Vice-President’s Address. 279 
ness of strata referred to consists of 2000 feet of the 
Wealden and 2400 feet of the Jurassics. This estimate 
does not take into account the thickness of the Lias 
(1340 feet) nor that of the New Red (fully 2500 feet), 
because there would appear to be some evidence that these 
strata were contemporaneously denuded during the formation 
of the later-formed rocks. To these rocks so removed we 
might well add the thickness of the Lower Greensand; but 
as there appears to be still some doubt in the minds of a few 
geologists regarding the physical relationship of these rocks 
to the strata above and below, I have thought it better in 
this connection to leave them out of account. 
Now if we are to go upon the principle that we must, as 
far as possible, interpret the past by the present, we are 
justified in assuming that after the Wealden strata were 
formed, and before the Gault was laid down, strata to a 
thickness of 4400 feet were tilted and denuded. If this 
denudation went on at the rate of 1 foot in 3000 years, this 
implies an interval of time of 13,200,000 years, Further 
investigation may, possibly, lead to this estimate being some- 
what modified; but, in the meantime, what other estimate 
can be made? If we abandon the principle of interpreting 
the past by the present, we shall be ignoring one of the 
foremost principles upon which all modern geological reason- 
ing is based. I would much prefer to state my conclusions 
arrived at after due consideration, let them be in accordance 
with the beliefs at present in fashion or not, rather than make 
any departure from moderately uniformitarian principles. 
We now retrace our steps, and consider here the Z%me 
required for the Formation of the Lower Greensand.—Strata of 
this age occur between the Wealden and the Upper Cretace- 
ous Rocks in the south-east of England. They consist largely 
of glauconitic sands of undoubted marine origin, together 
with marine clays, and beds of chert, which are largely 
composed of sponge spicules. Whether regarded from a 
physical or from a biological point of view, these strata must 
have taken a long time to form. But in the absence of 
reliable estimates, I shall be content to assume that they 
were formed at the rate of 1 foot in 3000 years. This, at a 
