KINDS OF DISPLACEMENT 23 I 



covers up the minor inequalities of the ground and leaves a level 

 surface, so on the sea-bottom, the sediments are spread out in 

 nearly level layers, disregarding ordinary inequalities. We must 

 remember, however, that this original horizontality is not exact, 

 and departures from it are not infrequent. On a large scale, these 

 departures from the horizontal position are very slight, while those 

 that are conspicuous are always local. 



Examples of such original deviations from horizontality are the 

 following : (1) When a sediment-laden stream or current empties 

 abruptly into a deep basin with steeply sloping sides, the sediment 

 is rapidly deposited in oblique layers, which follow the slope of 

 the sides. (2) Alluvial cones, or fans (p. 137), have steeply 

 inclined layers, for a similar reason. Both of these cases resemble 

 the artificial embankments which are built out by dumping earth 

 or gravel over the end, until each successive section is raised to 

 the necessary level. In such embankments the obliquity of the 

 layers is often plainly visible. (3) Sand beaches often have a 

 considerable inclination, as much as 8 %, and newly added layers 

 follow this slope. (4) On a large scale, the great sheets of sedi- 

 ment that cover the sea-bottom generally have a slight inclination 

 away from the land, with a somewhat increased slope along lines 

 of depression. These slight original inclinations of sedimentary 

 masses, either as a whole, or along certain lines, are called initial 

 dips, and have an important bearing upon the results of subsequent 

 movements of displacement. 



The displacements to which strata have been subjected after 

 their formation are of two principal kinds : ( 1 ) In the first kind, 

 the strata have been lifted vertically upward, often to great heights, 

 without losing their horizontality. Over great areas of our Western 

 States and those of the Mississippi valley, the beds are almost as 

 truly horizontal as when they were first laid down. In some of 

 the lofty plateaus through which the Grand Canon of the Colorado 

 has been cut, almost horizontal strata are found 10,000 feet above 

 the sea-level. (2) More frequent and typical are the displacements 

 of the second class, by which the beds are tilted and inclined at 

 various angles, sometimes bringing the strata into a vertical posi- 



