EFFECTS OF WEATHER 



and clay, pressed harder and harder, might be converted 

 into mudstone or shale. But there is another agency at 

 work. We have all seen mortar hardening and binding 

 bricks together; or cement hardening into concrete. 

 Similarly sedimentary deposits are bound together by 

 cements, of which there are many which exist naturally. 

 For example, silica is a natural cement ; and so is carbon- 

 ate of lime ; and so is peroxide of iron. All these will 

 bind other particles together. But how do they arrive at 

 the layers of particles ? By the same action which lays 

 down the particles themselves. They are rubbed off the 

 places where they exist by the wind or by water. Per- 

 haps they were laid down among the deposited particles 

 of mud or sand. Perhaps they were brought to them by 

 streams or rivers or lakes, and sank with the water into 

 them. In a red sandstone, for example, the quartz grains 

 of the rock may be often observed to be coated with 

 earthy iron peroxide, which serves to bind them together 

 into a rather hard stone. On the other hand, the process 

 is often being reversed. The weather frequently con- 

 spires by frost and wind and rain to remove the binding 

 cement, and thereby to allow the stone to return to its 

 original condition of loose sediment. 



For millions of years the winds have blown over the 

 surface of the earth, the rain has fallen on it, the sun 

 heated it by day, the frost cracked it. Consider the 

 winds that have circled the earth. All movements of the 

 air are due in the first place to the sun which heats the 

 atmosphere and causes it to expand. The sun's rays 

 passing through the air do not heat it at once, or 



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