44 Soil- Carriers 



sea. Fresh water being lighter than salt, flows over 

 the latter at first, before the two mingle ; and, as sea- 

 water abounds in these minute forms of life to a much 

 greater extent than river-water does, and as fresh 

 water is as deadly to the one as salt to the other, the 

 destruction wherever the two come in contact must be 

 wholesale. 



At present it is the delta of the future which chiefty- 

 benefits; but the present delta was slowly formed in, 

 like manner under water, and has therefore received 

 an abundant share of this fertilizing matter in ages 

 past, some amount of which is also brought and left 

 with the mud every year. 



But where rivers are affected by the tide, there the 

 salt water flows up under the fresh, for many a mile 

 above their mouths, carrying with it a living freight, 

 which must to a large extent perish and be left behind ; 

 while the microscopic inhabitants of the river-water 

 are destroyed with equal certainty wherever they come 

 in contact with that which is salt, or even brackish 

 that is, partially salt. 



Hence the great fertility of large districts on both 

 sides of the Thames, at the mouth of the Humber, on 

 the shores of the Wash and Bristol Channel. 



The remarkably productive ' carses ' of Scotland 

 owe their fertility to the same apparently insignificant 

 cause; and the soil of Holland is rich for similar 

 reasons, having been to a large extent formed by the 

 Rhine, Meuse, and other rivers. 



The whole of Southern Louisiana, with its extensive 

 cotton and rice-fields, was made in like manner by the 

 Mississippi, which becomes a dense yellow torrent 

 after it is joined by the Missouri, and brings down 



