(1)  Action  of  Gravity.  —  Gravity,  especially  when  aided  by
the  lifting  power  of  frost,  may  locally  exert  no  insignificant
influence.  The  tremendous  power  of  landslides,  or  avalanches,
have,  owing  to  their  devastating  effects,  been  impressed  upon
us  from  the  beginnings  of  written  history.  There  are,  how-
ever, other  results,  due  to  similar  causes,  but  which,  going  on
on  an  almost  microscopic  scale,  are  wholly  overlooked  by  the
ordinary  observer,  and  the  full  meaning  of  which  can  be  dis-
covered only  when  the  results  of  years  are  taken  into  account.
Professor  W.  C.  Kerr,  in  1881,  described1  the  manner  in  which
the  superficial  cap  of  soil  from  the  decomposition  of  micaceous

1  Am.  Jour,  of  Science,  3d  Series,  Vol.  XXI,  p.  345.
286


ACTION   OF   WATER   AND   ICE  287

and  hornblendic  gneisses  near  Philadelphia  had  crept  down
the  inclined  surface  on  which  it  rested,  and  the  gradual  attenu-
ation of  the  bands  of  variously  colored  debris  of  Avhich  it  was
composed.  This  creeping  process  he  ascribed  wholly  to  the
expansive  action  of  included  water  passing  into  the  condition
of  ice,  the  expansion  taking  place  laterally  and  the  material
being  pushed  down  the  slope  along  the  line  of  least  resistance.
Mr.  C.  Davidson  has  since  taken  up  the  subject  experimentally


FIG.  2-L  —  Showing  direction  and  rate  of  motion  of  soil ;  the  arrows  showing,  by
their  relative  lengths,  the  rate  of  movement  at  various  points,  a,  soil;  b,  bed-
rock.
and  shown  that  the  amount  of  the  creeping  could  be  accounted
for  by  the  ordinary  laws  of  gravity,  the  frost,  by  its  expansion,
raising  the  individual  particles  a  slight  distance,  and,  on  thaw-
ing, allowing  them  to  drop  back  again  a  greater  or  less  distance
down  the  slope,  according  to  the  angle  of  inclination.  Dr.
