Ch. II.] HOEIZONTALITY OF STRATA. 15 



farther, and not deposited in the same places as the grains of quartz ; and 

 since the force and velocity of the stream varies from time to time, layers 

 of mica or of sand will be thrown down successively on the same area. 



Original horizontality. — It is said generally that the upper and 

 under surfaces of strata, or the planes of stratification, are parallel. 

 Although this is not strictly true, they make an approach to paral- 

 lelism, for the same reason that sediment is usually deposited at first 

 in nearly horizontal layers. The reason of this arrangement can by 

 no means be attributed to an original evenness or horizontality in the 

 bed of the sea ; for it is ascertained that in those places where no 

 matter has been recently deposited, the bottom of the ocean is often as 

 uneven as that of the dry land, having in like manner its hills, valleys, 

 and ravines. Yet if the sea should sink, or the water be removed near 

 the mouth of a large river where a delta has been forming, we should 

 see extensive plains of mud and sand laid dry, which, to the eye, would 

 appear perfectly level, although, in reality, they would slope gently from 

 the land towards the sea. 



This tendency in newly-formed strata to assume a horizontal position 

 arises principally from the motion of the water, which forces along par- 

 ticles of sand or mud at the bottom, and causes them to settle in hollows 

 or depressions, where they are less exposed to the force of a current than 

 when they are resting on elevated points. The velocity of the current 

 and the motion of the superficial waves diminish from the surface 

 downwards, and are least in those depressions where the water is 

 deepest. 



A good illustration of the principle here alluded to may be sometimes 

 seen in the neighborhood of a volcano, when a section, whether natural 

 or artificial, has laid open to view a succession of various-colored layers 

 of sand and ashes, which have fallen in showers upon uneven ground. 

 Thus let A B (fig. 1) be two ridges with an intervening valley. These 

 original inequalities of the surface have been gradually effaced by beds 

 of sand and ashes c, d, e, the surface at e being quite level. It will be 

 seen that although the materials of the first layers have accommodated 



themselves in a great degree to the shape 

 of the ground A B, yet each bed is thick- 

 est at the bottom. At first a £reat manv 



particles would be carried by their own 

 gravity down the steep sides of A and B, 

 and others would afterwards be blown by the wind as they fell off the 

 ridges, and would settle in the hollow, which would thus become more 

 and more effaced as the strata accumulated from c to e. This levellino- 

 operation may perhaps be rendered more clear to the student by sup- 

 posing a number of parallel trenches to be dug in a plain of moving 

 sand, like the African desert, in which case the wind would soon cause 

 all signs of these trenches to disappear, and the surface would be as 

 uniform as before. Now, water in motion can exert this levelling power 

 on similar materials more easilv than air, for almost all stones lose m 



