312 ANNUM. REPOET SMITHSONIAN INSTITUTION, 1913. 



left toward the two extremities, where, with the aid of the last waves, 

 it divides into a double stream creating, in consequence, two small 

 currents running in opposite directions parallel to the shore. 



One learns from this the manner in which is formed, at this point, 

 the deposit of sand grains carried by these currents running at 

 right angles with the current of the tide, which brings them from 

 the upper beach. Meeting in the pool water that possesses little 

 force they lose their speed and are deposited. But, while they are 

 falling to the bottom, the transverse current intervenes, turns them 

 aside and arranges parallel with the shore this unstable, changeable 

 mass, which is in course of formation rather than formed, and con- 

 sists of these sandy particles, for the most part still suspended in 

 the water. 



Thus appear the parallel lines of ripple marks which reveal by 

 their direction that of the secondary currents, and by their undula- 

 tions the serpentine windings which are impressed on them by the 

 vertical oscillations of the waves. Though very distinct where the 

 divergence of the two lines of currents from an angle of 90°. 

 the ripple marks lose little by little their regularity at a distance, 

 and in the same measure as the forces in operation become weakened. 

 They end by disappearing altogether in the drain where the trans- 

 verse current, definitely deflected toward the sea, loses itself in the 

 current of the tide. 



Thus is this phenomenon very simply explained. In every part 

 of the lower beach where a current runs more or less nearly at right 

 angles with the normal current of the ebb, their combined effects 

 inscribe themselves in ripple marks on the sand. 



We arrived at this conclusion on noting the appearance which 

 the ripple marks present when they have acquired their definite 

 form. More convincing than any argimient is the reproduction here 

 given of a photograph showing these currents, surprised while at 

 work and depicted in the immobility of a hundredth of a second at 

 the moment when, under their double action, little lines of shadow 

 stamp upon the smooth surface of the sands the first outline of 

 ripple marks. I sought for a long time to obtain this illustration, 

 and though a difficult task, it was finally secured. 



Let us suppose that at the moment of breaking on the shore, a 

 wave happens to strike a rock presenting a surface at an angle of 

 about 45°. The wave, carried along by its own energy, washes 

 this surface, and then falling down and turned aside by it, retreats 

 to join the sea, in an elliptical course (fig. 10). 



The sandy particles which it carries, as in the pool previously 

 mentioned, are urged forward in two directions, " OA" and " OB," 

 the, one perpendicular to and the other parallel with the shore. 

 Eipple marks should be formed there. This is confirmed by the 



