toward the base, a longer travel path with its greater penetration 
should receive a greater relative speeding up or lead than a shorter, 
shallower path. Where refraction lines of varying lengths are 
available on a dome previously shot, a study of this sort may 
serve to point out some very interesting general conclusions as to 
the change in horizontal salt section with depths. 
In several cases already investigated from previous refraction 
data, a considerable variance has been found to exist between 
relative leads on long and short lines. At least one case has been 
noted where salt leads of considerable magnitude were obtained 
on a short line refraction fan, while a long line fan across the 
same dome showed so little lead as to question the probability that 
the dome would have been picked up in this manner. Assuming 
the data itself to be satisfactory, the logical analysis is that in 
this particular case the salt section likely decreases with depth, 
at least within the limits of penetration of the lines used. Though 
no detailed conclusions may be drawn from such data as to the 
particular depth at which the change takes place, nor accurate 
estimate be made as to the relative horizontal sections of salt, at 
least a more favorable prospect can be selected for further work 
of detailed nature. Conversely, and as an indication that all domes 
should not be considered as necessarily overhanging, are cases of 
domes which show relatively as great or greater leads when shot 
with long lines as with short. 
A further general analysis may be made by a comparison of 
the “shadow” or edge leads or “sideswipe’”’ as shown by different 
salt domes. More specifically, if a line shot tangent to the surface 
projection of a dome shows more or less relative lead than a sim- 
dlar line on another dome, the line of less lead may reasonably be 
supposed to have had less section to ‘“‘sideswipe” than the lines 
of equal penetration, or approximately the same length. 
It may be of interest to ncte that the refraction work on the 
dome discussed above showed an extremely sharp cut-off of lead 
on adjacent lines of a fan spreading over the edge (See Fig. 5), 
.30 sec. »80 sec 
.30 sec .25 gec 

eO Sec. sec 
eC $6C. 
-05 sec 
-O sac6 
Fig. 5 

S .P Fig. 6 
14 
