﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  79 
  

  

  represented 
  in 
  the 
  Champlain 
  area, 
  Ulrich 
  correlating 
  the 
  dove, 
  

   reef 
  limestone, 
  only 
  a 
  few 
  feet 
  in 
  thickness, 
  which 
  forms 
  the 
  

   basal 
  member 
  of 
  the 
  upper 
  Chazy 
  there, 
  with 
  the 
  Pamelia 
  hori- 
  

   zon. 
  In 
  the 
  much 
  more 
  complete 
  sections 
  about 
  Chambersburg, 
  

   Pa., 
  a 
  200 
  foot 
  thickness 
  of 
  limestone 
  with 
  an 
  upper 
  Chazy 
  fauna, 
  

   separates 
  the 
  Pamelia 
  horizon 
  from 
  the 
  Lowville. 
  Subsidence 
  

   apparently 
  ceased 
  in 
  the 
  Champlain 
  basin 
  during 
  the 
  time 
  of 
  

   Pamelia 
  depression 
  and 
  deposit 
  in 
  this 
  district, 
  and 
  as 
  this 
  ceased 
  

   here, 
  upper 
  Chazy 
  depression 
  was 
  renewed 
  there, 
  the 
  uncon- 
  

   formity 
  between 
  the 
  Pamelia 
  and 
  Lowville 
  representing 
  this 
  

   upper 
  Chazy 
  interval. 
  Knowledge 
  of 
  this 
  led 
  Ulrich 
  to 
  predict 
  

   the 
  unconformity 
  and 
  induced 
  the 
  search 
  for 
  it. 
  Otherwise 
  it 
  

   might 
  easily 
  have 
  escaped 
  our 
  notice. 
  

  

  Mohawkian 
  series^ 
  

  

  The 
  Mohawkian 
  series 
  comprises 
  the 
  Black 
  River 
  and 
  Trenton 
  

   groups. 
  The 
  Black 
  River 
  group 
  is 
  composed 
  of 
  the 
  Lowville 
  beds 
  

   including 
  the 
  Leray 
  limestone, 
  and 
  the 
  Watertown 
  limestone. 
  In 
  

   giving 
  to 
  the 
  Black 
  River 
  group 
  this 
  larger 
  scope, 
  we 
  return 
  tO' 
  the 
  

   original 
  conception 
  of 
  several 
  of 
  the 
  geologists 
  of 
  the 
  First 
  Geo- 
  

   Icgical 
  Survey 
  of 
  New 
  York, 
  i. 
  e. 
  Hall, 
  Vanuxem 
  and 
  Mather, 
  with 
  

   the 
  exception 
  that 
  the 
  Black 
  River 
  then 
  also 
  included 
  the 
  Chazy 
  

   limestone. 
  Emmons, 
  however, 
  to 
  whose 
  district 
  the 
  Black 
  River 
  

   region 
  belonged, 
  did 
  not 
  use 
  the 
  term 
  " 
  Black 
  River." 
  He 
  dis- 
  

   tinguished 
  the 
  " 
  Birdseye 
  limestone 
  " 
  and 
  the 
  " 
  Isle 
  La 
  Motte 
  

   marble 
  " 
  employing 
  the 
  latter 
  term 
  for 
  a 
  bed 
  locally 
  'the 
  main 
  

   object 
  of 
  the 
  quarrying 
  industry, 
  and 
  known 
  as 
  the 
  " 
  Seven 
  foot 
  

   tier." 
  Hall, 
  in 
  the 
  first 
  volume 
  of 
  the 
  Palaeontology 
  of 
  New 
  York, 
  

   restricted 
  the 
  term 
  Black 
  River 
  to 
  this 
  " 
  Seven 
  foot 
  tier 
  " 
  and 
  

   through 
  his 
  influence 
  and 
  the 
  description 
  of 
  a 
  very 
  striking 
  

   cephalopod 
  fauna 
  from 
  the 
  bed, 
  the 
  term 
  '' 
  Black 
  River 
  " 
  was 
  

   quite 
  generally 
  accepted 
  for 
  the 
  " 
  Seven 
  foot 
  tier." 
  Since, 
  how- 
  

   ever, 
  mainly 
  through 
  the 
  investigations 
  of 
  Dr 
  Ulrich, 
  the 
  fact 
  has 
  

   become 
  apparent 
  that 
  beds 
  which 
  in 
  the 
  Mohawk 
  valley 
  and 
  the 
  

   Lake 
  Champlain 
  region 
  have 
  been 
  referred 
  to 
  the 
  Black 
  River 
  

   limestone, 
  are 
  both 
  older 
  and 
  younger 
  than 
  the 
  Black 
  River 
  as 
  re- 
  

   stricted 
  by 
  Hall, 
  but 
  fall 
  within 
  the 
  limits 
  of 
  the 
  original 
  concep- 
  

   tion 
  of 
  Black 
  River, 
  it 
  has 
  become 
  advisable 
  to 
  revive 
  this 
  original 
  

   usage 
  of 
  the 
  term 
  to 
  avoid 
  confusion. 
  The 
  " 
  Seven 
  foot 
  tier 
  " 
  or 
  

   Black 
  River 
  limestone 
  of 
  Hall 
  has 
  then 
  to 
  be 
  renamed 
  and 
  the 
  term 
  

   "Watertown" 
  is 
  here 
  used 
  for 
  this 
  formation 
  [see 
  ^. 
  84]. 
  

  

  1 
  By 
  R. 
  Ruedemann. 
  

  

  