THE EARTH'S ROCK FOUNDATIONS 75 



the sandstone, though the sand grains are indistinct through 

 partial fusion. Marble is crystalline, fairly hard though still 

 scratched with a knife, and effervesces with acid, though not as 

 violently as limestone unless the latter contains much silica, a 

 siliceous or cherty limestone. 



Commercially the most valuable of all the sedimentary rocks 

 is the coal (Fig. 32). We are very fortunate in possessing such 

 vast quantities of it in this country. It is estimated that we 



FIG. 32. Entrance to a coal mine 



have mined some 14,000,000,000 tons thus far in our history, and 

 that we still have left 17,000,000,000 tons of anthracite, 

 1,500,000,000,000 tons of bituminous, and 2,000,000,000,000 tons 

 of lignite, a coal of inferior quality but still usable. We are using 

 our coal at a much faster rate than ever before, for the industrial 

 demand for it is ever increasing. In 1921 we mined nearly two- 

 thirds of a billion tons. Just how long the available supply will 

 last it is very difficult to say or even to make an approximate 

 guess as there are so many factors involved. Some of the coal 



