226 PRINCIPLES OF STRATIGRAPHY 



ened with complete destruction by the great storm flood of Christ- 

 mas, 1894, and is at present protected from the waves by artificial 

 constructions. (Fig. 35.) 



Rounding and Sorting of Detritus by Wave Action. Wave work 

 on sand grains is limited to those of larger size. Shaler (63) has 

 called attention to the fact that between the smaller grains of sand 

 on the beach a cushion of water exists, due to capillary attraction, 

 and that to this the wet beach sand owes its firmness. It is this 

 cushion of water which prevents the sand grains from rubbing 

 against each other, and thus the finer grains remain angular. Ex- 

 periments by Daubree {16: 2j6) showed that granules o.i mm. in 

 diameter will float in feebly agitated water, and that hence grains 

 of this size and less could not be mechanically rounded by water. 

 Destructible material, such as feldspar grains, is slowly eliminated 

 in wave-churned sands. On the shores of eastern Moray the sands 

 contained only 10 per cent, of feldspar, whereas, at the river mouths 

 which furnished the sands, 18 per cent, of feldspar grains is still 

 found. The rounding, purity and assortment according to size is 

 never as great as in the case of wind-blown sand, but probably in 

 most cases better than river sands. (See the tables and discussion 

 on p. 256.) 



Pure sands, i. e., sands consisting of one mineral — generally 

 quartz — are sometimes found on the seashore, though, as a rule, 

 other minerals are present. An unusually pure beach sand is found 

 at West Palm Beach, on the Atlantic coast of Florida, nearly every- 

 thing but quartz being eliminated. The grains are, however, 

 mostly subangular, though the material has been transported for 

 many miles along shore from the Piedmont region to the north. 

 Shaler (63) calls attention to the fact that these sands, though some- 

 what rounded, are not much smaller than those in the region of 

 that coast about Cape Hatteras, whence they come. 



Large rock fragments, on the other hand, are rapidly rounded 

 by the waves. At Cape Ann, cubes of granites of a kind which 

 forms excellent blocks for paving city streets are worn by the surf 

 in the course of a year to spheroidal forms, with an average loss of 

 more than an inch from their peripheries (Shaler-63 : ^o<?), while 

 fragments of hard-burned bricks are reduced to half their size by 

 a year of moderate beach-wearing. 



Tides. 



Tides are the periodic rise and fall of the ocean waters, due to 

 combined attraction of sun and moon, and occur twice in every 



