222 TKANSACTIONS OF THE HORTICULTUEAL 



within reach of the roots; but, as the surface is continually wasting, 

 the roots are enabled to penetrate deeper and deeper and draw their 

 supplies from the fresher soil beneath. 



But all soils are not formed like those of which we have been 

 speaking — by the decomposition of the rocks which immediately 

 underlie them. Many of our most fertile ones have been trans- 

 ported long distances. 



A good example of soils of this class is found in our bottom 

 lands. These owe their wonderful fertility not alone to the quantity 

 of humus they contain, or to their mechanical condition, but 

 largely to the fact that their materials are gathered from a wide 

 expanse of country. No river flows through its entire length over 

 the same kind of rock, but its bed usually contains many varieties, 

 and, as each of them contributes its part, the soil of these bottom 

 lands must contain more elements of fertility than ordinary soils 

 formed in situ. 



Another example of transported soils is our glacial drift. Gla- 

 ciers have their origin in immense snow-fields, which cover large 

 elevated plateaus of crystalline rocks. These plateaus, hemmed in 

 on every side as they are by highlands, form great catch-basins, in 

 which the snows of successive seasons accumulate, until the great 

 weight of the mass forces it out through some pass or opening in 

 its side and down into the valley below. Their weight, and the irre- 

 sistible force that drives them forward, cause them to break off and 

 grind up large quantities of these crystalline rocks, and carrying 

 them down the mountain side, to deposit them at the lower levels, 

 where the glacier melts. This is the course of glacial action as we 

 see it exemplified to-day in the Alps, the Scandinavian mountains or 

 in our own coast range, but in former times, during the glacial 

 epoch, the great British-American plateau, elevated then much 

 higher than now, formed a catch-basin from which a glacial sheet 

 spread southward nearly to the latitude of Cairo. This ice sheet, as 

 it retreated, left behind the material which it had gathered in its 

 course, coveriug the prairies of Illinois with a coating of ground 

 crystalline rock, often one hundred feet in thickness. Now it has 

 been shown that the crystalline rocks contain the elements of all 

 others combined, and hence this soil must have in it everything that 

 can be needed by any plant. This soil differs again from others in 

 that it was produced by grinding rather than chemical decomposi- 

 tion, and contains much undecomposed silicate. These silicates are 

 protected from decomposition by the fact that clay is at the same 

 time a great absorber and a poor transmitter of water, but they 

 break up readily whenever means are employed which cause water to 

 pass through the soil more freely. It is for this reason that the soil 

 of our prairies is, when properly cultivated, practically inexhaustible. 



