Vice-President’s Address. 275 
than that. As for the lower Invertebrata, it cannot be too 
strongly insisted upon, as I pointed out many years ago, that 
the Gasteropoda and Lamellibranchiata of the beds next 
below the Chalk are very closely related to those which 
reappear in the Lower Eocenes, and especially in the Thanet 
Sands. Need I repeat that the same is substantially true of 
both the fishes and the plants ? 
Upper Greensand and Gault.—The base of the Chalk 
graduates downward in places into a glauconitic sand similar 
to that which is now being slowly accumulated in deep 
water off the east coast of Africa. This, in turn, graduates 
downward into the fine-textured marine clay, the Gault. 
The numerous biological changes to which the fossils in these 
rocks testify, necessarily denote long intervals of time. In 
the absence of any reliable data, we may perhaps assume 
that these rocks, as a whole, were formed at the rate of 
1 foot in 5000 years. The Gault is 300 feet in thickness 
(or more), which, at the hypothetical rate suggested, gives 
us an interval of 1,500,000 years. 
An important unconformity occurs at the base of the 
Cretaceous Series—one which implies so vast a lapse of time 
that it would appear to be desirable here to consider, first of 
all in general terms, to what conclusion phenomena of this 
kind are likely to conduct us. 
Chronological Value of Unconformities.— There exists 
amongst geologists a considerable diversity of opinion re- 
garding the time-value which should be put upon uncon- 
formities. On the one hand, there are those geologists who 
regard every unconformity as representing an interval of 
time which, in every case, is proportional to the thickness 
of strata missing, which interval of time they think should 
be added to the time required for the deposition of the 
quantity of rock removed. On the other hand are those 
who are disposed to regard all unconformities in the light 
of mere local, and purely accidental, phenomena, to which, 
in estimating the Age of the Earth, no importance whatever 
need be attached. There are others again, who, in spite of 
perfectly clear field-evidence, will persist in either inserting 
hypothetical unconformities between two adjoining groups 
VOL. XIII. U 
