DETERMINING THE STRUCTURE OF BEDDED ROCKS 161 
arbitrarily adhered to, there can be but slight variation in the 
interpretation of any structure represented by a series of isolated 
elevations. 
In figure 3, as in figures 1 and 2, the same data are again 
utilized, but here the principle of intersecting planes is recog- 
nized. The observation points are made to outline the several 
triangular planes A to L, and each plane is contoured indepen- 
dently. The attitude of the triangles A, B, C, D, K, and L is 
evident. It is further evident that the corners of the tri- 
angles, C, jD, and G are tilted down to form an inclosed depres- 
sion. Likewise, the point represented by the ^^60” corners of 
the triangles E, F, G, H, /, and J represents an elevation. It is 
obvious then, that there can be but one interpretation of the 

30 
Fig. 4. Structural contour map of the same data as that presented in figures 
I and 2 as it would appear after the sharp intersections of the triangular planes 
have been ‘‘smoothed out,” 
gross structure of the region. The only possible error is that in 
the variation in size and the exact position of the small depres- 
sion marked by the 50 foot contour and the small elevation 
encircled by the 60 foot contour. 
The division of the surface into triangles either graphically or 
mentally gives the resulting contours a decidedly stiff’ ^ or 
mechanical appearance. It is evident that the planes must be 
warped and their intersections smoothed out” so that any 
cross section except where broken by faulting shall be free from 
abrupt changes in direction. 
For some time the writer has been practicing a system of 
modeling which includes all the advantages of the triangle system 
and at the same time eliminates most of its crudities. 
