SORBY — ON THE STRUCTURES PRODUCED BY CURRENTS. 139 



of course an horizontally stratified mass wonld result, each layer of 

 which would have a grained and striped surface. Sandstones of this 

 character, which may be distinguished by the term grained or 

 striped horizontal stratification," are very common in the lower coal- 

 strata ; and they are so distinctly grained and striped that there is 

 no difficulty whatever in determining with certainty the line in which 

 the ancient current moved, while the side from which the current 

 came can usually be learned from other structures. 



If in a modem water-course the depth of the water increase to 

 such an extent that the velocity of the current is diminished so much 

 that it becomes too slow to wash the sand forward, the sand is then 

 transported only to a certain point, where it falls down and accu- 

 mulates* on a slope. This will be best understood from the following 

 diagram (fig. 1) — 



Fig. 1. 



b a 



representing, by a vertical section in the line of the current, what 

 takes place at the bottom of the water. This kind of structure 

 can only be formed where there are particular relations between the 

 depth of the water and the velocity of the current. The velocity 

 above the part a h must be sufficient to drift forward the sand when 

 the sand is in motion, or else there would be no increase in the 

 dimensions of the bed for lack of material carried forward and thrown 

 down at 6 c ; but the velocity must not be so great as to wash up the 

 sand from a state of rest along the surface a b, else the bed would not 

 remain permanent. As might be expected, there cannot be a great 

 difference in the velocity of the currents producing these difierent 

 effects j but still it is easy to convince one's-self by experiment that 

 there is a decided difference. In one case the velocity need be only 

 sufficient to cause the resistance offered by grains of sand to the 

 motion of the water to be rather more than equal to the mere friction 

 of the transported sand on that lying unmoved at the bottom; 

 whereas, in the other case, it must be enough to likewise overcome 

 the inertia of the grains of sand. Above the part c e, the velocity 



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