338 NATURAL SCIENCE. May. 



Ranges," have become fixed in their positions through the upheavals 

 of mountain ranges ; and the modifications they have undergone in 

 their courses have been mainly confined to the hydrographic basins 

 in which they exist, the most fluctuating portion of the rivers being 

 on their own deltas. In cases where the levels of the watershed may 

 not have been very marked, alterations of the drainage basins them- 

 selves have arisen through differential subsidence or elevation. 

 Changes such as have been noted in the Oxusand Jaxartes in Central 

 Asia have hardly involved a rearrangement of drainage basins 

 considered on the large scale. The Ganges and Brahmapootra have 

 rolled their floods into the Bay of Bengal since the final upheaval 

 of the Himalayas. But it is unnecessary to multiply these instances.'^ 



A Future Period. 



It is now time to ask ourselves what is the meaning of this 

 constancy and the persistence of these important physical features. 

 Though differential subsidence and elevation on the large scale are 

 marked on the one hand by raised beaches on the land, and on the 

 other by deep-cut river beds below the level of the sea, as in the 

 Congo and the great rivers on the North American Continent, what 

 may be called the rugosities of surface represented by mountains 

 never disappear excepting by denudation. Further rugosities get 

 developed by faulting and mountain elevation in the locus of moun- 

 tain ranges, by a persistence of the movements which have initiated 

 them ; but the great levellers, the waters and the atmosphere, with 

 their combined chemical and dynamic action, bring their materials 

 again under the displacing action of gravity, whereby, like water 

 itself, they finally find their lowest level. Hence sediments ac- 

 cumulate and have been accumulating through Quaternary time, 

 and were these actions to continue long enough without compensatory 

 elevation, the whole of the land, as has been pointed out again and 

 again, would disappear. On the other hand, were compensatory 

 elevation to go on long enough, the whole of the sedimentary deposits 

 would be stripped from the land, and we should be permitted to 

 see what no one is certain he has ever yet seen, that is, the original 

 crust of the earth. 



It thus appears that, though these denudations are of long con- 

 tinuance, they must in the course of geologic history come to an end, 

 for we find most of the land-areas covered with sedimentary deposits. 



It follows, then, if the geologic history of our planet is to continue 

 in the form in which it has done in the past, and not to terminate in 

 stripped land-areas or universal levelling, that the sediments on the 

 coasts and seas bounding our continents, which have been accumu- 

 lating since Tertiary upheavals, must be themselves eventually raised 

 above the waters, and joined on to the dry land. In what way has 

 12- Ramsay's " River Courses of England and Wales," Q.J.G.S., 1872, and 

 "On the Physical History of the Rhine," Royal Inst ii., 1874, may be studied with 

 advantage in this connection. 



