208 N. S. SHALER PHENOMENA OF BEACH AND DUNE-SANDS. 



RESISTANCE TO ABRASION. 



One of the most noticeable features which is exhibited by beach 

 sands is their extraordinary endurance of the beating of the waves. On 

 examining any pebbly beach exposed to the ocean-surges we readily per- 

 ceive that the masses of stone wear at a very rapid rate. Thus, at cape 

 Ann, cubes of granite of a kind which forms excellent blocks for city 

 pavements are, when exposed to the surf, worn in the course of a year to 

 spheroidal forms, with an average loss of more than an inch from their 

 peripheries. Experiments with fragments of hard burned brick have 

 shown me that in a year of moderate beach-wearing they may be re- 

 duced by the abrasion to half their original size. On the other hand the 

 sand derived from these pebble-beaches endures for an unlimited time, 

 evidently with little wearing. Though subjected for ages to the beating 

 of the waves, with perhaps a hundred times as much energy applied to 

 the surface of which it forms a part as would suffice to reduce a granite- 

 bowlder containing a cubic foot of material to a granular or powdery 

 state, the beach sands remain unworn. 



An excellent example going to show the endurance of sand-grains on 

 the shore is afforded by the beaches of the Atlantic coast from New York 

 southward to near cape Florida. Collections along this line show that 

 the waste from the northern part of the coast is' slowly journeying south- 

 ward, partly along the beach-strip and partly in the shallow water at a 

 little distance from the shores, yet when these sands arrive on the 

 southern coast of Florida, though their quartz grains are somewhat 

 rounded, they are not much smaller than those on the region of the 

 coast about cape Hatteras. 



CAUSE OF THEIR ENDURANCE. 



On examining the conditions of the sand on a wet beach we find the 

 reason for the slight amount of wearing to which the grains are subjected 

 from the action of the waves. Owing to the small size of the fragments 

 and to the fact that they are generally provided with angular faces a film 

 of water is held by capillarity between the adjacent bits so that they, so 

 long as the beach is full of water, do not touch each other. Thus the 

 blow of the waves is used up in compressing the interstitial water and 

 is converted into heat without wearing the mineral matter in an appre- 

 ciable degree. A simple experiment will illustrate the extent to which 

 the water is held between the wet grains. By pressing the foot on the 

 surface of a flat sand beach just above the waterline we may observe that 

 the sand usually whitens around the field of pressure, the change in hue 



