and under and around each other, taking off a corner here 

 and there until they are like little bullets and still each 

 day they lose a little more until they are ground to 

 dust. Then when this powder has been made it is washed 

 hither and thither until some time when there is high 

 water it will be cast up on the fields. Water-carried soil 

 is called alluvial soil. Frost breaks up the rocks and the 

 winds blow particles from place to place and in some 

 countries even earthworms play an important part in help- 

 ing Nature in her soil factory. The air too wears the rocks 

 away; the surface of them becoming soft and scaling off. 

 This is repeated until after long years perhaps there is 

 nothing left of the original stone. But perhaps the most 

 wonderful way of making rock dust is that by which most 

 of the soil of eastern North America and northern Europe 

 is made. Aofes a^o these regfions were covered with ofla- 

 ciers or sheets of ice, as Greenland is. Prof. Tarr, of 

 Cornell University, tells the story of glacial soils in Na- 

 ture Study Leaflet No. 15. He says: 



''The bottom of the ice is like a huge sand paper 

 being dragged over the bed rock with tremendous force. 

 It carries a load of rock fragments, and as it moves ob- 

 tains more by grinding or prying them from the rocks 

 beneath. These all travel in towards the edge of the ice, 

 being constantly ground finer and finer as wheat is ground 

 when it goes through the mill. Indeed the resemblance 

 is so close that the clay coming from this grinding action 

 is often called rock flour." 



This soil which we have been considering, and from 

 which plants obtain their food supply is usually very 

 shallow, sometimes not more than a few inches deep, but 

 beneath it is what is known as the sub (or under) soil. 

 Into this sub-soil the roots of most horticultural plants go 

 for moisture and some little food, but mainly for the pur- 

 pose of support. Plants, such as apple trees must hold 



