W.  S.  Tangier-Smith.     A  Geological  Sketch  of  San  (Z!lemente  IaUiA|

18th  Ann.  Rept.  U.  S.  Geol.  Surv.,  Pt.  ii,  1898,  pp.  459-496,  pis.
R.  S.  Tarr  and  L.  Martin.     Recent  Changes  of  Level  in  the  Takat

Bay  Region,  Alaska,  Bull.  Qeol.  Soc.  Am.,  vol.  17,  1906,  pp.
pis.  12-23.
CHAPTER  XX

THE    GLACIERS    OF    MOUHTAIM    AND    CONTINENT

londitionB  essential  to  glaciation,  —  Wherever  for  a  suffi-
itly  protracted  period  the  annual  snowfall  of  a  district  is  in
esB  of  the  snow  that  is  melted,  a  residue  must  remain  from
:h  season  to  be  added  to  that  of  succeeding  ones.  Eventually
much  snow  will  have  accumulated  that  under  its  own  weight
in  obedience  to  its  peculiar  properties,  a  movement  will  begin
khin  the  mass  tending  to  spread  it  and  so  to  reduce  the  slope
its  upper  surface  {Frontispiece  plate).  The  conditions  favorable
ion  are,  therefore,  heavy  precipitation  and  low  annual
If  the  precipitation  is  scanty,  the  small  snowfall
Ited ;  and  if  the  temperature  be  too  high,  the  moisture
:ated  not  in  the  form  of  snow  but  as  rain.  It  is  impor-
here  to  keep  in  mind  that  snow  is  a  poor  heat  conductor
itself  protects  its  deeper  layers  from  melting.
