210 N. S. SHALER — PHENOMENA OF BEACH AND DUNE-SANDS. 



they are attached from its place on the floor ; it is then quickly urged 

 by the surges to the beach. Attaining this position, it is at once stripped 

 of the plants which bore it ashore. 



On the beaches of eastern Massachusetts a mile of ocean front, in a 

 time of heavy storms, often receives in the course of a day from ten to 

 twenty tons of pebbly material, borne in by seaweeds. In some cases 

 the rate of the importation much "exceeds this amount. The pebbles 

 thus delivered to the shores are not infrequently from three to six inches 

 in diameter. Where pebbles do not abound on the bottom, as is the 

 case along most sandy shores, the shells of the larger moUusks are in the 

 same way uplifted and brought upon the beach. 



Floating Pumice. — A certain contribution of debris to the seabeaches, 

 the amount of which is not yet determined, arises from the stranding of 

 floating pumice. Observations which I have made on the shores from 

 Eastport to Key West show that every part of this coastline receives a 

 certain share of this volcanic matter. On the Florida shore the quantity 

 of the material appears to be much greater than elsewhere, this prob- 

 ably because of the strong and far journeying current which passes by 

 that promontory. On this part of the coast it appeared to be easy at 

 certain points on the strand to gather an identifiable bit of pumice in 

 each square yard. As we go to the northward, passing away from the 

 margin of the Gulf stream, these pumiceous fragments become much 

 more rare until on the New England coast it may require a careful search 

 to reveal a bit of the material. 



Owing to the frail nature of pumice, as Avell as to the chemical unsta- 

 bility of its composition, it appears rapidly to break up ; it is thus not 

 easy to determine what part of the beach-matter is from this source. 

 Inasmuch as these volcanic materials enter into a mass which, as we 

 have seen, is in a way protected from erosion, a small annual contribu- 

 tion may not be unimportant. At present my inquiries concerning the 

 distributions of pumice along the eastern coast of the United States are 

 in a way checked by the need of carefully discriminating between the 

 pumiceous material thrown out from volcanic vents and the similar 

 materials contributed from the boiler furnaces of steamships ; the dis- 

 crimination is possible, but it cannot in all cases be readily effected. 



DuNE-SANDS. 



OBJECT OF THEIR INVESTIGATION. 



The origin of dune-sands has been well determined, as has also the 

 general principles of their movement; there are, however, many details 



