310 PHYSICAL FEATURES OF THE UNITED STATES 



the minerals of the land were mingled in one complex but homo- 

 geneous substance, the problem of civilization would be a problem 

 of separation and would be chemical ; but the gods have classi- 

 fied and arranged, sorting the more abundant materials into 

 broad layers, and gathering the rarer into crevices and pockets ; 

 and so the problem of civilization is a problem of exploration 

 and discovery, or a problem of geographic distribution. 



Pluto sorts by creating a slow circulation of water. As far as 

 mines and borings have penetrated the earth the pores of the 

 rocks are full of water, and the downward limit of this satura- 

 tion is unknown. The upper rocks are comparatively cool ; the 

 lower rocks are hot; and the contrast sets the water in motion. 

 The upper water, denser because cold, tends downward ; the 

 under water, expanded and made lighter by heat, is forced up- 

 ward, and though motion is exceedingly slow, there is a contin- 

 uous circulation. The chemistry of the upper water is different 

 from the chemistry of the lower. Each can dissolve certain sub- 

 stances, but the substances are not the same. The properties of 

 water change as heat and pressure increase, and again as heat 

 and pressure decrease. So the slow-moving water picks up cer- 

 tain substances in one region, and in another deposits them so 

 as to receive other substances, and in this way it sorts out many 

 of the rarer things, gathering together or concentrating ores of 

 gold, silver, platinum, mercury, lead, zinc, copper, and iron. 



Apollo sorts by the free circulation of water at the surface. 

 The soil that is washt away from mountains and uplands and 

 spread by the streams in lowlands and submerged plains is not 

 deposited in one promiscuous mass, but is classified according 

 to kinds — marl in one place, clay in another, and sand in an- 

 other — and in time these become limestone, shale, and sandstone. 

 The tissues of plants are gathered in swamps and changed to 

 peat, then buried under shale and sandstone, and finally trans- 

 formed to coal. The tissues of plants and animals, intimately 

 mingled with mud that changes underground to shales, are 

 slowly distilled in after ages to fill rock reservoirs with oil and 

 gas. In other places and by other special processes iron, salt, 

 gypsum, and phosphates are separated; and where Plutonic 

 stores of the metals are ravaged by storm and stream, the gold is 

 separated by its weight and gathered in the river gravels. 



The origin of the features of all lands having been thus briefly 

 sketcht, we may now consider in a broad way the physical 



