ASYMMETRY OF SOUND VELOCITY 75 
sarily gives a structural rather than a topographic profile; for whereas the 
refraction method yields a structural profile under certain conditions, it is 
obvious that in the case of an eroded and buried land surface, for example, 
it merely records the old topography. Second, the method does not presup- 
pose the existence of any cover over the formation being investigated. In 
this case investigation with the seismograph sounds rather paradoxical, but 
the condition, nevertheless, is a real one and is frequently met with in 
tropical work. For example, shale areas which have been folded and sub- 
sequently leveled may be covered with just sufficient surface soil to make pit- 
ting difficult and expensive while heavy vegetation obscures all the outcrops. 
Here the seismograph, using the dip method, can profile the structure or 
work out the contours with almost as much facility as surface geology dis- 
plays in exposed regions. On the other hand, the presence of considerable 
recent overburden does not interfere with the use of the method. 
A third point which may be noted here as bearing on the theory of inter- 
pretation of the records, which we shall refer to later, is that the refraction 
system of profiling does not concern itself with the sound wave as it travels 
through the rock being profiled, but only with its emergence into the overly- 
ing formation. On the other hand, the dip method must take into account the 
phenomena attending the travel of the wave at a considerable depth within 
the body of the stratified rock itself. 
EVIDENCE OF VELOcITY ASYMMETRY 
The idea that stratification might influence velocity in a way to be of 
practical value was first suggested to the authors by J. E. Brantly. It re- 
quired considerable field work and the accumulation of extensive data, how- 
ever, before it was possible to sift the velocity variations due to stratification 
from the variations due to several other causes. Some of these other causes 
will at once occur to anyone who has used the seismograph. Among them we 
might list such factors as varying velocity in the overburden, dip in the over- 
burden compounding with the dip effect from the subsurface, differences of 
suriace elevation, slope of the subsurface as distinguished from its dip in 
cases where the subsurface was eroded before being buried, etc. 
After making due allowance for such contributory causes as the foregoing, 
however, it was found that the sound wave travels faster parallel to the bed- 
ding planes than it does perpendicular to them. 
There are two lines of évidence to show that this is the case. The first is 
direct velocity measurement both parallel and normal to the bedding. 
Measurements of this kind were carried out recently on some exposures of 
the Lorraine Shales in the Province of Quebec, Canada. These shales have 
been highly folded along the Lake Champlain-Quebec Fault, and ina number 
of places are to be found standing on edge. This afforded an opportunity to 
measure the velocity perpendicular to the laminations of the shale and com- 
pare it with the velocity of the same formation in a horizontal position 
nearby. The following are some of the velocity readings. 
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