Vice-President’s Address. 277 
these terrestrial undulations progress, and roll, like great 
terrestrial tide- waves, from one part of the earth’s 
surface to another, in the course of long ages, much of the 
difficulty that has been experienced in dealing with the 
subject of unconformities would no longer be felt. In each 
case, where it is possible to do so, we have to determine 
the amplitude, the wave-length, the locus of each wave- 
crest or wave-sinus at any particular time, the frequency 
of the undulation, and, finally, the direction of propagation. 
Geological evidence in a few cases supplies us with infor- 
mation upon one or more of these points, sometimes to an 
extent sufficient to warrant us in coming to fairly definite 
conclusions regarding the chronological value of particular 
unconformities. In other cases the information is very 
scanty, and we can only form our conclusions upon this 
point after studying a considerable number of collateral facts. 
In pursuing the study of unconformities, there are many 
pitfalls into which the unwary inquirer may readily slip, 
and in this matter perhaps more than in most other geological 
subjects it is necessary to proceed with the greatest possible 
caution. In the case of one unconformity which will be 
considered in some detail presently, a particular stratum can 
be shown to lie across, or overstep, the edges of various 
strata whose collective thickness amounts to several miles. 
In a case of this kind we have to be quite sure (1) that the 
collective thickness referred to actually represents the thick- 
ness of strata which are known to have covered the lowest 
stratum overstepped at some time prior to the deposition 
of the overstepping stratum. (2) We have to assure our- 
selves that the unconformity in question is not wholly or 
in part contemporaneous, 7.¢, whether in the case referred 
to the denudation of the strata did not commence at one 
part, while the deposition of the higher and newer strata 
of the same series went on in another—the areas of denuda- 
tion and deposition lying within the area now overstepped 
by the unconformable bed. (5) We have further to assure 
ourselves that the unconformity in question belongs wholly 
to the period following the close of the formation of the 
highest bed overstepped, or whether it may with more 
