10 AGRICULTURE 



If we go back far enough in the history of 

 the earth we reach a time when there can 

 have been no soil, for the reason that there 

 was not even rock from which to form it. 

 The theory with regard to the evolution of 

 our earth, that finds most acceptance, assumes 

 that at one time it was subjected to so high 

 a temperature that everything of which it is 

 composed was in a state of vapour. In the 

 course of ages, as heat was lost, many sub- 

 stances hitherto held in a state of vapour be- 

 came liquefied, and at this stage we can imagine 

 a globe of essentially the same shape as at 

 present, but in a more or less plastic con- 

 dition, and containing no atmosphere as we 

 understand the term. Gradually, as more 

 heat was lost, the surface of this semi-fluid 

 mass solidified, and, as cooling proceeded 

 further, the superficial crust became thicker. 

 When the temperature fell sufficiently it 

 was possible for some of the water vapour to 

 assume the liquid form, and directly this point 

 was reached the formation of soil became 

 possible. Hitherto the rocks composing the 

 earth were all of the class called igneous, that 

 is to say, they had crystallized out of a 

 molten mass, or were of the ashy character of 

 which lava is a modern representative. But 

 directly water assumed the liquid form it 



