THE BUILDING OF AN ISLAND. 



65 



shoulders of the hills, and these again have still smaller valleys leading into 

 them, so that during rains the hills are drained by a multitude of small rills, 

 each of which is a tiny chisel, cutting the valleys deeper and deeper, and slowly, 

 but continually, lowering the slopes. It is to the action of such tools that we 

 owe the valleys. And while it is true that we owe the hills as masses to the 

 elevating forces, it is nevertheless mainly to the action of rain that we owe 

 their special forms, for the forms of the valleys decide that of the remaining 

 stuff which constitutes the hills. Nevertheless, the nature of the stuff itself has 

 much to say in deciding how the surface is carved out, as we shall presently 

 see. For there is another force at work, preparing the rocks for removal, and 

 that is the chemical action of air and water in breaking up or decomposing the 

 rocks, and the rate of this process varies greatlv, according to the nature of 

 the rock. Water and air find their way into the rocks through the cracks, 

 which, as we have already seen, abound in most kinds, and gradually change 

 their character. Finally, a soil is formed, generally full, however, of the small 

 fragments which result from the multitudinous cracks in the solid material, 

 for the most part, therefore, gravelly in the blue-beach formation, and often 

 filled with small limestone fragments in the marl formation. E.xamples of 

 both kinds maybe seen in the upper parts of gravel pits and marl pits in many 

 parts of the islam!. The soil thus formed is further divided bv the roots of the 



Fi( 



26. 



A MARL PIT. 



1. b. Limestone beds. 



m. Marl (altered edges of the beds.) 



s. Soil (covering a layer of limestone fragments.) 



countless plants which grow in it, and, dying in it, leave their decaying roots 

 to darken the colour of the soil with the carbon which the plants have col- 

 lected from the air. It is from these soils, of various degrees of poverty or 

 richness, that the little chisels running down the hillsides and along the bot- 

 toms of the valleys scoop out the modicum of the island which each is stealing 



