﻿THE 
  LANDSLIPS. 
  113 
  

  

  sufficiently 
  violent 
  earthquake 
  the 
  other 
  factor 
  that 
  comes 
  into 
  play 
  is 
  

   the 
  natural 
  tendency 
  of 
  the 
  hill 
  to 
  slip. 
  This 
  obviously 
  varies 
  with 
  

   the 
  slope, 
  a 
  gentle 
  slope 
  being 
  much 
  less 
  liable 
  to 
  slip 
  than 
  a 
  steep 
  

   one 
  and 
  the 
  nearer 
  the 
  slope 
  reaches 
  that 
  critical 
  angle, 
  at 
  which 
  the 
  

   soilcap 
  would 
  slip 
  away 
  of 
  its 
  own 
  accord, 
  the 
  less 
  the 
  impulse 
  required 
  

   to 
  set 
  it 
  in 
  motion. 
  

  

  There 
  is, 
  however, 
  another 
  factor 
  which 
  appears 
  to 
  be 
  of 
  almost 
  

   equal 
  importance 
  with 
  the 
  angle 
  of 
  the 
  slope, 
  and 
  that 
  is 
  its 
  height 
  

   from 
  base 
  to 
  crest. 
  While 
  travelling 
  in 
  the 
  Assam 
  hills 
  during 
  the 
  

   cold 
  weather 
  of 
  1897-98, 
  I 
  had 
  many 
  opportunities 
  of 
  observing 
  how 
  

   steep 
  slopes 
  and 
  scarps 
  of 
  a 
  low 
  height, 
  had 
  remained 
  uninjured, 
  

   while 
  gentle 
  slopes, 
  when 
  forming 
  part 
  of 
  a 
  hill 
  rising 
  to 
  several 
  

   hundred 
  feet 
  had 
  been 
  scarred 
  with 
  landslips. 
  

  

  This 
  is 
  well 
  seen 
  on 
  the 
  road 
  from 
  Shillong 
  to 
  Gauhati 
  ; 
  for 
  the 
  

   first 
  14 
  or 
  15 
  miles, 
  where 
  the 
  hills 
  are 
  high 
  and 
  cut 
  by 
  deep 
  valleys, 
  

   landslips 
  are 
  common, 
  but 
  for 
  the 
  rest 
  of 
  the 
  way, 
  where 
  the 
  height 
  of 
  

   the 
  hills 
  above 
  the 
  valleys 
  is 
  much 
  smaller, 
  landslips 
  are 
  almost 
  un- 
  

   known 
  though 
  the 
  earthquake 
  was 
  on 
  the 
  average 
  equally 
  severe 
  

   over 
  the 
  whole 
  of 
  this 
  tract. 
  

  

  The 
  explanation 
  of 
  this 
  connection 
  between 
  the 
  height 
  of 
  the 
  

   hills 
  and 
  the 
  degree 
  to 
  which 
  they 
  have 
  been 
  scarred 
  by 
  land- 
  

   slips 
  is 
  doubtless 
  the 
  greater 
  swing 
  which 
  was 
  imparted 
  to 
  the 
  

   higher 
  hills. 
  In 
  part, 
  this 
  is 
  due 
  to 
  the 
  fact 
  that 
  an 
  equal 
  angular 
  

   motion 
  would 
  result 
  in 
  a 
  greater 
  linear 
  movement 
  at 
  the 
  top 
  of 
  a 
  

   high 
  hill 
  than 
  of 
  a 
  low 
  one, 
  but 
  mainly 
  it 
  is 
  due 
  to 
  the 
  greater 
  

   elastic 
  play 
  of 
  the 
  high 
  hill, 
  especially 
  when 
  steep 
  sided 
  ; 
  just 
  as 
  the 
  

   end 
  of 
  a 
  long 
  switch 
  jerked 
  to 
  and 
  fro 
  describes 
  a 
  larger 
  are 
  than 
  

   that 
  of 
  a 
  short 
  one. 
  

  

  There 
  is 
  yet 
  another 
  factor 
  in 
  the 
  production 
  of 
  landslips, 
  and 
  

   that 
  is 
  the 
  mineral 
  constitution 
  of 
  the 
  hill. 
  When 
  this 
  is 
  composed 
  

   of 
  crystalline 
  rock, 
  the 
  surface 
  layer 
  of 
  disintegrated 
  and 
  weathered 
  

   material 
  is 
  either 
  thin, 
  or 
  it 
  passes 
  down 
  into 
  the 
  unweathered 
  

   rock 
  in 
  a 
  gradual 
  manner. 
  In 
  the 
  former 
  case, 
  the 
  thin 
  skin 
  may 
  

   ' 
  ( 
  "3 
  ) 
  

  

  