SOKBY — ON THK STRU0TUBE8 PRODUCED BY CURRENTS. 139 
of course an horizontally stratified mass would result, each layer of 
which would have a graiued aud 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 doterminiug 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 modern 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 aud the velocity of the current. The velocity 
above the part a b must be sufiScieut 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 b 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 gi-eat 
difference in the velocity of the currents producing these different 
effects ; 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 ; 
whex-eas, 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 
L 2 
