﻿142 
  NEW 
  YOEK 
  STATE 
  MUSEUM 
  

  

  In 
  like 
  manner 
  plant 
  life, 
  beginning 
  with 
  marine 
  forms 
  of 
  low 
  

   type, 
  gradually 
  developed 
  to 
  the 
  large 
  tree 
  ferns, 
  sigillaria, 
  lyco- 
  

   pods 
  and 
  equisetae 
  of 
  the 
  coal 
  measures. 
  

  

  CAMBRIAN 
  a 
  

  

  Subdivisions 
  or 
  periods 
  

  

  I 
  Sandstone 
  around 
  the 
  Adirondacks 
  

   Potsdam 
  •< 
  Limestone 
  in 
  Dutchess, 
  Washington 
  and 
  Saratoga 
  

   ( 
  counties 
  

  

  Acadian 
  Limestone 
  in 
  Dutchess 
  county 
  

  

  /-, 
  . 
  J 
  Rooiing 
  slates 
  of 
  Washington 
  county 
  

  

  ^ 
  ( 
  Quartzite 
  in 
  Dutchess 
  county 
  

  

  The 
  first 
  and 
  lowest 
  Palaeozoic 
  system 
  known 
  in 
  New 
  York 
  is 
  

   the 
  Cambrian, 
  so 
  called 
  from. 
  Cambria, 
  the 
  latin 
  name 
  of 
  Wales, 
  

   where 
  rocks 
  of 
  this 
  age 
  abound 
  and 
  were 
  first 
  studied 
  by 
  the 
  

   British 
  geologist, 
  Adam 
  Sedgwick, 
  Our 
  knowledge 
  of 
  the 
  Cam- 
  

   brian 
  of 
  New 
  York 
  is 
  largely 
  due 
  to 
  the 
  labors 
  of 
  C. 
  D. 
  Walcott,, 
  

   William 
  B. 
  Dwight, 
  and 
  S. 
  W. 
  Ford. 
  

  

  The 
  base 
  of 
  the 
  Cambrian 
  system 
  in 
  New 
  York 
  and 
  New 
  Eng- 
  

   land 
  rests 
  directly 
  upon 
  the 
  Archaean 
  rocks 
  and 
  its 
  limit 
  can 
  be 
  

   recognized 
  by 
  this 
  fact, 
  as 
  well 
  as 
  by 
  its 
  containing 
  the 
  earliest 
  

   known 
  fauna. 
  But 
  the 
  termination 
  of 
  the 
  uppermost 
  division 
  is 
  

   not 
  so 
  apparent, 
  as 
  it 
  grades, 
  both 
  in 
  sediment 
  and 
  fauna, 
  into 
  

   formations 
  of 
  the 
  Lower 
  Silurian 
  system, 
  thus 
  showing 
  that 
  there 
  

   was 
  no 
  great 
  physical 
  change 
  to 
  influence 
  the 
  tran'sition. 
  North 
  

   of 
  the 
  Adirondacks 
  the 
  delimitation 
  is 
  more 
  clearly 
  defined. 
  

  

  The 
  strata 
  of 
  the 
  Cambrian 
  system 
  are 
  classified 
  as 
  follows: 
  

  

  Upper 
  Cambrian, 
  or 
  Potsdam. 
  

  

  The 
  type 
  rock 
  is 
  the 
  sandstone 
  of 
  the 
  northern 
  and 
  eastern 
  

   borders 
  of 
  the 
  Adirondack 
  mountains, 
  and 
  correlated 
  with 
  it 
  are 
  

   certain 
  limestones 
  on 
  the 
  south 
  side 
  of 
  the 
  Adirondacks, 
  near 
  

   Whitehall 
  and 
  Saratoga 
  Springs, 
  and 
  in 
  Dutchess 
  county 
  near 
  

   Poughkeepsie. 
  The 
  characteristic 
  fossils 
  are 
  the 
  DikelocephaluB^ 
  

   trilobites. 
  ' 
  

  

  «The 
  descriptions 
  of 
  til 
  e 
  Georgjian 
  and 
  Acadian 
  gronps 
  are 
  chiefly 
  from 
  tha 
  

   work 
  of 
  C. 
  D. 
  Walcott, 
  Bulletin 
  No. 
  81, 
  U. 
  S. 
  Geological 
  Survey. 
  

  

  