548 STRATIFICATION OF DUNES, 



ble remains between successive deposits, and tliis I imagine to be 

 more frequent than has been generally supposed. Some geol- 

 ogists have thought that the sand strata of dunes are of annual 

 formation ; * but the autumnal deposit of foliage from neighbor- 

 ing trees and shrubs furnishes a more probable explanation of 

 the division of the sand-heaps into regular layers. 



A late distinguished American admiral communicated to me 

 an interesting observation made by him at San Francisco, which 

 has an important bearing on the arrangement of the particles of 

 sand in dunes and other irregular accumulations of that substance. 

 In laying out a navy-yard at that port, a large quantity of earthy 

 material was removed from the dunes and other hillocks and 

 carted to a low piece of ground which required filling up. Sand 

 of various characters, fine and coarse gravel, and common earth 

 were dropped promiscuously by the carts as accident or conveni- 

 ence dictated, and of course they were all confusedly intermixed. 

 Some time after, when the new ground was consolidated, various 

 excavations were made in it, and the different materials of which 

 the filling was composed were found to be stratified with con- 

 siderable regularity, according to their specific gravity. 



Two explanations of this remarkable fact suggest themselves 

 to me, which, however, do not perhaps exclude others. San 

 Francisco is subject to eai-thquakes, and though violent or even 

 sensible shocks are not very frequent, it is highly probable that, 

 as is shown to be the case in many other countries by late seis- 

 mological observations, there are, in the course of the year, a 

 great number of slight shocks which escape unscientific observa- 

 tion. A frequent repetition of slight trembhngs of the earth 

 would, like any other moderate mechanical agitation, probably 

 produce the separation of a miscellaneous mass, like that de- 

 scribed, into distinct layers. Again, the Pacific coast, like all 

 others upon an open sea, is exposed to incessant concussion from 

 the shock of the waves, which is repeated many thousand times 

 a day. This concussion is often sensibly felt by the observer, 

 and it seems not in the least improbable that the agitation may 

 have tended to produce a stratified arrangement in the case at 



* ScHOMAim, Oeologische Wanderungen durch die Preusmehen Ost-Bee Pro- 

 virnen, 1869, p. 81. 



