﻿H. 
  S. 
  Williams 
  — 
  Southern 
  Devonian 
  formations. 
  393 
  

  

  less 
  ; 
  thus 
  the 
  error 
  from 
  the 
  heating 
  of 
  the 
  wire 
  may 
  be 
  neg- 
  

   lected. 
  

  

  In 
  order 
  to 
  show 
  that 
  common 
  electrolytic 
  polarization 
  does 
  

   not 
  interfere 
  with 
  the 
  accuracy 
  of 
  our 
  method, 
  we 
  measured 
  

   with 
  the 
  help 
  of 
  our 
  20,000 
  volt 
  storage 
  battery 
  and 
  condensers 
  

   the 
  resistance 
  between 
  two 
  bright 
  platinum 
  plates 
  similar 
  in 
  

   size 
  to 
  the 
  copper 
  ones 
  used 
  before, 
  in 
  a 
  cupric 
  sulphate 
  solution. 
  

   This 
  was 
  found 
  to 
  be 
  four 
  ohms 
  ; 
  and 
  after 
  plating 
  the 
  elec- 
  

   trodes 
  with 
  copper 
  the 
  resistance 
  remained 
  unchanged. 
  Kohl- 
  

   rausch's 
  method 
  gave 
  no 
  satisfactory 
  result 
  with 
  both 
  electrodes 
  

   free 
  from 
  copper 
  ; 
  but 
  when 
  both 
  were 
  plated 
  it 
  indicated 
  a 
  

   resistance 
  of 
  3*9 
  ohms. 
  

  

  Our 
  method 
  may 
  therefore 
  be 
  a 
  useful 
  one 
  for 
  the 
  approxi- 
  

   mate 
  determination 
  of 
  conductivities 
  in 
  cases 
  where 
  impurities 
  

   or 
  polarization 
  render 
  Kohlrausch's 
  method 
  unsatisfactory. 
  

   For 
  accuracy, 
  of 
  course, 
  pains 
  must 
  be 
  taken 
  to 
  develop 
  all 
  

   the 
  photographs 
  in 
  the 
  same 
  fashion, 
  and 
  in 
  general 
  to 
  arrange 
  

   the 
  conditions 
  of 
  the 
  exposure 
  alike 
  in 
  all 
  cases. 
  

  

  Our 
  conclusion, 
  that 
  the 
  conductivity 
  of 
  electrolytes 
  is 
  not 
  

   essentially 
  affected 
  by 
  great 
  changes 
  in 
  current 
  strength, 
  only 
  

   emphasizes 
  all 
  the 
  more 
  strongly 
  the 
  conclusion 
  of 
  our 
  last 
  

   paper, 
  that 
  the 
  conductivity 
  of 
  gases 
  is 
  very 
  much 
  affected 
  by 
  

   changes 
  in 
  the 
  current 
  strength. 
  

   Harvard 
  University, 
  March 
  8, 
  189?. 
  

  

  Art. 
  XXXIX. 
  — 
  On 
  the 
  Southern 
  Devonian 
  formations 
  ; 
  by 
  

   Henry 
  S. 
  Williams. 
  

  

  [Read 
  before 
  the 
  Geological 
  Society 
  of 
  America, 
  Dec. 
  31st, 
  1896.] 
  

  

  One 
  of 
  the 
  most 
  remarkable 
  contrasts 
  of 
  geological 
  correla- 
  

   tion 
  is 
  met 
  with 
  on 
  passing 
  from 
  the 
  Devonian 
  system, 
  as 
  it 
  is 
  

   typically 
  expressed 
  in 
  New 
  York 
  state, 
  to 
  the 
  expression 
  of 
  the 
  

   same 
  system 
  as 
  found 
  in 
  Tennessee 
  and 
  Alabama. 
  

  

  In 
  the 
  north, 
  following 
  the 
  Silurian 
  formations 
  in 
  regular 
  

   sequence, 
  there 
  is 
  a 
  series 
  of 
  varying 
  sediments, 
  reaching 
  several 
  

   thousand 
  feet 
  in 
  thickness, 
  made 
  up 
  of 
  numerous 
  separate 
  

   formations, 
  sharply 
  differing 
  in 
  the 
  kind 
  of 
  materials 
  compos- 
  

   ing 
  them 
  and 
  containing 
  well-differentiated 
  faunas. 
  In 
  the 
  

   south 
  a 
  uniform 
  black 
  shale, 
  with 
  perfectly 
  even 
  sedimentation, 
  

   the 
  grain 
  of 
  the 
  rock 
  presenting 
  almost 
  no 
  difference 
  from 
  top 
  

   to 
  bottom, 
  containing 
  a 
  meagre 
  fauna 
  and 
  evidence 
  of 
  a 
  com- 
  

   mon 
  plant 
  vegetation 
  so 
  long 
  and 
  so 
  far 
  as 
  it 
  may 
  be 
  distributed 
  

   in 
  its 
  purity, 
  and 
  varying 
  in 
  thickness 
  from 
  500 
  to 
  900 
  feet 
  in 
  

   some 
  sections 
  to 
  a 
  few 
  inches 
  in 
  thickness 
  in 
  some 
  of 
  the 
  Ala- 
  

   bama 
  sections, 
  and 
  often 
  resting 
  unconformably 
  upon 
  Niagara, 
  

  

  