PROTECTORS FROM COXTIXEXTAL DESTRUCTION. 209 



being due to a partial expulsion of the water ; on withdrawing the weight 

 the sand resumes its original color. 



Those who will observe the condition of the water along a pebble- 

 beach in times of heavy rains may readily note the fact that it contains 

 a considerable amount of mud derived from the grinding action of the 

 stones as they are drawn over each other by the surf. A similar obser- 

 vation made on a normal sandbeach will show that the fragments yield 

 no waste. 



PROTECTORS FROM COXTINENTAL DESTRUCTION. 



Important geologic consequences arise from this peculiar feature in 

 the action of sand on the seashores. To it, in the main, is due the very 

 effective protection which sandbeaches afford the land areas against the 

 assaults of the waves. Probably more than four-fifths of the shores of 

 the continents which face the open sea are thus protected from the 

 surges by finely divided rock-material. If the agents of wear could 

 deal with the masses of these tiny fragments as easily as they do with 

 rock-cliffs, the history of our continents would have been quite other 

 than that which we trace. The shores, especially those composed of 

 friable materials, would have been easily driven back into the land. As 

 it is, the waves, not being aljle to grind up the sands, have to deposit 

 them in deep wat.'r or in em])ayments of the shore before they can con- 

 tinue to erode the cliffs which yield the detritus. 



Those who have examined the condition of small islands may have 

 remarked the fact that sandbeaches are rarely found along their shores, 

 the reason being that the limited field of erosion is not likely to afford a 

 sufficient supply of the material to make considerable accumulations of 

 that nature. It is partly, at least, in consequence of this lack of sand- 

 barriers that small islands are generally in a process of relatively rapid 

 shore-erosion, the rate of this destruction being in most cases evidently 

 greater than it is on the mainlands. 



SOME SOURCES OF SUPPLY. 



Seaweeds. — As the supply of sand on the shores is a matter of much con- 

 sequence in determining the effectiveness of the wave-action against the 

 land, I venture to note, in passing, two ways in which the deposits of the 

 shore line are augmented. The first of these is effected by a peculiar 

 action of our larger seaweeds. These plants have the habit of attaching 

 themselves to a pebble or shell, it may be, in water so deep that the 

 waves can have no scouring effect on the bottom. As they grow, these 

 plants gradually expose so much surface to the waves and are so upborne 

 by their air vesicles that in the end they often pull the body to which 



