DETERMINING THE STRUCTURE OF BEDDED ROCKS 159 
Granting, however, that all the observed elevations do indicate 
the true effective structure, there is no assurance that the data will 
be interpreted in a like manner by two different workers. In 
the writer^s experience, a dozen students using the same data 
will present nearly as many variations in the interpretation of 
the structure. With common methods there seem to be always 
several possible interpretations of the same data, although but 
rarely more than one logical interpretation presents itself if the 
observation points are in any sense adequate. 
An excellent illustration is to be found in a discussion by 
K. C. Heald.i In this discussion it is pointed out that two 
Fig. 1. Structural contour map of a region in which data are lacking at cer- 
tain critical points. Cross marks with figures are observation points on the 
datum bed. (After Heald.) 
Fig. 2. Structural contour map of the same data as presented in figure 1. 
Under the methods commonly employed this almost opposite interpretation of 
the data is permissible if not even logical. (After Heald.) 
almost opposite interpretations may be made from the same data. 
Figures 1 and 2 show the same elevations with an entirely dif- 
ferent interpretation. Aside from the fact that the structure 
shown in figure 2 is a very exceptional expression of deforma- 
tion, there is apparently no reason why either of these inter- 
pretations might not be considered correct. 
In determining the structure of a region much must always be 
taken for granted. It is assumed, for instance, that between two 
adjacent observation points the dip is essentially uniform. This 
1 Geologic structure of the northwestern part of the Pawhuska Quadrangle, 
Oklahoma. U. S. G. S. Bull 691 C, p. 80-81, 1918. 
