SORBY ON THE STRUCTUBES PRODUCED BY CURRENTS. 143 



as far as the dotted line, by the current (supposed to., be moving in 

 the direction indicated by the arrow), and the material thrown down 

 on the protected side, in a small bed, as at e. Sand is usually also 

 washed up by the current from the exposed side of the ripple, and 

 thrown down on the protected side j so that ripples like / gradually 

 progress. In this case it is very easy to ascertain the direction 

 from which the current came ; for, of course, it must have flowed in a 

 line perpendicular to the trend of the ripples, and from the opposite 

 side to that towards which the small beds of the ripple dip, as shown 

 by the arrow in fig. 2. 



If no actual deposition be taking place, and there be merely a 

 drifting forward of sand along the bottom, when the ripples progress 

 there is necessarily as much washed up from one side of each ripple as 

 is thrown down on the other ; and, therefore, nothing but an advanc- 

 ing series of laminated ripples is formed. If, however, more sand be 

 deposited than is washed up, so that there is an actual accumulation 

 of material, the lower part of the advancing ripples is necessarily left 

 behind. Fig. 3 will make my meaning apparent, and represent in 

 section that which occm-s when deposition takes place at a uniform 



Fig. 3. 



rate. For all structures that are the effect of the action of ripples 

 on drifted material, I have employed the term "ripple-drift." This 

 necessarily includes some cases of the well-known ripple-marks, which 

 term I would, however, restrict to those instances where the upper 

 surfaces of the ripples are more or less perfectly preserved. When 

 this is not the case, the resulting structure might easily be con- 

 founded with " drift-bedding," and no doubt often has been classed 

 with it as " false-bedding," being much more like it than like ripple- 

 marks. It will much facilitate my explanations if we also adopt the 

 following descriptive terms, and call the whole thickness, a c, a " bed " 



