On. II.] 



CAUSES OF DIAGONAL STRATIFICATION. 



17 



brown, and the lines a, b, c, mark some of the principal planes of strati- 

 fication, which are nearly horizontal. But the greater part of the sub- 

 ordinate laminae 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 laminae can- 

 not possibly be accounted for by any rearrangement of the particles ac- 

 quired during the consolidation of the rock. In what manner then can 

 such irregularities be due to original deposition ? We must suppose 

 that at the bottom of the sea, as well as in the beds of rivers, the mo- 

 tions 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 passages through them, just as a river forms 

 its bed. Suppose the bank A (fig. 4) to be thus formed with a steep 



C D 



sloping side, and the water being in a tranquil state, the layer of sedi- 

 ment No. 1 is thrown down upon it, conforming nearly to its surface. 

 Afterwards the other layers, 2, 3, 4, may be deposited .in succession, so 

 that the bank BCD is formed. If the current then increases in ve- 

 locity, it may cut away the upper portion of this mass down to the 

 dotted line e (fig. 4), and deposit the materials thus removed farther on, 

 so as to form the layers 5, 6, 7, 8. We have now the bank BCDE 

 (fig. 5), of which the surface is almost level, and on which the nearly 



Fig. 5. 



"~^^ 



-^>- 







11 









i° 









3 





^>^ 



-<^< 



J^-^-<^ 



^■'■'a\ 



Fig. 6. 



horizontal layers, 9, 10, 11, may then accumulate. It was shown in fig.. 

 3 that the diagonal layers of successive strata may sometimes have an 

 opposite slope. This is well seen in some cliffs of loose sand on the 



Suffolk coast. A portion of one of 

 these is represented in fig. 6, where 

 the layers, of which there are about 

 six in the thickness of an inch, are 

 composed of quartzose grains. This 

 arrangement may have been due to 

 the altered direction of the tides and 

 cuff between Mismer and Dnnwich. currents in the same place. 



