PHYSICO-CHEMICAL PROCESSES 151 



The earth appears to be composed of twelve main elements : 

 oxygen, silicon, aluminum, calcium, magnesium, potassium, 

 sodium, carbon, hydrogen, sulphur, chlorine, and iron. Of 

 course many other elements, such as the precious rnetals, 

 are present but are found in small quantities only. Most 

 of the rocks found in the earth are mixtures of two or more 

 minerals. Granite formed from volcanic eruption, for ex- 

 ample, is a mixture of three minerals feldspar, quartz, 

 and mica; sandstone consists of particles of silica or sand; 

 limestone consists of a carbonate of lime; slates consist 

 of silicates of aluminum; and clay consists principally of 

 aluminum compounds. The minerals are held together in 

 the stone by some binding substance, like carbonate of lime, 

 iron oxide, or silica. The color of the clay, rocks, and differ- 

 ent parts of the earth is due to the presence of small quanti- 

 ties of iron and other metals. Changes in temperature cause 

 the rocks to expand and contract and consequently they 

 gradually split and crack. The rain then washes into the 

 valley the loose parts of the rocks. Thus the soft, loose soil 

 found on the surface of the earth is the result of the breaking 

 up of the rocks in this way, and the process by which such 

 soil is made is termed weathering or erosion. 



Stones or rocks are designated as sedimentary, igneous, 

 or metamorphic, the classification depending upon their 

 origin. 



Sedimentary rocks are remains of older rocks which have 

 been deposited under water, layer by layer. Limestone 

 and sandstone are examples of this class. Igneous rocks 

 are formed by the solidifying in a crystalline state of lava 

 from a volcano. Granite and allied stones are examples of 

 this kind of rock. Metamorphic rocks are rocks that have, 

 after formation, changed their original forms because of 



