﻿Vol. 
  53.] 
  ANNIVERSARY 
  ADDRESS 
  OF 
  THE 
  PRESIDENT. 
  lxxi 
  

  

  The 
  pre 
  -Cambrian 
  Era. 
  

  

  Since, 
  therefore, 
  the 
  rocks 
  of 
  pre-Cambrian 
  age 
  which 
  are 
  exposed 
  

   to 
  view 
  are 
  either 
  of 
  igneous 
  origin 
  or 
  so 
  much 
  changed 
  that 
  their 
  

   origin 
  is 
  doubtful, 
  it 
  seemed 
  to 
  me 
  that 
  valuable 
  evidence 
  bearing 
  on 
  

   the 
  pre-Cambrian 
  era 
  might 
  be 
  obtained 
  by 
  tabulating 
  with 
  care 
  

   all 
  the 
  fragments 
  of 
  rocks 
  which 
  had 
  been 
  found 
  in 
  the 
  Cambrian 
  

   conglomerates 
  in 
  the 
  several 
  areas 
  ; 
  and 
  in 
  the 
  year 
  1890 
  I 
  read 
  a 
  

   paper 
  at 
  the 
  meeting 
  of 
  the 
  British 
  Association, 
  giving 
  the 
  results 
  

   which 
  I 
  had 
  obtained 
  up 
  to 
  that 
  time. 
  In 
  each 
  area 
  it 
  was 
  found 
  

   that 
  fragments 
  derived 
  from 
  igneous 
  rocks 
  occurred 
  in 
  considerable 
  

   abundance, 
  and 
  it 
  was 
  clear 
  that 
  intrusive 
  rocks 
  of 
  acid 
  and 
  

   basic 
  character, 
  lavas, 
  and 
  volcanic 
  tuffs 
  were 
  near 
  at 
  hand 
  when 
  the 
  

   Cambrian 
  conglomerates 
  were 
  deposited. 
  Quartzites 
  and 
  grits, 
  such 
  

   as 
  would 
  be 
  derived 
  from 
  the 
  destruction 
  of 
  granitic 
  rocks, 
  were 
  also 
  

   found 
  to 
  be 
  abundant 
  ; 
  but 
  the 
  hornstones 
  and 
  porcellanites, 
  which 
  

   are 
  mainly 
  made 
  up 
  of 
  fine 
  volcanic 
  dust, 
  occur 
  very 
  frequently 
  in 
  

   some 
  areas, 
  while 
  clay- 
  slates 
  and 
  calcareous 
  fragments 
  were 
  com- 
  

   paratively 
  rare 
  — 
  showing, 
  as 
  I 
  have 
  already 
  stated, 
  that 
  in 
  these 
  

   pre-Cambrian 
  areas 
  we 
  can 
  hardly 
  expect 
  to 
  meet 
  with 
  rocks 
  which 
  

   could 
  have 
  been 
  deposited 
  under 
  conditions 
  favourable 
  to 
  life. 
  

  

  It 
  may 
  now 
  be 
  useful 
  to 
  give 
  short 
  descriptions 
  of 
  the 
  pre- 
  

   Cambrian 
  rocks 
  found 
  in 
  the 
  different 
  areas 
  in 
  Britain, 
  with 
  lists 
  of 
  

   the 
  fragments 
  which 
  have 
  been 
  obtained 
  from 
  the 
  Cambrian 
  con- 
  

   glomerates 
  which 
  repose 
  upon 
  them, 
  or 
  which 
  are 
  found 
  in 
  their 
  

   immediate 
  neighbourhood 
  ; 
  for, 
  in 
  addition 
  to 
  the 
  testimony 
  which 
  

   they 
  furnish 
  regarding 
  the 
  nature 
  of 
  the 
  rocks 
  of 
  the 
  pre-Cambrian 
  

   era, 
  it 
  will 
  be 
  seen 
  in 
  a 
  very 
  marked 
  manner 
  how 
  little 
  change 
  

   has 
  taken 
  place 
  in 
  them 
  since 
  the 
  commencement 
  of 
  Cambrian 
  

   time, 
  when 
  the 
  fragments 
  were 
  broken 
  off. 
  

  

  Pembrokeshire. 
  

  

  I 
  have 
  already 
  referred 
  to 
  the 
  fact 
  that 
  it 
  was 
  in 
  this 
  county 
  that 
  

   we 
  first 
  obtained 
  clear 
  evidence 
  of 
  pre-Cambrian 
  volcanic 
  rocks, 
  

   and 
  it 
  is 
  here 
  perhaps 
  that 
  most 
  care 
  has 
  been 
  taken 
  in 
  defining 
  

   the 
  fragments 
  obtained 
  from 
  the 
  Cambrian 
  conglomerates. 
  In 
  

   the 
  slides 
  of 
  rocks 
  near 
  St. 
  David's 
  submitted 
  by 
  me 
  in 
  1884 
  to 
  

   Mr. 
  T. 
  Davies 
  he 
  detected 
  pebbles 
  of 
  a 
  granite 
  consisting 
  of 
  dirty 
  

   quartz 
  with 
  two 
  felspars, 
  quartz 
  both 
  dirty 
  and 
  clear 
  and 
  individual 
  

   felspars 
  both 
  orthoclase 
  and 
  plagioclase, 
  pebbles 
  of 
  felsite, 
  quartz- 
  

   felsite, 
  quartz-schist, 
  quartzite, 
  basic 
  igneous 
  rocks, 
  porcellanite, 
  

   and 
  mica 
  much 
  altered. 
  In 
  referring 
  to 
  the 
  fragments 
  of 
  granite 
  

  

  vol. 
  liii. 
  f 
  

  

  