RELATIONS OF GEOPHYSICS TO GEOLOGY 5 
knowledge of mathematical theory, but I shall try to present analogies 
which will make the discussion nonmathematical as far as applica- 
tions are concerned. 
A geophysicist may be likened to a civil engineer who accompanies 
an explorer around the world. If the two are in a big city, such as New 
York, where there are many high buildings close together, the ex- 
plorer may ask the engineer to calculate the height and capacity of a 
building on a crooked street where there is heavy traffic and where 
the building is sandwiched between others. Data for such calculations 
may comprise not only measurements along the street, but also meas- 
urements from some window of adjoining buildings, and the latter 
may be necessary for accurate results. 
Now suppose the two pass across the country to the Colorado 
Rockies and the problem is to construct an aerial tramway from a 
mine. At the mine high on the mountain side, the engineer looks across 
a canyon to a sharp rock pinnacle which he can not occupy himself, 
and he wishes to determine the span from the mine to the pinnacle. 
The explorer and the engineer continue westward in a car and cross 
the mountains to the edge of Death Valley. They have no map and the 
explorer wishes the engineer to determine the shortest distance across 
the Valley to a water hole. 
These three problems are similar to problems which the geologists 
present to the geophysicists. The first problem is one where measute- 
ments can be taken a relatively short distance from the object to be 
measured, and where any point at which measurement is desired is 
accessible. There is some difficulty in determining vertical angles and 
in using the stadia on account of the crowded streets, but the problem 
can be worked out to any degree of accuracy. 
The second problem is one where measurements are somewhat 
restricted, namely, to a certain position, but where clear atmosphere 
gives very great precision to such measurements as are made, and 
enables the engineer, with an accurate transit, to measure distance 
from a very short base line. 
The third problem is the difficult one. As the engineer stands on 
the edge of the desert valley, instead of clear view to the opposite 
hills, he sees a mirage. If he is so fortunate as to obtain a vantage 
point from which part of the opposite range is clearly visible, he is 
too far to identify vegetation which might be characteristic of a water 
hole. In most cases, the best he can do is to indicate the nearest point 
of the mountains on the opposite side of the valley and can tell noth- 
ing as to whether they contain at that point the water which is neces- 
sary to the traveler. The desert interferes with accurate measurements 
527 
