132 GEOLOGY. 



various inequalities of bottom, of the shear-zone, and of resistance to creep, 

 certain blocks of the shell may well be supposed to settle away from other blocks, 

 leaving them protuberant as horsts. Certain other blocks might drop to levels 

 lower than the adjacent ones, to fill the vacancy made by the spreading apart of 

 neighboring blocks. Tiltings, rift-valleys, and other phenomena common to 

 tensional areas would be natural accompaniments. 



Relations to sedimentation. — Lateral creep from continental to oceanic areas 

 should gradually lower the continents, particularly at their borders, and so induce 

 a landward creep of the sea, giving rise to tracts of shallow water (epi-continental 

 seas) well adapted to the shallow-water sedimentation which has prevailed in 

 the continental areas during the course of known geological history. It should, 

 furthermore, give rise to slight changes of elevation, by reason of the lateral move- 

 ment over inequalities of base. Gentle warpings, undulations, and crevassings, 

 should occur, as in glaciers, and for like reasons. Movements of these kinds, 

 because of their extreme gentleness, would be admirably adapted to promote 

 those common phases of stratification in which a nearly constant but slightly 

 changing depth of water is maintained for long periods. The movement would 

 also constitute an element in base-leveling. 



On the borders of the continents, such supposed creep should give rise to 

 several peculiar results that will be discussed under the Pliocene and later 

 periods, since they seem to have specially manifested themselves at these times. 



