THE CRUST OF THE EARTH 183 



the colder crust, not being able to fit the shrinking interior, 

 wrinkled and formed what we call mountains. The once 

 horizontal rocks were folded up, while igneous material of 

 the interior was squeezed up into these folds. 



206. Dikes and Veins. Sometimes, in this process of 

 folding, the rocks break, forming deep cracks or fissures 

 through which comes melted rock. This melted rock cools 

 and solidifies, completely filling the fissure with material 



FIG. 92. DIKES; SPANISH PEAKS, COLORADO 



The wall-like elevations are of hard lava which once filled fissures in 

 a less hard rock. The latter has been eroded and the lava has been left 

 in ridges from 50 to 100 ft. high. What shows that the ridges have been 

 somewhat eroded? 



different from the rock on either side of it. Such a forma- 

 tion is called a dike. Its width may be a few inches or 

 hundreds of feet. Its resemblance to an artificial dike is 

 shown when it is harder than the rock near it, for it remains 

 as a ridge after the softer rock has been worn away. If the 

 dike is softer or full of cracks, it will be worn away faster 



