INSIDE OF THE EARTH 51 



ment of differences between the forces of heat and of 

 pressure. As a result, these layers, and others, may have 

 been forced up, pressed together, broken and distorted, 

 and the result was a new range of mountains (see Fig. 7). 

 And in such mountains to-day we may see what was once 

 mud changed to a kind of rock called shale, what was once 



FIG. 20. Igneous rock; a granite quarry. 



a layer of shells changed to limestone, and what was once 

 sand changed to sandstone. 



5. As to their origin, all rock or formations of rock may 

 be classed as igneous or sedimentary. Igneous means 

 related to fire (ignis, fire), and igneous rocks are those 

 whose substance was once molten. They have come 

 to their present state through cooling, pressure also usually 

 having a good deal to do with it. Granite is an example 

 of igneous rock (see Fig. 20). Sedimentary rock is rock 



