Rocks and Soil 291 



Igneous rocks are those which cooled from a molten mass. 

 Ign, the Latin stem of the word igneous, means fire. Lava, the 

 molten rock that comes from the volcano, is igneous rock. Some 

 of the most durable rocks we have, such as granite, belong to 

 this class. Igneous rocks never have a layer-like structure. 



Sedimentary rocks are those which have been formed by ma- 

 terial deposited in layers, usually by water but sometimes by 

 wind. These sediments are pressed together by the weight of 

 material that collects above them and, with the. aid of certain 

 chemicals in them that act as a cement, they are formed into a 

 solid mass that shows a layer formation. Sandstone and shale are 

 sedimentary rocks. 



Metamorphic rocks are formed from sedimentary rocks when 

 they become buried and are placed under high pressure due to 

 weight of other rocks above them. The high pressure causes a high 

 temperature, because the energy used in the pressure is changed 

 into heat. The high pressure and the temperature cause the struc- 

 ture of the rocks to change very much. In some cases it is 

 hard to tell from what sedimentary rocks the metamorphic was 

 made. A good example of metamorphic rock is marble. This 

 is formed from limestone under high temperature and pressure. 

 The material in the limestone came from the shells of small 

 animals. 



Experiment to Show Formation of Layers in Sedi- 

 mentary Rock. Place small amounts of sand, gravel, clay and 

 humus in a jar, fill the jar with water, stir the contents and allow 

 the material to settle undisturbed. The different materials have 

 different weights and will settle at different rates. The heavy 

 materials will settle at once, then the lighter, and at last the very 

 lightest. If the water is then carefully poured off and a hole 

 scooped in the center of the material left in the bottom of the jar, 

 layers will be clearly noticeable. 



Fossils. Sedimentary rocks often contain forms which show 

 the animal and plant life of the time when they were made. A 

 plant or animal body became covered with the rock- forming 

 material and when the body decayed new rock material came in 

 to fill the space left. Sometimes this material is the same as 



