhat diffident experimentation, it has evolved 
into a standard working procedure. ''Soil scientists now utilized air-photos in 
all phases of soil mapping, and are generally aware of the many clues to place- 
ment of soil boundaries and identification of the soils that can be interpreted 
from them. Air-photos serve to increase both speed and accuracy in soil mapping. 
In fact, without air-photos modern detailed soil maps of many places could not 
be produced except at prohibitive cost’ (Rourke and Austin, 1951). 
Soil science has its roots in geology, and photogeologic interpretation of 
bedrock and/or surficial deposits may throw considerable light on the parent 
material of soil. Interpretation of the soil itself, however, is commonly subject 
to greater limitations than is the geology, and the need for field examination 
of the soil is more manifest and continuous. According to the Soil Survey Man- 
ual issued by the U. S. Department of Agriculture (1951, p. 81), "It must always 
be recalled, however, that accurate photo-interpretation depends on familiarity 
with ground conditions. Ability to interpret pictures accurately in one area is not 
necessarily followed by similar accuracy in another area with different condi- 
tions." It is said also that ''land-form interpretations, though useful in locating 
soil boundaries, are not dependable criteria for defining soil types within the 
areas delineated."' '*In few places can the air-photo give all of the clues needed to 
identify the soil type, because it reveals mainly surface features, not internal 
or soil profile characteristics essential for identification of the soil . . . field study 
is always essential to proper classification of soils and correct placement of 
many of the boundaries on the soil map” (Rourke and Austin, 1951). 
Coming now to applied earth science, the practical applications of the fore-