60 THE WONDERS OF GEOLOGY. Lect. I. 



months, when the currents are feeble, and the rivers 

 shallow. In that part of the stream affected by the tides, 

 there is also a constant flux and reflux of the waters, and 

 from this cause the sediments must, to a certain degree, be 

 periodical. Accordingly we find the silt and sand disposed 

 in strata or layers, from the partial consolidation of the 

 surface of one bed of mud, before the superincumbent 

 layer was precipitated upon it. Thus wherever a fresh 

 break takes place in a bank of consolidated silt, in a delta, 

 as for example in that of the Nile, it is easy to trace the 

 deposits of each successive year, by means of the lighter 

 earth on the top of each. When a portion is taken into 

 the hand, it separates into layers ; and on closely examining 

 the edges of these, very delicate thin lines are perceptible, 

 showing a laminated structure, like those observable in the 

 coal-shales. Judging from these layers, the annual deposits 

 from the Nile appear to vary considerably, but the average 

 thickness is little more than a quarter of an inch.* 



Where a river terminates in an extensive estuary, the 

 sea throws over the layer of mud brought down by the 

 river, a covering of sand ; and frequently these alternate 

 with the greatest regularity, the receding of the tide 

 allowing the fresh water to deposit its mud, and the ad- 

 vance of the sea discharging sand and marine exuvise over 

 the surface. 



23. Ripple-marks. — And here we may notice another 

 phenomenon. Every one must have observed, when walking 

 by the banks of a river at low water, or on the sands of 

 the sea-shore, that when the water has been agitated by 

 the wind, the surface of the mud, or sand, is undulated, or 

 furrowed over by the rippling of the waves, the marks 

 presenting various appearances, according to the force and 



* Letter to Professor Silliman from an American who visited Egypt 

 in 1834. 



