276 Proceedings of the Royal Physical Society. 
of strata whose fossils happen to differ materially (as, for 
example, between the Chalk and the Thanet Sands, or between 
the Dyas and the Trias), or, otherwise, will ignore with 
equal persistence the time-value of an extensive uncon- 
formity such as that between the Lower New Red or Dyas 
and the Carboniferous and other rocks beneath, merely 
because a few fossils of low biological grade happen to have 
a sufficiently wide range in time to extend across the 
unconformity. 
That unconformities have very different time-values in 
different cases, no field-geologist would be prepared to deny. 
All he would say regarding the general question is that 
each case must be judged entirely on its own merits, and 
that due regard must be paid in each case to both the 
physical and the biological aspects of the question. All 
unconformities must necessarily be, in one sense, local pheno- 
mena, and there must be, in every case, strata of some kind 
or other which have been formed in one part during the 
interval when waste and -destruction were going on in 
another. This is only another way of stating the fact 
that the deposition of strata must have gone on continu- 
ously, and absolutely without break, over one part or another 
of the earth’s surface, from the very beginning of its geological 
history down to the present day. It is only that the areas 
of deposit have gradually changed their positions, and that 
areas which at one time have been areas undergoing depres- 
sion, have gradually changed phase and have become for a 
time areas of upheaval. There are many geologists who 
are of opinion that movements of these kinds have at one 
time or another affected every part of the earth’s crust— 
in the course of long ages changing continental areas into 
oceans, or oceanic areas into land. However much geologists 
may differ upon this particular point, they are all agreed 
that earth-movements take the form of undulations. They 
would probably also mostly agree that these undulations 
differ widely in different cases in their amplitude, in their 
wave-length, and in their frequency. If, bearing these 
points in mind, they would take into account the fact 
(to which fuller reference will be made further on) that 
