OEIG IN OF TOPOGRAPHIC FOEMS. HI 



plateau has been produced its remarkably complex drainage system. From 

 enormous plateaus have been carved the great ranges of Colorado, with 

 their peaks, canyons, and cliffs. From the plateaus of the Colorado drain- 

 age system thousands of feet of rock have been worn away, leaving here 

 and there great cliffs and high plateaus to show the magnitude of its work, 

 while the great canyons dividing the lower plateaus, some of them a mile in 

 depth, though the least among its works, are the topographic wonders of 

 the world. From the moment the land rose above the sea, this agency of 

 destruction has been at work, and its labors will not cease until the land 

 again sinks beneath the waves. 



The action of water on rocks may be divided into three parts — weather- 

 ing, transportation, and corrasion. The rocks of the general surface of the 

 land, or the terrain, are disintegrated and converted into soil by weathering. 

 The material thus loosened is transported by streams, and while thus being 

 transported it helps to corrade other material from the channels of the 

 streams. In weathering, the chief agents are solution by water, frost, the 

 mechanical beating of rain, gravity, and vegetation. Some rocks, particu- 

 larly limestones, are entirely dissolved by water, especially when it is charged 

 with carbonic acid; others are dissolved only in part and the remaining part 

 is thus disintegrated. Rocks are cracked and broken by the freezing of 

 water in their interstices. When the foot of a cliff is undermined by ei*osion, 

 the upper portion, failing of support, breaks off in fragments by its own 

 weight. The roots of plants pushing their way into the interstices of rocks 

 pry them apart and thus aid in disintegration. In general, soft rocks disin- 

 tegrate more rapidly than hard rocks and soluble rocks more rapidly than 

 insoluble rocks. Disintegration is more rapid in a moist than in a dry climate. 



The product of disintegration is soil, and this may be regarded in future 

 discussion as a soft bed subject to the same laws of corrasion and transpor- 

 tation as other beds, with only such modifications as its want of cohesion 

 recpiires. 



TRANSPORTATION AND CORRASION. 



Rain falls upon the surface, a portion of it sinks and reappears in springs, 

 while another portion flows down the surface and collects in water courses, 

 which, joining one another, produce, finally, large streams. During a rain 



