[1] Russell, _River Development_ (John Murray, 1888).
10
weight of transported sediment in six out of the nine rivers does
not vary widely. The mean is 2,730 to 1. But this is not the
required average. The water-discharge of each river has to be
taken into account. If we ascribe to the ratio given for each
river the weight proper to the amount of water it discharges, the
proportion of weight of water to weight of sediment, for the
whole quantity of water involved, comes out as 2,520 to 1.
Now if this proportion holds for all the rivers of the
world--which collectively discharge about 27 x 1012 tonnes of
water per annum--the river-born detritus is 1.07 x 1010 tonnes. To
this an addition of 11 per cent. has to be made for silt pushed
along the river-bed.[1] On these figures the minor limit to the
age comes out as 47 millions of years, and the major limit as 188
millions. We are here going on rather deficient estimates, the
rivers involved representing only some 6 per cent. of the total
river supply of water to the ocean. But the result is probably
not very far out.
We may arrive at a probable age lying between the major and minor
limits. If, first, we take the arithmetic mean of these limits,
we get 117 millions of years. Now this is almost certainly
excessive, for we here assume that the rate of covering of the
primary rocks by sediments was uniform. It would not be so,
however, for the rate of supply of original sediment must have
been continually diminishing
[1] According to observations made on the Mississippi (Russell,
_loc. cit._).
11
during geological time, and hence we may assume that the rate of
advance of the sediments on the primary rocks has also been
diminishing. Now we may probably take, as a fair assumption, that
the sediment-covered area was at any instant increasing at a rate
proportionate to the rate of supply of sediment; that is, to the
area of primary rocks then exposed. On this assumption the age is
found to be 87 millions of years.