158 
MAURICE G. MEHL 
One cannot always be sure, for instance, that an elevation on 
a bed indicates its structural attitude. Within a region of even 
moderate relief, if the rock series has the normal complement of 
shales, there is abundant opportunity for their creep or slump 
with a consequent down-slope bending of the competent beds. 
One has only to observe this tendency of limestones to conform 
to the slope of the hill, as it is shown in a fresh cut across a 
ridge, to realize the possibility of error in this direction. 
As a rule, the top of some conspicuous bed is selected as the 
datum plane and the observations are confined to this horizon as 
fully as possible. Few beds maintain their full thickness for any 
considerable distance, however, and often they are notably 
irregular. In Wilson County, Kansas, the writer has observed 
the Stanton limestone reduced from more than 30 feet in thick- 
ness to nothing within comparatively short distances. Or again, 
in Anderson County, Kansas, a Sub-Allen limestone which 
forms a conspicuous cap of as much as 40 feet in thickness 
over the ridges of a considerable area is very thin or entirely 
lacking within half a mile from what is essentially its greatest 
development. 
Obviously, closely spaced observations on such an irregular 
surface do not give a true picture of the structure of the region. 
If the datum horizon is displaced 5 feet by slump or the irregu- 
lar surface of the bed, a decidedly abnormal dip is indicated 
between this point and another observation point nearby which 
is taken on the properly related plane. An error of 5 feet if dis- 
tributed over a large distance is not far from the true configura- 
tion of the datum bed. It would appear that observation points 
can be spaced too closely for the accurate determination of the 
structure. 
There is also some question as to the desirability of showing 
other than the larger features in the configuration of the beds in 
an attempt to show the structure. Even if one could be sure of 
the correctness of the closely spaced observations it is doubtful 
if the minor changes in dip which these observations show would 
affect the generally parallel beds at any appreciable distance 
above or below the datum bed. 
