﻿224 
  OLDHAM 
  : 
  GREAT 
  EARTHQUAKE 
  OF 
  1897. 
  

  

  while 
  it 
  was 
  noticeable 
  that 
  this 
  was 
  not 
  the 
  case. 
  As 
  a 
  rule, 
  only 
  

   certain 
  parts 
  showed 
  signs 
  of 
  crushing, 
  while 
  others 
  were 
  left 
  sharp- 
  

   edged 
  and 
  intact. 
  This 
  might, 
  of 
  course, 
  be 
  due 
  to 
  unequal 
  powers 
  

   of 
  resistance 
  of 
  different 
  parts 
  of 
  the 
  borders 
  of 
  the 
  plane 
  of 
  fracture 
  

   and 
  twisting, 
  but 
  is 
  more 
  probably 
  due 
  to 
  differences 
  in 
  the 
  pressure 
  

   brought 
  to 
  bear 
  on 
  them. 
  

  

  In 
  the 
  foregoing 
  passages 
  I 
  have, 
  for 
  simplicity's 
  sake, 
  assumed 
  

   a 
  circular 
  section 
  of 
  the 
  column 
  twisted, 
  but 
  the 
  explanation 
  is 
  

   equally 
  applicable 
  to 
  objects 
  of 
  square 
  or 
  angular 
  section. 
  In 
  this 
  

   case 
  the 
  first 
  tilt 
  would 
  necessarily 
  be 
  on 
  one 
  side 
  or 
  one 
  angle, 
  

   and 
  Gray's 
  principle 
  would 
  come 
  into 
  play, 
  but 
  all 
  succeeding 
  tilts 
  

   would 
  take 
  place 
  along 
  one 
  of 
  the 
  sides 
  of 
  the 
  lower 
  part 
  of 
  the 
  

   column, 
  or 
  at 
  one 
  of 
  the 
  points 
  of 
  intersection 
  of 
  the 
  sides 
  of 
  the 
  

   lower 
  and 
  upper 
  portion, 
  after 
  the 
  former 
  had 
  been 
  twisted 
  from 
  its 
  

   original 
  position. 
  In 
  this 
  way 
  may 
  be 
  explained 
  the 
  fact 
  that 
  

   some 
  of 
  the 
  corners 
  show 
  quite 
  clear 
  , 
  cut 
  and 
  uncrushed 
  fractures,, 
  

   while 
  on 
  the 
  sides 
  close 
  by 
  there 
  may 
  be 
  very 
  palpable 
  signs 
  

   of 
  the 
  effect 
  of 
  great 
  pressure. 
  

  

  It 
  may 
  be 
  noticed 
  that 
  the 
  explanation 
  which 
  has 
  been 
  proposed 
  

   is 
  but 
  an 
  amplification 
  of 
  Mallet's 
  second 
  explanation. 
  Instead 
  of 
  

   only 
  two 
  shocks 
  it 
  postulates 
  a 
  series 
  of 
  successive 
  shocks 
  in 
  suc- 
  

   cessively 
  varying 
  directions, 
  and 
  by 
  regarding 
  the 
  rotation 
  as 
  due, 
  

   not 
  to 
  a 
  single 
  twist 
  round 
  a 
  single 
  centre, 
  but 
  to 
  the 
  accumulated 
  

   effect 
  of 
  a 
  succession 
  of 
  small 
  twists 
  round 
  a 
  succession 
  of 
  different 
  

   centres, 
  it 
  allows 
  a 
  considerable 
  rotation 
  to 
  be 
  accompanied 
  by 
  a 
  

   much 
  smaller 
  shifting 
  of 
  the 
  centre 
  of 
  gravity 
  than 
  could 
  be 
  the 
  case 
  

   if 
  the 
  same 
  amount 
  of 
  rotation 
  was 
  due 
  to 
  a 
  single 
  effort 
  of 
  twisting. 
  

  

  If 
  we 
  modify 
  Gray's 
  explanation 
  in 
  so 
  far 
  as 
  to 
  regard 
  the 
  rota- 
  

   tion 
  as 
  due 
  not 
  to 
  a 
  single 
  tilt, 
  but 
  to 
  a 
  series 
  of 
  successive 
  tilts 
  on 
  

   opposite 
  sides 
  of 
  the 
  base, 
  we 
  might 
  get 
  a 
  large 
  rotation 
  accompanied 
  

   by 
  a 
  small 
  shifting 
  of 
  the 
  centre 
  of 
  gravity, 
  but 
  it 
  fails, 
  even 
  with 
  this 
  

   modification 
  to 
  explain 
  how 
  two 
  similarly 
  oriented 
  objects 
  close 
  

   together 
  may 
  be 
  rotated 
  in 
  opposite 
  directions. 
  We 
  hr.ve 
  two 
  cases 
  

   ( 
  224 
  ) 
  

  

  