﻿Appalachian 
  Geosyncline. 
  441 
  

  

  east, 
  the 
  Chemung 
  on 
  the 
  west, 
  holding 
  similar 
  relations. 
  

   The 
  Skunnemunk 
  conglomerate, 
  an 
  isolated 
  Upper 
  Devonian 
  

   outlier, 
  twenty-five 
  miles 
  east 
  of 
  the 
  main 
  Devonian 
  areas, 
  is 
  

   held 
  to 
  represent 
  a 
  fragment 
  of 
  the 
  gravel 
  plain 
  once 
  lying 
  

   between 
  the 
  delta 
  flats 
  and 
  the 
  ancient 
  mountains. 
  

  

  Mention 
  must 
  also 
  be 
  made 
  of 
  the 
  Rensselaer 
  grit, 
  another 
  

   outlier 
  in 
  eastern 
  New 
  York 
  near 
  the 
  Massachusetts 
  boundary, 
  

   although 
  this 
  may 
  not 
  be 
  of 
  Upper 
  Devonian 
  age. 
  Finally, 
  

   the 
  Focono 
  sandstone, 
  the 
  lowest 
  Mississippian 
  formation, 
  must 
  

   be 
  briefly 
  described, 
  since 
  it 
  overlies 
  the 
  Catskill, 
  was 
  formed 
  

   under 
  somewhat 
  similar 
  conditions, 
  and 
  the 
  Catskill 
  grades 
  

   into 
  it 
  by 
  beds 
  of 
  passage. 
  

  

  The 
  Oneonta 
  formation. 
  

  

  The 
  Catskill 
  type 
  of 
  sedimentation 
  began 
  in 
  eastern 
  New 
  

   York, 
  as 
  determined 
  first 
  by 
  James 
  Hall, 
  at 
  the 
  close 
  of 
  the 
  

   Hamilton 
  and 
  in 
  northeastern 
  Pennsylvania 
  apparently 
  some- 
  

   what 
  later, 
  during 
  the 
  Fortage 
  epoch.* 
  This 
  lower 
  portion, 
  

   the 
  Oneonta 
  formation 
  in 
  New 
  York, 
  is 
  largely 
  separated 
  from 
  

   the 
  Catskill 
  proper 
  by 
  an 
  eastwardly 
  penetrating 
  wedge 
  of 
  

   Chemung. 
  The 
  equivalent 
  horizon 
  may 
  exist 
  in 
  northeastern 
  

   Pennsylvania, 
  but 
  the 
  Oneonta 
  has 
  not 
  there 
  been 
  discriminated 
  

   from 
  the 
  Catskill. 
  It 
  consists 
  dominantly 
  of 
  red 
  shales 
  but 
  

   with 
  a 
  considerable 
  content 
  of 
  gray, 
  flaggy 
  sandstones. 
  It 
  

   attains 
  a 
  thickness 
  of 
  a 
  thousand 
  feet 
  in 
  the 
  Catskill 
  Mountains. 
  

   The 
  sections 
  by 
  Darton 
  show 
  how 
  in 
  passing 
  westward 
  it 
  shades 
  

   by 
  transition 
  into 
  the 
  thin-bedded 
  sandstones 
  and 
  hard 
  dark 
  

   shales 
  of 
  the 
  Fortage 
  group,f 
  to 
  which 
  in 
  geologic 
  time 
  it 
  is 
  

   the 
  equivalent. 
  

  

  The 
  Portage 
  Beds. 
  

  

  This 
  group 
  of 
  formations 
  extends 
  from 
  the 
  Genesee 
  black 
  

   shale 
  below 
  to 
  the 
  Chemung 
  olive 
  shales 
  and 
  sandstones 
  above, 
  

   measuring 
  in 
  southwestern 
  New 
  York 
  from 
  1,200 
  to 
  1,500 
  feet 
  

   in 
  thickness. 
  It 
  constitutes 
  the 
  Senecan 
  group 
  in 
  the 
  present 
  

   New 
  York 
  classification. 
  It 
  is 
  the 
  upper 
  part 
  which, 
  accord- 
  

   ing 
  to 
  recent 
  correlations, 
  corresponds 
  especially 
  with 
  the 
  

   Oneonta 
  to 
  the 
  east. 
  At 
  the 
  base 
  of 
  the 
  group 
  black 
  shales 
  

   and 
  some 
  limestone 
  testify 
  to 
  the 
  slackness 
  of 
  sedimentation. 
  

   Above 
  they 
  become 
  alternating 
  shales 
  and 
  flags 
  which, 
  accord- 
  

   ing 
  to 
  the 
  dominance 
  of 
  one 
  or 
  the 
  other, 
  permit 
  subdivision 
  

   into 
  shale, 
  flag, 
  or 
  sandstone 
  formations. 
  The 
  color 
  as 
  a 
  whole 
  

   is 
  gray, 
  the 
  sandstones 
  lighter 
  than 
  the 
  shales. 
  The 
  bedding 
  

  

  * 
  C. 
  S. 
  Prosser. 
  The 
  Devonian 
  System 
  of 
  eastern 
  Pennsylvania 
  and 
  New- 
  

   York, 
  Bull. 
  120, 
  U. 
  S. 
  Geol. 
  Surv., 
  1894. 
  

  

  f 
  The 
  Stratigraphic 
  Eelations 
  of 
  the 
  Oneonta 
  and 
  Chemung 
  formations 
  in 
  

   eastern 
  Central 
  New 
  York. 
  This 
  Journal 
  (3), 
  xlv, 
  1893, 
  pp. 
  204, 
  205. 
  

  

  