﻿Vol. 
  53.] 
  Or 
  AUGUST 
  1892 
  AND 
  NOVEMBER 
  1 
  893. 
  173 
  

  

  Geological 
  Survey 
  map. 
  In 
  the 
  map 
  of 
  the 
  epicentral 
  area 
  (p. 
  158), 
  

   the 
  principal 
  faults 
  are 
  indicated 
  by 
  broken 
  lines. 
  They 
  are 
  founded 
  

   on 
  the 
  Index-map 
  of 
  the 
  Geological 
  Survey 
  (on 
  the 
  scale 
  of 
  4 
  miles 
  

   to 
  1 
  inch), 
  on 
  the 
  map 
  which 
  accompanies 
  the 
  paper 
  of 
  Messrs. 
  Marr 
  

   and 
  Roberts 
  on 
  ' 
  The 
  Lower 
  Palseozoic 
  Rocks 
  of 
  the 
  Neighbourhood 
  

   of 
  Haverfordwest,' 
  1 
  and 
  on 
  information 
  kindly 
  given 
  to 
  me 
  by 
  

   Mr. 
  Marr. 
  The 
  two 
  faults, 
  whose 
  positions 
  are 
  inferred 
  from 
  the 
  

   seismic 
  phenomena, 
  are 
  shown 
  by 
  dotted 
  lines 
  cm, 
  j3)3. 
  So 
  far 
  as 
  I 
  

   am 
  aware, 
  no 
  geological 
  evidence 
  of 
  their 
  existence 
  has 
  been 
  dis- 
  

   covered, 
  but 
  in 
  this 
  part 
  of 
  the 
  district 
  the 
  exposures 
  are, 
  I 
  believe, 
  

   insufficient 
  to 
  give 
  much 
  force 
  to 
  negative 
  evidence. 
  

  

  Earthquakes 
  of 
  1892. 
  — 
  Several 
  of 
  the 
  earthquakes 
  of 
  1892, 
  

   including 
  the 
  principal 
  earthquake, 
  are 
  apparently 
  connected 
  with 
  

   a 
  transverse 
  fault 
  cm, 
  hading 
  to 
  the 
  west. 
  There 
  is 
  not 
  sufficient 
  

   evidence 
  to 
  determine 
  whether 
  this 
  was 
  the 
  case 
  with 
  the 
  first 
  

   shock 
  (a), 
  but, 
  assuming 
  it 
  to 
  be 
  so, 
  there 
  must 
  have 
  been 
  a 
  slight 
  

   slip 
  a 
  short 
  distance 
  north 
  of 
  Pembroke. 
  This 
  was 
  succeeded 
  in 
  

   less 
  than 
  an 
  hour 
  by 
  another 
  slip 
  farther 
  south, 
  the 
  centre 
  of 
  the 
  

   focus 
  being 
  close 
  to 
  the 
  coast. 
  The 
  focus 
  may 
  have 
  extended 
  under 
  

   both 
  land 
  and 
  sea, 
  but 
  the 
  amount 
  of 
  slip 
  must 
  have 
  been 
  very 
  

   small, 
  for 
  no 
  tremor 
  was 
  perceptible 
  on 
  the 
  surface. 
  

  

  The 
  function 
  of 
  these 
  preliminary 
  slips 
  seems 
  to 
  have 
  been 
  to 
  

   relieve 
  the 
  stress 
  at 
  two 
  isolated 
  spots, 
  and 
  thus 
  to 
  equalize 
  the 
  

   residual 
  stress 
  over 
  a 
  great 
  length 
  of 
  the 
  fault. 
  The 
  total 
  relief 
  

   was, 
  however, 
  very 
  slight, 
  for, 
  2 
  minutes 
  after 
  the 
  second 
  slip, 
  

   the 
  most 
  important 
  movement 
  took 
  place. 
  Owing 
  to 
  the 
  incom- 
  

   pleteness 
  of 
  the 
  isoseismal 
  7, 
  no 
  estimate 
  can 
  be 
  formed 
  of 
  the 
  

   length 
  of 
  the 
  focus, 
  but 
  it 
  was 
  certainly 
  many 
  miles. 
  Its 
  centre 
  

   coincided 
  approximately 
  with 
  that 
  of 
  the 
  earth-sound 
  (6), 
  and 
  the 
  

   focus 
  itself 
  lay 
  partly 
  under 
  land 
  and 
  partly 
  under 
  sea. 
  The 
  

   double 
  nature 
  of 
  the 
  shock, 
  and 
  the 
  greater 
  intensity 
  of 
  the 
  

   second 
  series 
  of 
  vibrations 
  in 
  practically 
  all 
  parts 
  of 
  the 
  disturbed 
  

   area, 
  show 
  that 
  the 
  slip 
  was 
  interrupted, 
  the 
  sliding 
  mass 
  almost 
  

   halting 
  for 
  a 
  few 
  seconds 
  ; 
  but 
  whether 
  the 
  second 
  and 
  more 
  

   important 
  part 
  of 
  the 
  slip 
  occurred 
  in 
  the 
  same, 
  or 
  a 
  different, 
  region 
  

   of 
  the 
  fault, 
  there 
  is 
  no 
  evidence 
  to 
  show. 
  The 
  latter 
  alternative 
  

   seems, 
  I 
  think, 
  the 
  more 
  probable, 
  when 
  we 
  consider 
  the 
  dimensions 
  

   of 
  the 
  displaced 
  rock-mass. 
  

  

  However 
  great 
  the 
  amount 
  of 
  slip 
  may 
  have 
  been 
  near 
  the 
  centre 
  

   of 
  the 
  focus, 
  it 
  must 
  have 
  died 
  out 
  before 
  reaching 
  the 
  surface, 
  for 
  

   no 
  trace 
  of 
  a 
  fault-scarp 
  is 
  recorded 
  within 
  the 
  epicentral 
  tract. 
  

   The 
  water-waves 
  observed 
  at 
  Bulwell 
  and 
  near 
  Langwm 
  can 
  hardly 
  

   have 
  been 
  of 
  the 
  nature 
  of 
  a 
  ' 
  great 
  sea-wave/ 
  such 
  as 
  would 
  be 
  

   produced 
  by 
  a 
  sudden 
  elevation 
  or 
  subsidence 
  of 
  the 
  sea-bed, 
  but 
  

   rather 
  what 
  Mallet 
  would 
  have 
  called 
  ' 
  forced 
  sea- 
  waves.' 
  They 
  

   were 
  no 
  doubt 
  due 
  to 
  the 
  wave 
  of 
  direct 
  vibrations 
  transmitted 
  

   .through 
  the 
  water. 
  

  

  '& 
  1 
  

  

  1 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xli. 
  (1885) 
  pp. 
  476-491. 
  

  

  