SORUV — ON THE aTIlUCXUBES PUODUCEU BY 0U1UIENT8. 143 
as far as the dotted line, by the current (supposed to bo 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 hy 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 curreut 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 pai-t of the advancing ripples is necessarily left 
behind. Fig. 3 will make my meaning apparent, and represent in 
section that which occurs 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 pi-eserved. 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" 
