260 



ROCKS AND SOIL 



out of artificial stone, men shape the rock material while it is soft and 

 plastic, and then bake, or "fire" it. The people of ancient Babylonia 

 and Assyria made their ordinary records, such as notes, deeds, and 

 mortgages, as well as the records of their history and literature, upon 

 clay tablets, and then baked the clay in the sun. 



Earthenware articles, such as bricks, jars, and tiles, are made of 

 common clay, hardened by heat. If salt is put into the heating furnace 

 (kiln), the articles receive a glazed surface. 



Porcelain and china are obtained by the "firing" of a mixture of 

 kaolin (a pure, white clay), quartz, and feldspar. The feldspar melts 

 first, and cements the mixture together. The first "firing" of porce- 

 lain and china articles takes place at a rather low temperature; then the 



glaze is added to the surface, 

 and the article is heated to a 

 high temperature, often for 

 several days. 



Stoneware is much like por- 

 celain, but it has not been 

 heated to so high a tempera- 

 ture. As a result, it is opaque, 

 while porcelain is translucent. 

 The science of making articles 

 out of artificial stone is called 

 keramics. 



Concrete is another artifi- 

 cial stone, made out of cement, 

 gravel, and water. The 

 cement and the water unite, 

 and cause the mixture to 

 "set." Cement is formed by 

 the heating of a mixture of 

 limestone and clay. 



Fig. 225. 



Niagara Falls are being undermined all the 

 time, because the soft shale is being worn 

 away by the water, and the hard lime- 

 stone falls into the Gorge. From 

 Hopkins, after Gilbert. 



286. Classes of Rocks. If we examine many different 

 rocks, we shall find that we can divide them into two great 

 classes. One consists of rocks having a more or less 



