RELATIONS OF GEOPHYSICS TO GEOLOGY II 
For example, there are certain localities along the Dos Bocas- 
Alamo anticline in Mexico where the upper surface of the producing 
limestone shows deep, narrow cafions. These have been discovered 
because a very close spacing of wells happened to be made at their 
locality. If the wells had been spaced farther apart, the existence of 
such narrow canons in the limestone would probably have been un- 
suspected. Similarly, the usual geophysical survey of this ridge would 
miss these cafions. It therefore is very important for the geologist to 
advise the geophysicist whether the history of an area gives any clues 
to the existence of very irregular formation boundaries, because such 
a condition may not be found by surveying unless very close spacing 
is used, which will not generally be the case in prospecting work. 
In the foregoing discussion, an endeavor has been made to show 
the limitations in geophysical surveying which attempts to give con- 
tour maps of an area, and suggestions have been made for the codpera- 
tion of the geologist in reducing the amount of computation to be 
made by the geophysicist. It is not intended to suggest that the ge- 
ologist should lead the geophysicist into a particular interpretation, 
but rather that he should suggest the most probable type of structure 
and variation in physical characteristics of the rocks on such struc- 
tures so that the geophysicist can arrange his survey to give the great- 
est light with a minimum of expense and with the greatest probability. 
In summary, with regard to geophysical methods which depend 
on the formulae of the potential function, profiles with numerical 
values or contour maps can only be made by geophysical surveying 
under certain favorable conditions. Although such numerical results 
are theoretically obtainable by the seismic method, which is not a 
potential function method, there are uncertainties in many localities 
regarding the regularity of the formations which give rise in turn to 
uncertainties in the contour maps made by the seismic method. 
Geology and geophysics represent two aspects of scientific labor 
which have been well explained by J. B. Dumas:* 
The art of observation and that of experiment are very distinct. In the 
first case, the fact may either proceed from logical reasons or be mere good 
fortune; it is sufficient to have some penetration and the sense of truth in 
order to profit by it. But the art of experimentation leads from the first to 
the last link of the chain, without hesitation and without a blank, making 
successive use of reason, which suggests an alternative, and of experience, 
which decides on it, until, starting from a faint glimmer, the full blaze of 
light is reached. 
4 Letter to Louis Pasteur, quoted by R. Vallery-Radot, The Life of Pasteur, trans- 
lation by Mrs. R. L. Devonshire (Garden City Publishing Company), p. 122. 
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