DETERMINING THE STRUCTURE OF BEDDED ROCKS 165 
Aside from these advantages, the models have a distinct phase 
of usefulness. It is recognized by all geologists that some of the 
data are at times more or less questionable. It is sometimes im- 
possible to determine in the field whether beds have been prop- 
erly correlated or whether an outcrop shows the true structural 
attitude. When the structural data of a region are modeled 
indiscriminately, it is not uncommonly found that certain of the 
observation points are decidedly out of harmony with those 
nearby. In other cases there is evident a distinct offset in the 
surface represented by adjacent sets of observations. It is at 
once suggested that the elevations of the adjacent regions have 
been taken on different beds, beds that have been correlated as 
the same, or the offset may be of such a nature as to be highly 
suggestive of a fault which was not recognized in the field. 
In this manner then, the modeling offers a further check on the 
field valuation of the observation points. 
Of special benefit are these suggestions in the interpretation of 
well-log data. One recognizes the difficulty of interpretation of 
such data because of the personal equation injected into the record 
by the driller. His record, if not actually compiled from memory 
at the end of his tower, is based on the hardness of the materials 
as indicated by the difficulty of drilling, the color of the sludge 
and, very rarely, the actual examination of the cuttings. Any 
fall of material in an ^^open hole’’ will modify his record materially 
and his estimates of the thickness or depth of any formation 
rarely fits well with the steel tape check. 
An illustration of the manner in which the models may be 
useful is found in checking the data given in connection with the 
structural map of the Bothwell-Thamesville oil district of 
Canada.2 This map, reproduced with slight modifications in 
plate XX, shows a portion of the structure which has evidently 
been contoured from well log data from which has been deter- 
mined the varying elevation of the Delaware limestone. It is 
clear that unless other data were available than those men- 
tioned by its author, there are many phases of the map which 
2 Williams, M. Y., Oil prospects of Southwestern Ontario, Canadian Dept. 
Mines, Summary Kept., 1917, pt. E, plate 1. 
