842 DYNAMICAL GEOLOGY. 



VIII. Valleys. 



1. By Erosion. — 1. Through fresh-water streams, this is the great source of the 

 valle} r s and gorges in mountainous regions : sometimes, though seldom, the direction 

 is predetermined by fractures, p. 641. 



2. Through marine currents and waves, removing dikes that intersect coast rocks, 

 or portions of yielding rock; a process which produces small cuts or excavations, but 

 not true valleys, p. 686. 



3. Through the action of glaciers, either by the tearing action of the ice, where 

 descending at bottom into cavities in the rocks, or by abrasion carried on by means of 

 the stones in the bottom and sides of the glaciers, p. 539. 



2. By movements of the Earth's crust. — l. Producing parallel ranges of 

 mountains of which the " Mississippi valley " is an example, pp. 23, 825. 

 2. Producing monoclinal uplifts, and consequently intervening depressions, p. 792. 

 See further, pp. 824, 825. 



IX. Lake Basins. 



1. Through glacial action, the glacier plowing deep where the rocks are soft, and so 

 making a deep depression, and then ceasing the excavation where there is a change 

 to a hard rock, p. 539. 



2. Through a dam thrown across a valley, by (1) a moraine from a glacier, p. 701; 

 (2) a slide of gravel, or avalanche; (3) a flow of lava; or (4), of a temporary character, 

 through damming by a glacier. 



3. Through a dam or dike of sand or gravel made along a seashore, by the waves 

 and tidal currents, shutting off a region of water from connection with the sea, which 

 may finally become fresh, if it receives the drainage of the back country. 



4. Through uplifts of mountains surrounding intervales or low plains, for which 

 subsequent erosion provides no complete drainage. 



5. Through the elevation of a country producing level regions, over which depres- 

 sions remain without a drainage channel, because the waters are too sluggish in move- 

 ment for much erosion, as about the headwaters of the Mississippi. 



6. Through the undermining of the surface deposits of a country by the action of 

 water, p. 665. 



7. Through the ejection of lavas from a volcano, leaving, when the volcano becomes 

 extinct, a crater as a basin-like depression. 



8. Through the contraction of the rocks beneath a region, in consequence of cooling, 

 causing a depression of the surface. 



9. Through pressure of overlying beds or of a glacier mass forcing out a softened 

 stratum underneath, p. 666. 



X. Markings on Rocks. 



1. Scratches. — 1. By the movement of glaciers, pp. 538, 699 ; or of icebergs, p. 

 701; or of any floating ice, carrying stones at bottom. 



2. By the mutual friction of the opposite walls of a fissure, at the time of the making 

 *f the fissure (the usual way), or afterward, pp. 90, 774. 



3. By the sliding of beds on one another, either as a consequence of gravity, p. 666, 

 or of lateral pressure, p. 90. 



4. By the drifting of sands by winds, pp. 91, 631- 



5. Through the rapid transportation of stones by water. 



6. By land slides. 



2. Other markings. — 1. Ripple-marks, rill-marks, rain-drop impressions, p. 84. 

 2. Of organic origin, as footprints, etc. 



