262 ROCKS AND SOIL 



structure like glass, and show no crystals. Slag is an artificial unstrati- 

 fied rock (cf. 119, Fig. 94). Since unstratified rocks were probably 

 in a melted condition when deposited, and therefore hot, they are often 

 called igneous rocks, from the Latin ignis, "fire." 



Marble is an example of a third class of rocks. Marble 

 is calcium carbonate, like limestone ; but instead of show- 

 ing a layer structure, it is crystalline, like igneous rocks. 

 We believe that such rocks were originally stratified, but 

 that they have been changed by heat and pressure. They 

 are called altered, or metamorphic rocks. Anthracite 

 coal is metamorphic coal, and slate is metamorphic shale. 

 Often all the stages in the change from stratified rocks to 

 apparently igneous rocks can be seen. 



287. Origin of Stratified Rocks. In the preceding 

 section we learned that stratified rocks were formed as 

 sediment, under water, while igneous rocks were formed 

 by the cooling of melted minerals. How do we know this? 



The best method of finding out how changes took place 

 in the past is to study the changes taking place to-day. 

 We know that running water can carry sediment, and that 

 when a stream empties into a body of water, such as a 

 lake or pond, its velocity is checked, and it deposits its 

 sediment in more or less horizontal layers. 



Imagine a small stream emptying into a quiet pond. After a rain 

 the swollen stream will bring down mud, sand, and often fine gravel. 

 When it enters the pond, it drops its sediment, the largest particles 

 first, and the smallest ones last. The finest particles take a long time 

 to settle, and are spread out over the bottom of the pond. Then, after 

 a while, another rain comes, and a second deposit is placed over the 

 first one. The largest particles of the second deposit will be on top of 

 the finest particles of the first. In this way the sediment will take 

 on a layer structure, and a vertical cross section of it will show the 



