ON THE SMALLEST DELTA 345 



negative compared — The former shows the white clouds, 

 the latter the blue sky — It is the latter which has the form 

 of ripple-mark in sand — The clouds correspond, not to 

 ripple-mark but to the eddies of water which make the 

 ripple- mark. 



On the Transverse, and Longitudinal Ridges formed 

 in the Flow of Watery Sand, and on the 

 Smallest Delta. 



The great variety in the position, shape, and super- 

 ficial inequalities of submerged sandbanks is due 

 to the circumstance that sand can be carried in 

 suspension, but that it does not follow the line of 

 flow of the water. Wherever the current is checked 

 or deflected the sand tends to collect in a heap. 



In their lower course sandy streams often dry 

 up owing to the escape of water by percolation. 

 On the large scale and with gentle slopes this 

 process forms deltas. On the small scale, with 

 streamlets on steep slopes, the deltaic deposits have 

 curious ridges which are somewhat analogous to 

 the transverse ripples and longitudinal ridges 

 formed on the beds of sandy streams. The ridges 

 of these deltillcB, or miniature deltas, are due in 

 the first stage to the quick subsidence of sand in 

 water, but in the final stage to the rapid subsi- 

 dence of water through sand. These forms are 

 produced very imperfectly or not at all in mud- 



