170 On Ripple-MarJcs. 



marks will be preserved, and new ripple-marks may appear 

 above them. Such is the origin of those marks we observe in 

 various sandstones, from the most recent down to those of the 

 coal-measures. 



Dr Fitton informs rae, that he found the sand-hills on the 

 south of Etaples (in France), consisting of i-ipple-marks on a 

 large scale. They are crescent-shaped hillocks, many of which 

 are more than a hundred feet high. The height is greatest in 

 the middle of the crescents, declining towards the points ; and 

 the slope on the inner side of the crescent, which is remote from 

 the prevailing direction of the winds, is much more rapid than 

 that on which it strikes. 



Mr Lyell has observed and described this mode of formation 

 of ripple on the dunes of sand near Calais ; remarking, that in 

 that case there is an actual lateral transfer — the grains of sand 

 being carried by the wind up the less inclined slope of the rip- 

 ple, and falling over the steep scarp. I have observed the same 

 fact at Swansea. 



A similar explanation seems to present itself as the origin of 

 that form of clouds familiarly known as " a mackarel sky" — a 

 wave-like appearance, which probably arises from the passage of 

 a current of air above or below a thin stratum of cloudsi The air 

 being of nearly the same specific gravity as that of the cloud it 

 acts upon, would pi'oduce ripples of larger size than would 

 otherwise occur. 



The surface of the sun presents to very good telescopes a 

 certain mottled appearance, which is not exactly ripple, and which 

 it is difficult to convey by description. It may, however, be sug- 

 gested, that wherever such appearances occur, whether in plane- 

 tary or in stellar bodies, or in the minuter precincts of the dye- 

 house and the engine-boiler, they indicate the fitness of an in- 

 quiry, whether there are not two currents of fluid or semi-fluid 

 matter, one moving with a different velocity over the other, the 

 direction of the motion being at right angles to the lines of 

 waves. — Bfibbage's Bridgewater Treatise, p. 222. 



