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may be deposits which are inaccessible to us; or, again, an entire absence of deposits; either because not laid down in the areas we examine, or, if laid down, again washed into the sea.  These sources of error in part neutralise one another.  Some make our resulting age too long, 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.

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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 by measurements of the mass of the sediments, and then the method becomes more definite.  The new method is pursued as follows: