﻿224 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [Mar. 
  9, 
  

  

  the 
  effects 
  of 
  elevation, 
  denudation, 
  and 
  deposition 
  come 
  successively 
  

   into 
  view. 
  One 
  is 
  on 
  the 
  Upper 
  Mississippi, 
  and 
  the 
  other 
  on 
  the 
  River 
  

   Ohio 
  ; 
  and 
  they 
  both, 
  though 
  not 
  near 
  each 
  other, 
  are 
  on 
  the 
  same 
  

   horizon. 
  They 
  consist 
  of 
  prolonged 
  dome-shaped 
  elevations 
  of 
  rup- 
  

   tured 
  Devonian 
  rocks, 
  disclosing 
  great 
  masses 
  of 
  Lower 
  Silurian, 
  all 
  

   now 
  covered 
  by 
  Quaternary 
  beds. 
  Denudation 
  by 
  exposure 
  to 
  the 
  

   weather, 
  that 
  is, 
  to 
  air 
  and 
  water, 
  must 
  be 
  very 
  slow 
  ; 
  for 
  Delia 
  

   Marmora 
  gives 
  a 
  good 
  woodcut 
  of 
  Cap 
  de 
  l'Ours, 
  in 
  Sardinia, 
  which 
  

   is 
  so 
  named 
  from 
  having 
  from 
  time 
  immemorial 
  resembled 
  a 
  bear. 
  

   It 
  is 
  mentioned 
  by 
  Ptolemy, 
  and 
  therefore 
  is 
  still 
  older 
  than 
  his 
  

   time. 
  

  

  Mr. 
  Darwin 
  long 
  ago 
  showed 
  that 
  the 
  great 
  requisites 
  for 
  any 
  

   large 
  accumulation 
  of 
  sediment 
  are 
  three: 
  — 
  namely, 
  (1) 
  a 
  long- 
  

   continued 
  supply 
  of 
  sediment; 
  (2) 
  an 
  extensive 
  and 
  shallow 
  sea; 
  

   and 
  (3) 
  an 
  area 
  slowly 
  subsiding 
  to 
  a 
  great 
  depth. 
  How 
  

   seldom, 
  says 
  he, 
  in 
  the 
  present 
  day 
  do 
  these 
  conditions 
  concur 
  ! 
  

   Hence 
  the 
  general 
  want 
  of 
  that 
  close 
  sequence 
  in 
  fossiliferous 
  for- 
  

   mations 
  we 
  might 
  have 
  theoretically 
  anticipated. 
  

  

  A 
  gap 
  or 
  blank 
  is 
  sometimes 
  merely 
  an 
  omission 
  — 
  a 
  defect 
  in 
  

   stratigraphical 
  succession 
  — 
  and 
  does 
  not 
  include 
  any 
  idea 
  of 
  vertical 
  

   space. 
  The 
  expression 
  refers 
  to 
  time 
  as 
  well 
  as 
  to 
  deposits. 
  Gaps 
  

   are 
  met 
  with 
  from 
  the 
  beginning 
  of 
  geological 
  time, 
  as 
  far 
  as 
  that 
  

   is 
  known 
  ; 
  and 
  they 
  show 
  themselves 
  throughout 
  all 
  epochs 
  and 
  

   parts 
  of 
  epochs, 
  from 
  place 
  to 
  place. 
  (See 
  the 
  instances 
  we 
  have 
  

   given.) 
  

  

  It 
  may 
  be 
  remarked, 
  and 
  with 
  surprise, 
  that 
  few 
  examples 
  are 
  

   known 
  where 
  the 
  absence 
  of 
  a 
  set 
  of 
  strata 
  is 
  attended 
  by, 
  or 
  in 
  

   any 
  way 
  connected 
  with, 
  rupture 
  of 
  beds 
  from 
  beneath, 
  or 
  the 
  out- 
  

   burst 
  of 
  igneous 
  rocks 
  ; 
  the 
  occurrence 
  has 
  been 
  usually 
  due 
  to 
  

   broad 
  elevation 
  alone.' 
  Sections 
  (Muirkirk 
  Coal-field 
  and 
  Lesma- 
  

   hago), 
  however, 
  in 
  Western 
  Scotland, 
  by 
  Mr. 
  Geikie 
  *, 
  may 
  afford 
  

   instances 
  of 
  this. 
  

  

  Some 
  of 
  the 
  blanks 
  are 
  of 
  limited 
  extent, 
  as 
  when 
  caused 
  by 
  the 
  

   absence 
  of 
  a 
  bed 
  or 
  two 
  ; 
  but 
  they 
  are 
  usually 
  large, 
  and 
  in 
  certain 
  

   cases 
  occupy 
  an 
  important 
  portion 
  of 
  the 
  earth's 
  surface 
  — 
  one 
  or 
  more 
  

   millions 
  of 
  square 
  miles. 
  Such 
  are 
  the 
  Quaternary 
  deposits 
  resting 
  

   directly 
  on 
  Laurentian, 
  &c. 
  (North 
  America, 
  South 
  America, 
  Scan- 
  

   dinavia, 
  &c). 
  

  

  These 
  gaps 
  or 
  blanks 
  are 
  often 
  very 
  large 
  and 
  numerous 
  (existing 
  

   contemporaneously 
  and 
  long) 
  ; 
  they 
  arise 
  from 
  so 
  many 
  common 
  and 
  

   so 
  many 
  different 
  floors 
  or 
  beginnings, 
  that 
  we 
  may 
  safely 
  assume 
  

   that, 
  at 
  various 
  times 
  and 
  in 
  a 
  thousand 
  ways, 
  they 
  ran 
  together 
  

   and 
  coalesced 
  into 
  vast 
  spaces 
  of 
  dry 
  land 
  of 
  diversified 
  geological 
  

   structure, 
  in 
  the 
  form 
  of 
  continents, 
  peninsulas, 
  and 
  islands, 
  just 
  

   as 
  we 
  have 
  it 
  in 
  the 
  present 
  day, 
  and 
  probably 
  as 
  extensively. 
  

  

  2. 
  Summary. 
  — 
  The 
  greater 
  number 
  of 
  gaps, 
  and 
  especially 
  of 
  those 
  

   which 
  become 
  important 
  by 
  reason 
  of 
  duration 
  or 
  size, 
  spring 
  from 
  

   the 
  Silurian, 
  Carboniferous, 
  or 
  Jurassic 
  ages 
  ; 
  and 
  this 
  partly 
  from 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xvi. 
  p. 
  322, 
  pi. 
  18. 
  fig. 
  3 
  ; 
  (Murchison), 
  

   ibid. 
  vol. 
  xii. 
  p. 
  18. 
  

  

  