WATEE AS A MECHANICAL AGENT. 189 



tain needles ; but much of the granite of the world easily crumbles under 

 atmospheric influences, and makes the tamest of scenery. Slates standing 

 on end often bristle slopes with projecting ledges, and rise into lofty needles 

 that defy the elements, like the Matterhorn in the Alps ; but other slates are 

 fragile, and wear down into hills of gentle earth-covered slopes. 



8. Climatal effects. — Cliniatal causes also have great effect on the work 

 of rivers. A wet climate produces abundant vegetation, which is more or 

 less a protection from wear ; and in tropical regions it covers even precipices 

 with ferns and other foliage. It also occasions rapid decay by the chemical 

 and other weathering methods. Moreover, it sometimes makes deep, hard- 

 working rivers, torrents that sweep away roughly, degrade rapidly and per- 

 sistently and leave behind massive peaks, broad mountains, earth-covered 

 slopes ribbed or belted by the more enduring beds, with gently swelling out- 

 lines over the lower slopes, and foliage almost everywhere. 



A dry climate, on the contrary, as in the Colorado region, and that of 

 Yellowstone Park, makes small streams or streamlets in the mountain valleys, 

 many of which through much of the year are only threads of water, if not 

 wholly dried up. They hence finish off with sharp and delicate outlines. 

 All the variations of the beds in hardness are expressed in series of pro- 

 jecting edges beneath the broader shelves and entablatures. The jointed 

 structure of the thick, durable beds adds much to the diversity of surface, 

 instead of insuring the removal of the beds. The winds also aid with 

 lighter finger. 



In such regions, color from foliage may fail. But the dripping waters of 

 the occasional rains, or the oozings through the steep mountain-sides, transfer 

 to the surface the results of oxidations and deoxidations, and paint the walls 

 with various delicate tints. 



Even alternations of half-hardened clay-beds and sand-beds, under such 

 conditions, as Colorado scenery illustrates, may be cut into groups of pinnacles, 

 turrets, and columns finished with capitals and bases which will last indefi- 

 nitely ; for whatever the occasional supply of waters to the channels, it ends 

 in reproducing the same features in the soft beds. Appalachian rains, as 

 Powell says in his work on the Colorado Canon (1875), would soon oblit- 

 erate much of Colorado scenery. The excavation of the Colorado Canon 

 has been chiefly due to great floods ; but the finishing work carried on 

 within it has been of the gentler kind. 



Transportation and Deposition. 



Amount of material transported and deposited by rivers. — The materials 

 transported by running waters are (1) stones, pebbles, sand, and clay or 

 earth ; (2) logs and leaves from the forests, and sometimes trees that have 

 been torn up or dislodged by the current ; (3) Mollusks and their dead shells. 

 Worms, Insects, etc., attached to the logs or leaves ; (4) occasionally larger 



