﻿Yerrill 
  — 
  Notes 
  on 
  the 
  Geology 
  of 
  the 
  Bermudas. 
  319 
  

  

  excavated, 
  are 
  composed 
  of 
  shell-sand 
  drifted 
  from 
  the 
  beaches 
  

   and 
  sandy 
  flats 
  by 
  the 
  winds 
  in 
  former 
  times 
  into 
  hills 
  and 
  

   afterwards 
  consolidated 
  by 
  infiltration 
  and 
  exposure 
  to 
  the 
  

   atmospheric 
  influences 
  into 
  what 
  is 
  known 
  as 
  geolian 
  limestone. 
  

   A 
  few 
  local 
  deposits 
  of 
  beach-rock, 
  containing 
  broken 
  shells 
  

   and 
  corals, 
  are 
  the 
  only 
  certain 
  exceptions 
  to 
  this.* 
  

  

  1. 
  The 
  reefs 
  do 
  not 
  consist 
  of 
  coral, 
  but 
  of 
  seolian 
  rock, 
  

   made 
  up 
  of 
  shell-sand 
  with 
  very 
  little 
  coral. 
  f 
  The 
  living 
  

   corals, 
  though 
  abundant, 
  constitute 
  a 
  mere 
  coating 
  over 
  the 
  

   surface 
  of 
  the 
  reefs, 
  and 
  an 
  imperfect 
  coating 
  at 
  that. 
  Nor 
  do 
  

   they 
  contribute 
  very 
  much 
  material 
  to 
  the 
  sands 
  or 
  mud. 
  

   The 
  islands 
  and 
  reefs, 
  though 
  in 
  the 
  form 
  of 
  an 
  atoll, 
  do 
  not 
  

   form 
  a 
  true 
  atoll, 
  but 
  &jjseudatoll. 
  Although 
  having 
  the 
  form 
  

   of 
  an 
  atoll 
  they 
  are 
  not 
  coral 
  reefs, 
  — 
  they 
  are 
  sunken 
  wind- 
  

   drifteo^sand-dunes. 
  But 
  they 
  may, 
  perhaps, 
  rest 
  on 
  a 
  founda- 
  

   tion 
  of 
  coral 
  rock, 
  of 
  earlier 
  age, 
  now 
  deeply 
  submerged. 
  The 
  

   living 
  corals 
  help 
  to 
  protect 
  the 
  reefs 
  from 
  rapid 
  erosion. 
  

  

  5. 
  The 
  white 
  sand 
  and 
  mud 
  of 
  the 
  bottom 
  of 
  the 
  bays 
  

   and 
  sounds 
  and 
  of 
  the 
  beaches 
  consist 
  mainly 
  of 
  small 
  mollus- 
  

   can 
  shells, 
  mostly 
  broken, 
  with 
  a 
  few 
  foraminifera 
  and 
  coral- 
  

   lines, 
  and 
  a 
  variable 
  amount 
  of 
  detritus 
  from 
  the 
  erosion 
  of 
  

   the 
  cliffs. 
  This 
  is 
  a 
  matter 
  that 
  I 
  consider 
  of 
  importance 
  in 
  

   the 
  geology 
  of 
  the 
  islands, 
  because 
  most 
  of 
  the 
  writers 
  have 
  

   more 
  or 
  less 
  misunderstood 
  the 
  nature 
  of 
  the 
  material 
  of 
  these 
  

   sands 
  and 
  their 
  origin 
  4 
  The 
  limestones 
  consist 
  of 
  the 
  same 
  

   materials. 
  (See 
  tigs. 
  8, 
  9.) 
  

  

  6. 
  This 
  shell-sand 
  is 
  constantly 
  increasing 
  in 
  amount 
  chiefly 
  

   by 
  the 
  annual 
  growth 
  and 
  death 
  of 
  small 
  shells, 
  as 
  in 
  former 
  

   periods, 
  so 
  that 
  the 
  total 
  mass 
  of 
  the 
  islands 
  is 
  probably 
  still, 
  

   increasing, 
  beneath 
  the 
  sea. 
  

  

  7. 
  The 
  periods 
  of 
  most 
  rapid 
  and 
  extensive 
  formation 
  of 
  the 
  

   beach 
  shell-sand 
  and 
  of 
  the 
  great 
  sand-dunes 
  that 
  formed 
  the 
  

   hills 
  of 
  seolian 
  limestones 
  were 
  the 
  periods 
  of 
  progressive 
  eleva- 
  

  

  * 
  These 
  deposits 
  of 
  beach 
  rock 
  are 
  in 
  some 
  cases 
  of 
  very 
  recent 
  origin 
  ; 
  others 
  

   are 
  of 
  much 
  earlier 
  date; 
  but 
  all 
  contain 
  only 
  existing 
  species 
  of 
  marine 
  shells. 
  

   They 
  are 
  seldom 
  more 
  than 
  10 
  to 
  12 
  feet 
  above 
  high 
  tide. 
  The 
  materials 
  for 
  such 
  

   beds 
  may 
  have 
  been 
  washed 
  to 
  that 
  height, 
  or 
  even 
  higher, 
  in 
  exposed 
  

   places, 
  by 
  storms 
  like 
  that 
  of 
  last 
  September. 
  The 
  " 
  base 
  rock 
  " 
  so 
  called 
  appears 
  

   to 
  me 
  to 
  be 
  only 
  a 
  part 
  of 
  the 
  aeolian 
  limestone 
  altered 
  locally 
  by 
  infiltrations 
  of 
  

   calcite. 
  (See 
  p. 
  321.) 
  

  

  •f 
  One 
  reason 
  for 
  this 
  is 
  because 
  most 
  of 
  the 
  common 
  reef 
  corals 
  at 
  Bermuda 
  

   are 
  large 
  massive 
  species 
  of 
  Diploria, 
  JIaiandrina, 
  Orbicella, 
  Siderastrcea, 
  and 
  

   Porites 
  astrceoides, 
  all 
  of 
  which 
  attach 
  themselves 
  so 
  firmly 
  to 
  the 
  reefs 
  that 
  they 
  

   !dom 
  broken 
  off 
  even 
  by 
  violent 
  storms. 
  Millepora 
  is 
  the 
  most 
  common 
  

   branched 
  and 
  brittle 
  form, 
  and 
  is 
  the 
  most 
  common 
  as 
  fragments 
  in 
  the 
  sands 
  and 
  

   limestones. 
  The 
  star 
  coral 
  ( 
  OMcella 
  annularis) 
  is 
  common 
  in 
  large 
  masses 
  on 
  

   the 
  outer 
  reefs. 
  The 
  larger 
  0. 
  cavernosa, 
  is 
  less 
  common 
  in 
  the 
  same 
  places. 
  

   Diploria 
  forms 
  hemispheres 
  sometimes 
  5 
  or 
  6 
  feet 
  across, 
  and 
  is 
  abundant. 
  No 
  

   madrepores 
  occur 
  here. 
  

  

  discussion 
  of 
  this 
  on 
  pages 
  328-330. 
  

  

  