6 TROPICAL AGRICULTURE CHAP. 



ally slides down into the valleys, carrying away with it pro- 

 jecting masses of rock and grinding away the rocky bed over 



Formation which it glides. The mass of ice and snow thus formed is 

 termed a glacier ', and its slow and grinding movement down 

 the mountain slope is called glacial action. When the end 

 of the glacier gets down to the warmer regions of the plains 

 or valleys it melts and forms a stream of water ; the larger 

 portions of the rock it has broken off in its descent soon 

 find resting places, but the finer particles of the hard rocks 

 are made into mud by the water, and this is carried lower 

 down into the valleys and deposited on the level parts or 

 carried out to sea by the rivers. 



Transforma- l n past asres the earth was very different from what it is 



tions of the ..... 



earth's sur- now, the land was composed almost entirely of hard rocks, 



changes of which, in many places, were covered with ice and snow. 



climates. Indeed, in parts of the world where the climate is now 



warm and where the land is highly cultivated, there are 



evidences of glacial action still to be found, and most of 



the inhabitants little think that their rich soil has been, to 



a great extent, formed by the slow grinding action of the 



glaciers on the hard rocks which covered their country in 



past ages. 



THE ATMOSPHERE is one of the principal agents in the 

 formation of the soil. It is made up for the most part of 

 two gases, oxygen and nitrogen, with a small quantity of 

 carbonic acid. It is the latter, which, with the help of water 

 in the form of rain, snow, mist and dew, acts on limestone 

 and other rocks and causes them gradually to crumble away. 

 Rocks worn The softer portions are first attacked, and this accounts for the 

 atmosphere? holes and crevices that are frequently found in exposed rocks. 

 But even the hard portions are gradually worn away, and 

 the whole process is seen going on in stone buildings, where, 

 after a time, the sharp corners become rounded, and the 

 smooth surfaces are roughened, by the <f weathering" effect 

 of the atmosphere. Weathering action on rocky material 



