282 



GENERAL SCIENCE 



history have been solidified from a melted condition. If 

 on solidifying the cooling has been slow, these rocks will 

 have a crystalline structure such as is found in marble. 

 Lava is a form of igneous rock (Figure 239). 



Sedimentary rocks are those formed by the deposition 

 of sand, gravel, clay, and other materials, by water. The 



U. S. Geological Survey. 

 FIG. 241. Sedimentary Rock. 



Showing a fault in rock formation, Crawford, Nebraska. 



gravel, sand, or clay is washed from the lands into the 

 ocean and deposited on the ocean floor; the finest par- 

 ticles being carried farthest from the shore. In the course 

 of ages these layers of sediment are changed into solid rock 

 (Figures 240-241). Sandstone and limestone are classes 

 of sedimentary rocks. Sandstone is formed by particles 



