THE EARTH'S ROCK FOUNDATIONS 45 



Dig down into the soil anywhere and you finally come to the 

 solid rock on which the soil always rests (Fig. 25, p. 46). Some- 

 times the soil is only a thin cover for the rock that lies close to 

 the surface; again it may be a thick blanket. In the hill or 

 mountain regions the bare rock may cover miles of area with 

 scarcely a vestige of soil upon it. Bore down into the rock as 

 deeply as man has been able, or sink a mine shaft, and the going 

 is all the way through solid rock. True, our deepest mines and 

 borings explore only the outer part of the earth, penetrating but 

 a little over a mile. The J. H. Lake well at Fairmont, West 

 Virginia, is 7,579 feet deep. But they tell us that this outer por- 

 tion is all made of just such rock as we find somewhere at the 

 surface. Such rock may be a great mass of a single mineral or 

 it may be composed of grains or crystals of minerals all firmly 

 pressed together. 



By a mineral we mean any inorganic substance composed 

 throughout of one definite or nearly definite chemical substance. 

 Limestone, the common bed rock of the Chicago area, is a rock, 

 and the mineral of which it is made is called calcite, which is 

 chemically a carbonate of lime. Sandstone, another widespread 

 rock, is made of grains of the mineral quartz, cemented together 

 with more or less lime. Granite, on the other hand, is made up of 

 bits of several minerals, quartz and feldspar certainly, and fre- 

 quently others as well, all making the solid rock. 



Most minerals occur as solids, though a few are liquids in 

 a state of nature. Thus mercury may occur in drops, sulphur 

 in pools or even lakes of the molten mineral. If the term, 

 mineral, as defined is taken in its broadest sense it will include 

 certain gases like nitrogen, oxygen, and steam, but here it is 

 used with its more customary meaning. 



A few minerals are chemical elements; that is, they cannot by 

 ordinary means be broken up into simpler things. Such, for 

 instance, are certain metals like iron, copper, gold, silver. These 

 occur in the rocks at times in grains or even in good-sized chunks 

 of pure or native metal. Masses of such native copper were highly 



