﻿HI8T0BI0 
  GEOLOGY 
  127 
  

  

  rain 
  and 
  frost, 
  rivers, 
  waves 
  and 
  currents 
  through 
  all 
  time 
  has 
  

   led 
  to 
  the 
  deposition 
  of 
  a 
  succession 
  of 
  strata 
  which 
  on 
  the 
  

   whole 
  are 
  unbroken 
  in 
  their 
  sequence, 
  though 
  they 
  have 
  varied 
  

   so 
  much 
  in 
  the 
  areas 
  of 
  their 
  deposition 
  that 
  in 
  no 
  region 
  do 
  

   we 
  find 
  the 
  series 
  complete. 
  There 
  has 
  been 
  a 
  break 
  of 
  con- 
  

   tinuity 
  in 
  those 
  areas 
  which 
  for 
  a 
  time 
  were 
  elevated 
  above 
  the 
  

   sea, 
  but 
  the 
  continuity 
  of 
  the 
  geologic 
  series 
  has 
  always 
  been 
  

   maintained 
  in 
  one 
  area 
  or 
  another. 
  Contemporaneous 
  strata 
  

   are 
  found 
  only 
  in 
  those 
  areas 
  which 
  are 
  simultaneously 
  depressed 
  

   and 
  which 
  were 
  submerged 
  during 
  the 
  same 
  time. 
  

  

  Contemporaneous 
  strata 
  may 
  differ 
  widely 
  in 
  composition 
  

   owinig 
  to 
  differences 
  in 
  material 
  and 
  the 
  conditions 
  of 
  their 
  de- 
  

   position. 
  Thus 
  the 
  Potsdam 
  sandstone 
  in 
  northern 
  New 
  York 
  

   is 
  contemporaneous 
  with 
  a 
  limestone 
  in 
  Saratoga 
  and 
  Dutchess 
  

   counties. 
  

  

  As 
  a 
  result 
  of 
  the 
  alternating 
  invasions 
  and 
  retreats 
  of 
  the 
  

   ocean 
  over 
  the 
  land, 
  we 
  find 
  in 
  various 
  geologic 
  systems 
  what 
  is 
  

   known 
  as 
  a 
  trinity 
  of 
  formations,<^ 
  viz. 
  a 
  base 
  consisting 
  of 
  sand- 
  

   stone 
  or 
  conglomerate, 
  a 
  center 
  consisting 
  chiefly 
  of 
  limestone 
  and 
  

   a 
  summit 
  of 
  shale 
  or 
  mud 
  stone. 
  

  

  The 
  cause 
  of 
  this 
  alternation 
  is 
  not 
  fully 
  known. 
  The 
  sand- 
  

   stones 
  and 
  conglomerates 
  are 
  usually 
  solidified 
  beach 
  and 
  shoal 
  

   water 
  deposits. 
  The 
  shales 
  are 
  solidified 
  sea 
  bottom 
  de- 
  

   posits 
  consisting 
  of 
  the 
  finer 
  material 
  carried 
  from 
  the 
  shore 
  

   by 
  waves 
  and 
  currents 
  and 
  also 
  of 
  sediment 
  carried 
  into 
  the 
  sea 
  

   by 
  rivers. 
  

  

  The 
  limestones 
  were 
  probably 
  formed 
  in 
  many 
  cases 
  as 
  at 
  the 
  

   present 
  day, 
  in 
  warmer 
  waters, 
  which 
  permitted 
  the 
  luxuriant 
  

   growth 
  of 
  corals, 
  mollusks 
  and 
  other 
  marine 
  invertebrates 
  which 
  

   have 
  external 
  skeletons 
  composed 
  of 
  carbonate 
  of 
  lime. 
  In 
  New 
  

   York 
  there 
  were 
  coral 
  reefs 
  in 
  the 
  Trenton, 
  Niagara 
  and 
  Cornif- 
  

   erous 
  periods. 
  Whether 
  corals 
  in 
  Palaeozoic 
  time 
  required 
  the 
  

   same 
  warm 
  temperature 
  of 
  water 
  as 
  at 
  the 
  present 
  day, 
  we 
  do 
  

   not 
  know. 
  

  

  a 
  Geikie's 
  Text 
  Book 
  of 
  Geology, 
  Ilird 
  Ed. 
  p. 
  454. 
  

  

  