CHAP. X.] 



THE EAKTli'S AGE. 



207 



been made to determine the magnitude of these effects till Mr. 

 Alfred Tylor, in 1853/ pointed out that by measuring the 

 quantity of solid matter brought down by rivers (which can be 

 done with considerable accuracy), we 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. CroU 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 : 



^ Pliilosophical Magazine^ April 1853, 



