BloTvn  Sand.  —  Wherever  a  sandy  soil  occurs  unprotected  by
vegetation,  as  in  deserts  or  along  the  seacoast,  the  wind  drifts  the

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190


TERRESTRIAL  DEPOSITS


sand  and  piles  it  up  into  hills  or  sand  dunes.  The  dunes  are
roughly  divided  into  layers,  the  thickness  and  inclination  of  which
depend  upon  the  force  and  direction  of  the  wind,  and  often  imi-
tate the  confused  arrangement  of  sands  piled  up  by  waves  and
currents  under  water.  The  sand-grains  of  the  dunes  are,  however,
more  rounded  by  the  abrasion  which  they  have  undergone  and,
especially  in  deserts,  they  are  apt  to  be  smaller.     When  the  sands


Fig.  86.— Sand  dune;  Beaufort  Harbor,  N.C,


are  mixed  with  pieces  of  shells  and  other  calcareous  material,  per-
colating waters,  by  dissolving  and  redepositing  the  CaCOg,  may
c6ment  the  sands  into  firm  rock.  This  is  the  more  conspicuous
when  the  whole  material  is  calcareous,  as  in  the  shell  sands  of
Bermuda.  This  substance,  ground  up  by  the  surf,  is  transported
inland  by  the  wind  and  piled  up  into  dunes.  Rain-water  cements
the  loose  grains  together,  and  by  the  alternate  accumulation  by


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SPRING  DEPOSITS


191


mnd  and  cementing  by  rain  is  formed  the  stratified  aeolian  or
drift-sand  rock.
Spring  Deposits.  —  As  our  knowledge  of  microscopic  plants
increases,  many  processes  which  were  believed  to  be  purely  chemi-
cal, are  found  to  be  dependent  upon  the  activity  of  minute  plants.
At  present,  it  is  not  possible  to  distinguish  accurately,  in  all  cases,
between  the  two  kinds  of  processes.
