32 



LYELL'S ELEMENTS OF GEOLOGY. 



Diagonal St ratification, and its Causes. 



" false Stratification" has been given. Thus in the annexed sec- 

 tion (Fig. 3.) we see seven or eight large beds of loose sand, 



Section of sand at Sandy Hall, near Biggleswade, Bedfordshire, 

 Height twenty feet. (Green-sand formation.) 



yellovi^ and brown, and the lines a, c, mark some of the prin- 

 cipal planes of stratification, which are nearly horizontal. But 

 the greater part of the subordinate laminse do not conform to 

 these planes, but have often a steep slope, the inclination being 

 sometimes towards opposite points of the compass. When the 

 sand is loose and incoherent, as in the case here represented, the 

 deviation from parallelism of the slanting laminse cannot possi- 

 bly be accounted for by any rearrangement of the particles 

 acquired during the consolidation of the rock. In what manner 

 then can such irregularities be due to original deposition 1 We 

 must suppose that at the bottom of the sea, as well as in the beds 

 of rivers, the motions of waves, currents, and eddies often cause 

 mud, sand, and gravel to be thrown down in heaps on particular 

 spots, instead of being spread out uniformly over a wide area. 

 Sometimes, when banks are thus formed, currents may cut pas- 

 sages through them, just as a river forms its bed. Suppose the 

 bank A (Fig. 4.) to be thus formed with a steep sloping side, and 

 Fig. 4. B 



C D 



the water being in a tranquil state, the layer of sediment No. 1. 



