existing Physical Causes during stated Periods of Time, 265 



water equal to about 6 inches of rain, or 400,000 square miles, 

 or a much greater volume of water than the Mississippi pours 

 into the Gulf of Mexico, taking into consideration the difference 

 in size of the countries they drain. 



The alluvium suspended in the waters of the Ganges^ is as 1 

 to 858 by weight ; consequently the detrital matter removed in 

 suspension by the water in one year would cover the land from 

 which it is derived to the depth of yy jy of a foot ; that is to 

 say, the Ganges might pour out muddy water at its present rate 

 for 1751 years before the mean level of 400,000 square miles 

 would be reduced 1 foot in height. The great elevation of the 

 Himalaya range, or possibly a greater rain-fall, may probably 

 occasion the difference between the rates of denudation indicated 

 by the Ganges and the Mississippi. As there are also parts of 

 the earth^s surface drained by rivers flowing into lakes and 

 inland seas, and other tracts are entirely without rain, I propose 

 to estimate (as before mentioned) that only half the land contri- 

 butes detritus in suspension to rivers flowing directly into the 

 seat- If this area be annually reduced in level at the same rate 

 as the district through which the Mississippi flows, then the 

 mean level of the land on the globe would be reduced 3 feet in 

 54,000 years, and consequently the level of the ocean raised 1 

 foot in the same period by means of the detritus suspended in 

 river-water poured into the ocean J. 



But in addition to the sediment carried down by means of 

 rivers, we have also to take into consideration the amount of 

 debris washed into the sea from cliffs during so long a period as 

 that mentioned. It is difficult, however, to form any estimate 

 of what this would annually amount to, for old maps and charts 

 are hardly accurate enough to represent the waste of cliffs by 

 breaker-action even within the last 100 years. Captain Wash- 

 ington has, however, published a report § which gives an account 

 of the encroachment of the sea at intervals on one part of the 

 Suffolk coast. This will give a general idea of the contribution 

 of detritus that may be obtained from some points of a coast-line. 

 The following statements are collected from Captain Washing- 

 ton's Report on Harwich Harbour in 1844, from which also the 

 figures 4, 5, 6, 7 are copied. 



* Seep. 261. 



t The proportion of land without rain is about TaVo^^*^ of the whole. 

 Keith and Johnston say that nearly one-half the drainage-water of Europe 

 and Asia falls into the Black and Caspian Seas. The proportion for Africa 

 and America is not known. 



X It is not improbable that the solvent powers of rain and river-water 

 are as important agents in the removal of land as the agency above men- 

 tioned. Definite calculations on this subject remain to be made. 



§ Tidal Harbours' Commission, First Report of 1845. 

 Phil. Mag, S. 4. Vol. 5. No. 33. April 1853. T 



