30 LYELL'S ELEMENTS OF GEOLOGY. 



Horizontality of Strata. 



be thus understood. If we take a handful of quartzose sand, 

 mixed with mica, and throw it into a clear running stream, we 

 see the materials immediately sorted by the water, the grains of 

 quartz falling almost directly to the bottom, while the plates of 

 mica take a much longer time to reach the bottom, and are car- 

 ried farther down the stream. At the first instant the water is 

 turbid, but immediately after the flat surfaces of the plates of 

 mica are seen alone reflecting a silvery light, and they descend 

 slowly, to form a distinct micaceous lamina. The mica is the 

 heavier mineral of the two ; but it remains longer suspended, 

 owing to its great extent of surface. It is easy, therefore, to 

 conceive how the intermittent action of waves, currents, and 

 tides, may sort the sediment brought down from the waste of a 

 granitic country, and throw down the mica, layer after layer, 

 separately from the mud or sand. 



Original hcrizontality. It has generally been said that the 

 upper and under surfaces of strata, or the planes of stratification, 

 as they are termed, are parallel. Although this is not strictly 

 true, they make an approach to parallelism, 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 horizontally 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, al- 

 though, 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 particles 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 down- 

 wards, and is least in those depressions where the water is deep- 

 est. A good illustration of the principle here alluded to, may be 

 sometimes seen in the neighbourhood of a volcano, when a sec- 

 tion, whether natural or artificial, has laid open to view a suc- 

 cession of various-coloured 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 in- 

 equalities of the surface have been gradually effaced by beds of 



