CHAP. X.] 
THE EARTH’S AGE. 
207 
been made to determine the magnitude of these effects till Mr. 
Alfred Tylor, in 1853, 1 pointed out that by measuring the 
quantity of solid matter brought down by rivers (which can be 
done with considerable accuracy), w r e may obtain the amount of 
lowering of the land-area, and also the rise of the ocean level, 
owing to the quantity of matter deposited on its floor. A few 
years later Dr. Croll applied the same method in more detail to 
an estimate of the amount by which the land is lowered in a 
given period ; and the validity of this method has been upheld 
by Prof. Geikie, Sir Charles Lyell, and all our best geologists, as 
affording a means of actually determining with some approach 
to accuracy, the time occupied by one important phase of 
geological change. 
The quantity of matter carried away from the land by a river 
is greater than at first sight appears, and is more likely to be 
under- than over-estimated. By taking samples of water near 
the mouth of a river (but above the influence of the tide) at a 
sufficient number of points in its channel and at different 
depths, and repeating this daily or at other short intervals 
throughout the year, it is easy to determine the quantity of 
solid matter held in suspension and solution ; and if corre- 
sponding observations determine the quantity of water that is 
discharged, the total amount of solid matter brought down 
annually may be calculated. But besides this, a considerable 
quantity of sand or even gravel is carried along the bottom or 
bed of the river, and this has rarely been estimated, so that the 
figures hitherto obtained are usually under the real quantities. 
There is also another source of error caused by the quantity of 
matter the river may deposit in lakes or in flooded lands during 
its course, for this adds to the amount of denudation performed 
by the river, although the matter so deposited does not come 
down to the sea. After a careful examination of all the best 
records, Professor A. Geikie arrives at the following results, as 
to the quantity of matter removed by seven rivers from their 
basins, estimated by the number of years required to lower the 
whole surface an average of one foot : 
Philosophical Magazine , April 1853. 
