370 PHYSICAL GEOGRAPHY. 



CHAPTER X. 



CHANGES IN OCEANIC REGIONS. 



E have now arrived at a very interesting department 

 of Physical Geography the consideration of the 

 changes to which the surface of the globe is subject, 

 and of the causes which produce them, already indi- 

 cated in the preceding pages. From the experience 

 of many an individual life, it might be imagined, 

 that " all things continue as they were from the 

 beginning of the creation" that an immutable cha- 

 racter belongs to the earth's external aspect so 

 uniform are the appearances presented by nature 

 during the course of man's threescore years and ten. 

 The grandsire, trembling with age and infirmity, and 

 living in a country distant from the centres of volcanic action, sees no alteration in the 

 configuration of the hills and valleys with which he has been surrounded from his child- 

 hood. The stream wanders in the same channel, with as much transparency, and with 

 as many circling eddies, now that he is old and grey-headed, as when in youth he 

 romped upon its banks, and plucked with careless hand the daisy or the cowslip from its 

 grassy slopes. There is, however, no part of the globe free from physical change, 

 whether bare to the light and air of heaven, or lying a thousand fathoms deep below the 

 waters, though it may require the lapse of ages to discover the signs of alteration, and 

 though circumstances may forbid the mutation being the subject of sensible evidence. 

 The bed of the ocean must of necessity be constantly undergoing changes, extensive and 

 diversified, wrought in secret places, into which he inquisitive eye of man cannot pene- 

 trate, and which are often beyond the reach of his longest sounding-line. "All the 

 rivers run into the sea, yet the sea is not full : unto the place from whence the rivers 

 come, thither they return again." Denudation, or the carrying away a portion of the 

 solid materials of the land through which they flow, is one effect of their action. It 

 transpires with varying energy, according to the velocity of their current, and the nature 

 of the contiguous soil ; and the distribution of the material of which the land is robbed 

 takes place under the control of these two particulars. The heavier debris of rivers 

 may be generally deposited in their own channels, where there is a marked diminution in 

 the power of the stream, arising from its course lying through an extensive level ; but 

 the finer particles are transported to a more distant locality, and are either deposited at 

 the confluence of rivers with the sea, where the tides meet them with sufficient force to 

 produce stagnation, or they are conveyed to a more remote resting-place by the tremen- 

 dous rush of the fresh water into the bed of the deep, and the action of the oceanic 

 currents. 



According to Major Rennell, a glass of water taken from the Ganges in the flood- 

 season will yield about one part in four of mud. The mean quantity of water discharged 

 by the river throughout the year he estimated to amount to 80,000 cubic feet in a 

 second, but to be 405,000 cubic feet when the river is in flood. Calculating upon 

 these data, Mr. Lyell states, that if the mud be assumed to be equal to one half the 

 specific gravity of granite, a supposition below the truth, the weight of matter daily 



