CLAY. ALUMINUM. CALCAREOUS ROCKS. 79 



architect for the first solid representation of the ornaments 

 of pillars and other parts of buildings. 



275. Most of the slates are more or less aluminous. 

 The metal aluminum, which is the basis of clay, very 

 much resembles silver in color, brilliancy and hardness, 

 though far less beautiful. Alum, from which it derives 

 its name, is partly made of it. 



The oxide, alumina, is one of the most abundant mate 

 rials of the crust of the earth, forming not less than one 

 quarter of its substance. Two of the most beautiful of 

 the precious stones, the sapphire and the ruby, are alumina 

 tinged with a little oxide of iron. They are inferior only 

 to the diamond in hardness and brilliancy. Another very 

 beautiful precious stone, the topaz, is also an aluminous 

 mineral. When colorless, it possesses a lustre which has 

 often caused it to be mistaken for the diamond. 



276. (4.) Calcareous Rocks arc composed chiefly of 

 carbonate of lime, that is, lime chemically combined with 

 carbonic acid. There also enters into their composition 

 a greater or less proportion of silex or other sand, and of 

 clay and sometimes other mineral substances. In England 

 and some other countries, vast quantities of chalk, which 

 is carbonate of lime, are found, and in some places the 

 soil is almost wholly made of it. 



A soil consisting chiefly of calcareous earth is a very 

 poor soil. It has more tenacity than sand, but less than 

 clay, absorbs moisture readily, but easily parts with it, 

 and is liable to crack, when dry, like clay, and to parch 

 plants growing in it. Excessive moisture turns it into a 

 thick mud, and if, in this state, it be exposed to extreme 

 cold, it swells and cracks, and is apt to wound the roots 

 of plants and even throw them out of the ground. In its 



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