214 



ISLAND LIFE 



PART I 



of the country, must always pass from a higher to a lower 

 level, and must ultimately reach the sea, unless it is first 

 deposited in some lake, or by the overflowing of a river 

 goes to form an alluvial plain. The universality of this 

 subaerial denudation, both as regards space and time, 

 renders it certain that its cumulative effects must be very 

 great ; but no attempt seems to have been made to deter- 

 mine 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. 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 Sir A. 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 corresponding 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 

 ^ Philosophical 3Iagazine, April, 1853. 



