MINERALS AND ORES 



175 



In some places, as in parts of Arizona and the Yellowstone 

 Park, there are great areas covered with fragments of petri- 

 fied wood that was buried for long ages. Hundreds of 

 thousands of years ago a forest was there. In the course of 

 time, the trees became submerged, and while under water 

 the wood decayed and its molecules were replaced by mole- 

 cules of the mineral silica. The trees were petrified. Later 

 the region was elevated; the surface was removed by rain, 



FIG. 87. NATURAL BRIDGE 



This is a large petrified tree trunk in the petrffied forest at Adamana, 

 Arizona. This tree must have fallen, been covered with water containing 

 silica, and petrified while submerged. In the erosion which has since 

 occurred, earth has been taken from underneath, leaving it across a gully or 

 arroyo. 



wind, and running water; and now stone trunks of trees and 

 broken fragments lie on the surface of the sandy plain. 



200. Importance of the Study of Minerals. A descrip- 

 tion of minerals, some of which we have never seen, is rather 

 uninteresting. It seems of no use to know whether calcite 

 is soluble, whether feldspar crumbles, whether quartz is hard 

 or soft. When we associate these facts with others, however, 



