166 Mr. De la Beche's Notes on the Formation 



From what has been above stated, it will be seen that the 

 sea endeavours to throw back upon the land the detritus it has 

 received from it, and even, as in the case of corals and shells, 

 of hard substances that have been formed in it, and that the 

 pebbles or shingles are not likely to quit the coast under 

 ordinary circumstances * ; indeed the common velocity of 

 tides seems inadequate to transport them in moderate depths, 

 where the power of the waves on the surface of the sea 

 ceases. 



II. Action of Tideless Seas on their Coasts, 



The principal difference between these and those above 

 noticed, consists in the phsenomena attendant on the discharge 

 of rivers into them, which will be noticed under the head of 

 rivers. Shingle beaches are accumulated, and protect lands 

 behind them, but from the want of tide we do not see their 

 bases, and they appear of inferior dimensions to those on tidal 

 seas. From the want of tide, which should successively pre- 

 sent different portions of a cliff to the greatest action of the 

 breakers, the destruction of coasts is not so great, and the spaces 

 of open sea being more or less limited, the battering power of 

 the breakers is greatly inferior to that of the great ocean swell, 

 discharged on a tidal coast. Still the same rejection of detritus 

 derived from the land will be observed when it does not fall 

 into deep water, beyond the reach of the moving power of 

 such seas; and we know of no current sufficiently strong in tide- 

 less seas to distribute the gravel that has been thrown into 

 their deep waters. 



Large lakes present nearly the same phsenomena as to shin- 

 gle beaches as tideless seas ; and as most of them are lower at 

 one time than another, we may observe the shingle beaches 

 better ; and it is by no means uncommon to see a skirting of 

 shingle round them when their waters are low. 



III. Action of Rivers on their Beds. 

 Rivers most frequently, though not always, take their rise 

 among hills and mountains, and are supplied either by the 

 melting of snows or glaciers, the draining of rain waters, or 

 by springs. The two former particularly bring down frag- 

 ments formed by decomposition from the neighbouring rocks, 

 into the bed of the river. In mountainous regions fragments 

 of rocks of greater or less dimensions fall into the river from 

 the mountain sides. The river also undermines its banks, 

 and the loose decomposed surface of the rocks tumbles into it. 



* Even in the case of sands, which do not enter within the scope of this 

 memoir, there is a tendency in the sea to throw them upon the land. Wit- 

 ness the sandy Dunes, so common on various coasts. 



From 



