ong, others make it out too short. But we
do not know if a balance of error does not still remain. Here,
however, is a table of deposits which summarises a great deal of
our knowledge of the thickness of the stratigraphical
accumulations. It is due to Sollas.[1]

Feet.

Recent and Pleistocene - - 4,000
Pliocene - - 13,000
Miocene - - 14,000
Oligocene - - 2,000
Eocene - - 20,000
63,000

Upper Cretaceous - - 24,000
Lower Cretaceous - - 20,000
Jurassic - - 8,000
Trias - - 7,000
69,000

Permian - - 2,000
Carboniferous - - 29,000
Devonian - - 22,000
63,000

Silurian - - 15,000
Ordovician - - 17,000
Cambrian - - 6,000
58,000

Algonkian--Keeweenawan - - 50,000
Algonkian--Animikian - - 14,000
Algonkian--Huronian - - 18,000
82,000

Archaean - - ?

Total - - 335,000 feet.

[1] Address to the Geol. Soc. of London, 1509.

6

In the next place we require to know the average rate at which
these rocks were laid down. This is really the weakest link in
the chain. The most diverse results have been arrived at, which
space does not permit us to consider. The value required is most
difficult to determine, for it is different for the different
classes of material, and varies from river to river according to
the conditions of discharge to the sea. We may probably take it
as between two and six inches in a century.

Now the total depth of the sediments as we see is about 335,000
feet (or 64 miles), and if we take the rate of collecting as
three inches in a hundred years we get the time for all to
collect as 134 millions of years. If the rate be four inches, the
time is soo millions of years, which is the figure Geikie
favoured, although his result was based on somewhat different
data. Sollas most recently finds 80 millions of years.[1]

THE AGE AS INFERRED FROM THE MASS OF THE SEDIMENTS

In the above method we obtain our result by the measurement of
the linear dimensions of the sediments. These measurements, as we
have seen, are difficult to arrive at. We may, however, proceed