On Ripple-Murlis. 169 



2. On Ripple-Marks. 



The small waves raised on the surface of the water, by the 

 passage of a slight breeze, are called ripple ; and a series of 

 marks, very similar in appearance, which are sometimes seen at 

 low water, on the flat part of a sea-beach formed of fine sand, 

 are called ripple-marks. Such marks occur in various strata ; 

 and are regarded as evidence of their having been formed beneath 

 the sea. Similar appearances occur v-hcn a strong wind drives 

 over the face of a sandy plain. 



It appears that two fluids of diff*erent specific gravity, the 

 lighter passing over the surface of the former, always concur in 

 the formation of ripple. It seems also, that the lines of ripple- 

 mark are at right angles to the direction of the current which 

 forms them. 



If a fluid-like air, pass over the surface of perfectly quiescent 

 water, in a plain absolutely parallel, it will have no effect ; but 

 if it impinge on the surface of that water with the sHghtest in- 

 clination, it will raise a small wave, which will be propagated by 

 undulations to great distances. If the direction of the wind is 

 very nearly parallel to the surface of the water, this first wave, 

 being raised above the general surface, will protect that part of 

 the water immediately beyond it from the full eflect of the wind, 

 which will therefore again impinge upon the water at a little dis- 

 tance ; and, this concurring widi the undulation, will tend to 

 produce another small Avave, and thus again, new waves will be 

 produced. But the under surface of the air itself will also as- 

 sume the form of waves ; and so, on the sHghtest deviation at 

 any one point fi"om absolute parallelism in the two fluids, their 

 whole surfaces will become covered with ripples. 



If one of the fluids be water, and the lower fluid be fine sand, 

 partially supported in water, these marks do not disappear when 

 the cause ceases to act, as they do when formed by air on the 

 s^irface of water. These are the marks we observe when the 

 tide has receded from a flat sandy shore. 



If, after the formation of ripple-marks at the bottom of a 

 shallow sea, some adjacent river, or some current, deposit upon 

 them the mud which it holds in suspension, then the former 



