280 Proceedings of the Royal Physical Society. 
thickness of 800 feet, gives 2,400,000 years, an estimate, 
again, which paleontologists, taking the life-history of 
the period into account, will probably regard as far too 
small. 
Wealden-Jurassic Series.— Prior to the great Pre-Cretaceous 
unconformity, there was formed in the south of England 
more than 5700 feet of strata without any break—to wit, 
Wealden 2000, Oolites and Lias 3700. These largely con- 
sist of limestones, which show every sign of having been 
very slowly formed. It has been said that some of the 
limestones are simply old coral-reefs, and on that account it 
has been assumed that their rate of formation has been com- 
paratively rapid. But the important fact seems to have been 
overlooked that the Jurassic Limestones do not occur as great 
upstanding masses which end off all round with great abrupt- 
ness, but, on the contrary, they are distinctly well-stratified 
deposits, showing every sign of having been laid down upon 
the sea-bottom in precisely the same manner as the majority 
of other limestones; and, like these, are remarkably persistent 
over large areas. Corals occur in them in abundance; but 
there is an important difference between a limestone full of 
corals and a coral-reef. It may well be doubted whether 
the reef-building habit amongst corals can be traced back in 
time farther than the Miocene Period at the farthest. 
Taking the Jurassic-Wealden series as a whole (and 
including the marine Wealden of the south of France), more 
than one-third of the entire thickness (or 2000 feet) consists 
of limestone. Assuming, as before, that each foot of this 
marine limestone took 25,000 years to form, the time required 
for this alone amounts to 50,000,000 years. We have to add 
to that the time required for the marine clays (which may be 
regarded as constituting another 2000 feet), 1 foot in 3000 
years, to put it at the highest rate, another 6,000,000 years. 
To these add the time required for the formation of 1700 feet 
of sandstones. These Jurassic Sandstones I should be disposed 
to regard as having been formed at the rate of 1 foot in 2000 
years, which makes an additional 3,400,000 years. This, in 
all, makes the period required for the evolution of the many 
successive changes of animal and plant life, which are so 
