40 THE STORY OF THE EARTH. 



durability. There are three chief kinds of ce- 

 ment, which were originally of vegetable or ani- 

 mal origin. First, there is the iron sand which is 



commonly red, , or brown; and is rarely 



green. The iron was probably collected from the 

 sea-water by the growth of marine plants, and 

 liberated by their decay. Oxide ^i iron is lia 

 to accumulate in the planes in which water has 



flowed underground through the rock, which 



sometimes determined by strain. Examples of 



1 so coloured are seen in the Bagshot beds 

 Alum bay, in tin [s e of Wight; and theWealden 



beds of \Varbarr<>\\- bay, in the 1- Purbeck. 



The Wcalden sands of Kent and Sus so rich 



in iron that they were for a long time the main 

 Source Of the metal out of which English imple- 

 ments <»f war and ornaments of art were made. 

 Sands are often bound into < us sand- 



ies by a Cd ' carbonate of lime. It is 



sometimes derived originally from the evapora- 

 tion of water, but more frequently from the fall- 

 tO the bottom of Organisms which lived in the 

 Or by the accumulation of marine shells. 

 ■ tltish Rag, which forms the l<»wi-r i Kir t of the 



:id, is a familiar exam] 



;n the S 1 . land. I 



Is become dissolved by water flowii g through 

 the nate < >f lime i >f w inch they 



to till the inter- 



betwe< • i grains of quartz, ai d f< 



w huh bind the sand into a sandsfc 



i third ^md of sandst< modified by 



. is t lie m1k . dstone, 



in w hich th< quartz a nd t< ^< ther 



1 ■ : 



