﻿lxviii 
  PKOCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [May 
  1 
  897, 
  

  

  we 
  have 
  obtained 
  evidences 
  of 
  most 
  interesting 
  Lower 
  Cambrian 
  

   faunas 
  from 
  those 
  areas. 
  During 
  the 
  whole 
  time 
  when 
  the 
  Lower 
  

   Cambrian 
  rocks 
  were 
  deposited 
  it 
  is 
  quite 
  evident 
  that 
  the 
  general 
  

   conditions 
  were 
  favourable 
  to 
  marine 
  life 
  ; 
  but 
  as 
  the 
  deposits 
  must 
  

   have 
  accumulated 
  with 
  comparative 
  rapidity 
  and 
  in 
  fairly 
  shallow 
  

   water, 
  there 
  are 
  few 
  calcareous 
  zones, 
  and 
  the 
  range 
  of 
  typical 
  

   fossils 
  is 
  often 
  great. 
  The 
  close 
  of 
  the 
  pre-Cambrian 
  period 
  was 
  

   undoubtedly 
  accompanied 
  by 
  great 
  physical 
  changes, 
  which 
  produced 
  

   very 
  uneven 
  land- 
  surfaces, 
  and 
  the 
  encroachment 
  of 
  the 
  sea 
  over 
  the 
  

   land 
  was 
  therefore 
  necessarily 
  interrupted 
  and 
  irregular. 
  This 
  also 
  

   would 
  in 
  part 
  account 
  for 
  the 
  unusual 
  range 
  of 
  some 
  of 
  the 
  organisms, 
  

   for 
  those 
  which 
  hugged 
  the 
  shores 
  would 
  still 
  remain 
  in 
  some 
  areas 
  

   while 
  deeper-water 
  forms 
  might 
  be 
  almost 
  close 
  at 
  hand. 
  When 
  it 
  

   is 
  remembered 
  also 
  that 
  the 
  pre-Cambrian 
  land 
  contained 
  an 
  

   unusual 
  amount 
  of 
  loose 
  and 
  easily 
  disintegrated 
  volcanic 
  material, 
  

   which 
  would 
  be 
  carried 
  by 
  streams 
  or 
  rivers 
  into 
  narrow 
  bays, 
  the 
  

   cause 
  of 
  rapid 
  accumulation 
  becomes 
  easily 
  understood. 
  In 
  the 
  

   Lower 
  Cambrian 
  rocks 
  hundreds 
  of 
  feet 
  of 
  sandstones 
  and 
  grits 
  

   constantly 
  occur, 
  with 
  only 
  here 
  and 
  there 
  very 
  thin 
  seams 
  of 
  muddy 
  

   deposits. 
  The 
  beds 
  also 
  are 
  so 
  often 
  ripple- 
  marked 
  that 
  it 
  is 
  clear 
  

   that 
  the 
  depression 
  did 
  little 
  more 
  than 
  keep 
  pace 
  with 
  the 
  accumu- 
  

   lation. 
  When 
  finer 
  sediments 
  occur, 
  and 
  there 
  are 
  signs 
  of 
  a 
  pause 
  

   in 
  the 
  deposition, 
  fossils 
  are 
  usually 
  found 
  in 
  fair 
  abundance 
  and 
  

   with 
  evidence 
  of 
  fresh 
  arrivals 
  on 
  the 
  scene. 
  

  

  All 
  this 
  is 
  proof 
  to 
  my 
  mind 
  of 
  the 
  incoming 
  of 
  forms 
  from 
  

   adjoining 
  marine 
  areas 
  more 
  suitable 
  to 
  their 
  development 
  than 
  

   these 
  constantly 
  changing 
  shores, 
  where 
  the 
  accumulation 
  of 
  

   materials 
  was 
  going 
  on 
  so 
  rapidly. 
  Though 
  we 
  count 
  our 
  Cambrian 
  

   deposits 
  by 
  thousands 
  of 
  feet, 
  no 
  one 
  will 
  venture, 
  when 
  he 
  knows 
  

   the 
  full 
  circumstances 
  under 
  which 
  they 
  were 
  accumulated, 
  to 
  say 
  

   that 
  the 
  majority 
  of 
  the 
  animal 
  forms 
  now 
  classed 
  as 
  characteristic 
  

   of 
  that 
  period 
  were 
  not 
  equally 
  so 
  of 
  much 
  of 
  the 
  pre-Cambrian 
  

   period. 
  

  

  We 
  may 
  now 
  fairly 
  ask 
  why 
  it 
  is 
  that 
  the 
  pre-Cambrian 
  rocks 
  

   do 
  not 
  contain 
  evidences 
  of 
  the 
  earlier 
  faunas. 
  This 
  question 
  

   would 
  have 
  been 
  much 
  more 
  difficult 
  to 
  answer 
  a 
  few 
  years 
  

   ago 
  than 
  at 
  present. 
  Then 
  all, 
  or 
  nearly 
  all, 
  pre-Cambrian 
  rocks 
  

   were 
  looked 
  upon 
  as 
  metamorphosed 
  sediments. 
  Now 
  all 
  agree 
  

   that 
  the 
  pre-Cambrian 
  rocks 
  with 
  which 
  we 
  are 
  acquainted 
  are 
  

   mainly 
  of 
  igneous 
  origin 
  — 
  either 
  intrusive 
  or 
  extrusive 
  igneous 
  

   rocks. 
  .And 
  the 
  comparatively 
  few 
  rocks 
  of 
  sedimentary 
  origin 
  

  

  