SORBY — OX THE STRUCTUEES PRODUCED BY CURRENTS. 141 



causing it to be grained and striped, and some is even carried forward 

 and deposited beyond c ; and when the angle h c d becomes very 

 smallj 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 fi'om above, 

 and the decrease of that drifted forward, passes into simple horizontal 

 sti-atification, 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 h and c e, which must usually be in 

 inverse proportion to the relative depth. The thickness and other 

 characters 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. 



I 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 w^ater, small wave-like undulations are generated on the 

 surface of the semi-fluid mixture of sand and water, of which the 



