ESTTEEIOR STRUCTURE OF DUNES. 647 



Interior Structure of Dunes. 



The interior structure of tlie duues, the arrangement of their 

 particks, is not, as might be expected, that of an unorganized, 

 confused heap, but they show a strong tendency to stratification. 

 This is a point of much geological interest, because it indicates 

 that sandstone may owe its stratified character to the action of 

 other forces as well as of water. The origin and peculiar charac- 

 ter of these layers are due to a variety of causes. 



For example, a southwest wind and current may deposit upon 

 a dime a stratum of a given color and mineral composition, and 

 this may be succeeded by a northwest wind and current, bringing 

 with them particles of a different hue, constitution and origin. 

 Again, if we suppose a violent tempest to strew the beach with 

 sand-grains very different in magnitude and specific gravity, and, 

 after the sand is dry, to be succeeded by a gentle breeze, it is 

 evident that only the lighter particles will be taken up and car- 

 ried to the dunes. If, after some time, the wind freshens, heav- 

 ier grains will be transported and deposited on the former, and a 

 still stronger succeeding gale will roll up yet larger kernels. 

 Each of these deposits will form a stratum. If we suppose the 

 tempest to be followed, after the sand is dry, not by a gentle 

 breeze, but by a wind powerful enough to lift at the same time 

 particles of very various magnitudes and weights, the heaviest 

 will often lodge on the dune whUe the lighter will be carried 

 farther. This would produce a stratum of coarse sand, and the 

 same effect might result from the blowing away of hght particles 

 out of a mixed layer, while the heavier remained imdisturbed.* 

 Still another cause of apparent stratification may be found in the 

 occasional interposition of a thin layer of leaves or other vegeta- 



* The lower strata must be older than the superficial layers, and the parti- 

 cles which compose them may in time become more disintegrated, and there- 

 fore finer than those deposited later and above them. 



Hull ingeniously suggests that, besides other changes, fine sand intermixed 

 ■with or deposited above a coarser stratum, as well as the minute particles re- 

 sulting from the disintegration of the grains of the latter, may be carried by 

 rain in the case of dunes, or by the ordinary action of sea-water in that of 

 sand-banks, down through the interstices in the coarser layer, and thus the 

 relative position of sand and gravel may be changed. — Oorsprong der HoUand' 

 8che Duinen, p. 103. 



