﻿24 
  XEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  Gilbert 
  gulf.^ 
  The 
  shores 
  of 
  the 
  glacial 
  and 
  sea 
  level 
  waters 
  

   are 
  conspicuously 
  preserved 
  in 
  many 
  places, 
  and 
  specially 
  in 
  

   Jefferson 
  county 
  immediately 
  south 
  of 
  the 
  area; 
  while 
  their 
  sedi- 
  

   ments 
  occupy 
  the 
  valleys 
  [see 
  pi. 
  29]. 
  

  

  The 
  slow 
  tilting 
  uplift 
  of 
  this 
  part 
  of 
  the 
  continent 
  finally 
  

   raised 
  the 
  Thousand 
  Islands 
  district 
  above 
  the 
  ocean 
  level 
  and 
  

   then 
  Lake 
  Ontario 
  was 
  initiated. 
  The 
  uplifting 
  has 
  continued 
  

   until 
  the 
  outlet 
  and 
  lake 
  are 
  now 
  246 
  feet 
  above 
  tide. 
  

  

  As 
  the 
  lake 
  and 
  marine 
  waters 
  were 
  slowly 
  drained 
  awav 
  from 
  

   the 
  gently 
  sloping 
  surface 
  of 
  the 
  area 
  the 
  storms 
  and 
  streams 
  

   resumed 
  their 
  briefly 
  interrupted 
  work, 
  and 
  for 
  a 
  few 
  thousand 
  

   years 
  they 
  have 
  again 
  been 
  gnawing 
  at 
  the 
  rocks 
  and 
  land 
  surface 
  

   with 
  important 
  eft'ects. 
  

  

  THE 
  ROCKS^ 
  

   Precambric 
  rocks 
  

  

  The 
  Precambric 
  rocks 
  of 
  northern 
  Xew 
  York, 
  as 
  at 
  present 
  

   known, 
  may 
  be 
  most 
  conveniently 
  classed 
  in 
  four 
  groups, 
  (a) 
  

   a 
  series 
  of 
  old 
  sediments 
  or 
  rocks 
  laid 
  down 
  under 
  water, 
  the 
  

   Grenville 
  series; 
  (b) 
  a 
  series 
  of 
  granitic 
  gneisses 
  of 
  igneous 
  

   origin, 
  which 
  cut 
  the 
  Grenville 
  sediments 
  intrusively 
  and 
  hold 
  

   abimdant 
  inclusions 
  of 
  them 
  and 
  which, 
  in 
  so 
  far 
  at 
  least 
  as 
  the 
  

   immediate 
  region 
  is 
  concerned, 
  are 
  correlated 
  quite 
  confidently 
  

   ^vith 
  the 
  Laurentian 
  granite-gneisses 
  of 
  Canada; 
  (c) 
  a 
  series 
  of 
  

   somewhat 
  younger 
  igneous 
  rocks 
  which 
  cut 
  and 
  hold 
  inclusions 
  

   of 
  both 
  the 
  preceding 
  groups, 
  which 
  have 
  a 
  great 
  development 
  

   in 
  the 
  eastern 
  Adirondacks 
  but 
  occur 
  in 
  less 
  force 
  in 
  the 
  imme- 
  

   diate 
  region, 
  and 
  which 
  consist 
  of 
  anorthosites, 
  syenites, 
  granites 
  

   and 
  gabbros, 
  the 
  last 
  three 
  of 
  which 
  occur 
  here 
  in 
  masses 
  of 
  

   usually 
  small 
  size; 
  and 
  (d) 
  of 
  much 
  younger 
  igneous 
  rocks, 
  of 
  

   late 
  instead 
  of 
  early 
  Precambric 
  age, 
  which 
  appear 
  as 
  dikes 
  of 
  

   diabase 
  or 
  trap, 
  and 
  which 
  have 
  some 
  development 
  in 
  the 
  region, 
  

   though 
  less 
  abundant 
  than 
  in 
  the 
  eastern 
  Adirondacks. 
  

  

  The 
  Grenville 
  sediments 
  are 
  the 
  oldest 
  known 
  rocks 
  of 
  the 
  

   region, 
  and 
  the 
  fact 
  that 
  they 
  are 
  water-deposited 
  rocks 
  necessi- 
  

   tates 
  belief 
  in 
  the 
  existence 
  of 
  a 
  floor 
  of 
  older 
  rocks 
  on 
  which 
  

   they 
  were 
  laid 
  down. 
  Xo 
  certain 
  trace 
  of 
  this 
  old 
  floor 
  has 
  ever 
  

   been 
  discovered 
  in 
  Xew 
  York, 
  and 
  though 
  it 
  is 
  possible 
  that 
  

   fragments 
  of 
  it 
  may 
  be 
  contained 
  as 
  inclusions 
  in 
  the 
  granite 
  

   gneiss, 
  we 
  are 
  as 
  yet 
  unable 
  to 
  distinguish 
  such, 
  if 
  present, 
  from 
  

  

  ^Gilbert 
  Gulf 
  (Marine 
  waters 
  in 
  Ontario 
  basin). 
  Geol. 
  Soc. 
  Am. 
  Bui. 
  

   17:712-18. 
  

   2Bv 
  H. 
  P. 
  Cr.shins. 
  

  

  