SORBY — ON THE STBUCTUBES PllODUCKD BY CURRENTS. 141 
causing it to be grained and striped, and some is even carried forward 
and deposited beyond c ; and when the angle bed becomes very 
small, or of no appreciable magnitude, the whole passes into the 
horizontal grained and striped stratification. This, again, by the 
increase in the amount of the material actually deposited from above, 
and the decrease of that drifted forward, passes into simple horizontal 
stratification, with little or nothing to indicate the direction of the very 
feeble current. The production of the grained and striped, instead of 
the simple horizontal stratification, therefore, depends upon the actual 
velocity of the current ; whilst the formation of drift-bedding depends 
upon the relative velocity above a b and c e, which must usually be in 
inverse proportion to the relative depth. The thickness and other 
chai-acters of the bed must therefore be so intimately related to the 
actual depth of the water, that, when all the requisite data have been 
determined, this actual depth could be calculated from the thickness 
and peculiar characters of the bed a d. 
T have already made a number of experiments from which a first 
approximation to this very interesting problem can be deduced ; but, 
before everything can be determined in a perfectly satisfactory 
manner, it will be necessary to take into consideration many things 
requiring much further investigation. Even in the present state of 
the inquiry, however, we may draw several important conclusions. 
The existence of perfectly similar beds, separated by a considerable 
thickness of rock, clearly shows that, whatever the actual depth of 
water might have been, it must have been the same at both periods, 
which, of course, necessitates the notion of a considerable amount of 
subsidence having taken place ; whilst, in other cases, the upper beds 
indicate a less depth than the lower, as if owing to a decrease in the 
depth of water, caused by the accumulation of the deposits. 
We all know very well that when wind blows over the surface of 
water it gives rise to ripples and small waves, which trend perpendi- 
cular to the direction of the wind, and move forward in the line of its 
motion. We could thus readily determine the direction of the wind 
from the direction of the waves and ripples on the surface of the water. 
In a somewhat similar manner*, when a current of water moves over 
sand and water, small wave-like undulations are generated on the 
surface of the semi-fluid mixture of sand and water, of which the 
