﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  55 
  

  

  tion 
  since 
  it 
  is 
  very 
  exceptional 
  to 
  have 
  the 
  evidence 
  as 
  abundant 
  

   and 
  as 
  clearly 
  shown 
  as 
  it 
  is 
  here. 
  

  

  The 
  evidence 
  of 
  surface 
  irregularity 
  is 
  of 
  threefold 
  nature, 
  (a) 
  

   that 
  given 
  by 
  exposures 
  of 
  direct 
  contacts, 
  (b) 
  that 
  given 
  by 
  the 
  

   tracing 
  of 
  the 
  lines 
  of 
  contact 
  of 
  the 
  Potsdam 
  and 
  Precambric, 
  even 
  

   without 
  exposures 
  of 
  the 
  actual 
  contact, 
  and 
  (c) 
  that 
  given 
  by 
  the 
  

   topography 
  of 
  the 
  present 
  Precambric 
  surfaces, 
  since 
  it 
  can 
  be 
  

   shown 
  that 
  these 
  surfaces 
  are 
  substantially 
  those 
  upon 
  which 
  the 
  

   Potsdam 
  was 
  originally 
  deposited 
  ; 
  in 
  other 
  words 
  that 
  the 
  Potsdam 
  

   is 
  just 
  being 
  pared 
  away 
  from 
  the 
  Precambric 
  over 
  part 
  of 
  the 
  

   district, 
  its 
  numerous 
  outliers 
  testifying 
  to 
  its 
  former 
  presence 
  over 
  

   the 
  whole 
  and 
  to 
  the 
  recency 
  of 
  its 
  removal 
  where 
  now 
  absent. 
  

  

  Where 
  the 
  Potsdam 
  has 
  been 
  removed 
  the 
  Precambric 
  surface 
  

   disclosed 
  is 
  one 
  of 
  low 
  ridges 
  and 
  valleys, 
  with 
  general 
  northeast- 
  

   southwest 
  trend. 
  The 
  ridges 
  are 
  low 
  and 
  with 
  hummocky 
  surface, 
  

   and 
  the 
  valleys 
  are 
  broad 
  and 
  shallow, 
  and 
  developed 
  on 
  the 
  weak 
  

   rocks 
  (such 
  as 
  the 
  limestones 
  and 
  weak 
  schists) 
  or 
  on 
  lines 
  of 
  

   structural 
  weakness 
  (as 
  along 
  lines 
  of 
  sheared 
  and 
  shattered 
  gran- 
  

   ite). 
  The 
  extreme 
  of 
  relief 
  does 
  not 
  much 
  exceed 
  loo 
  feet, 
  and 
  is 
  

   generally 
  less. 
  The 
  quartzites, 
  resistant 
  gneisses 
  and 
  some 
  long 
  

   and 
  wide 
  granite 
  dikes 
  constitute 
  the 
  ridges. 
  In 
  the 
  relatively 
  ele- 
  

   vated 
  country 
  occupied 
  by 
  the 
  igneous 
  bathyliths 
  the 
  surface 
  is 
  of 
  

   the 
  knob 
  and 
  basin 
  type 
  [pi. 
  6 
  and 
  7]. 
  The 
  numerous 
  granite 
  

   dikes 
  and 
  small 
  bosses 
  which 
  cut 
  the 
  limestone 
  and 
  are 
  resistant 
  to 
  

   weathering, 
  diversify 
  the 
  valley 
  bottoms. 
  Hence 
  a 
  large 
  part 
  of 
  the 
  

   area 
  consists 
  of 
  slopes, 
  and 
  extensive 
  flats 
  do 
  not 
  appear. 
  

  

  The 
  surface 
  underneath 
  the 
  Potsdam 
  is 
  precisely 
  of 
  this 
  sort. 
  

   The 
  smaller 
  Potsdam 
  outliers 
  are 
  usually 
  mere 
  remnants 
  remaining 
  

   in 
  places 
  where 
  the 
  floor 
  was 
  lowest 
  and 
  the 
  sandstone 
  thickest. 
  

   The 
  larger 
  outliers 
  cover 
  both 
  high 
  and 
  low 
  ground. 
  The 
  Pots- 
  

   dam 
  resists 
  wear, 
  and 
  hence 
  usually 
  presents 
  cliff 
  fronts 
  at 
  its 
  mar- 
  

   gins, 
  showing 
  thicknesses 
  of 
  from 
  20 
  to 
  more 
  than 
  60 
  feet 
  of 
  sand- 
  

   stone, 
  yet 
  even 
  with 
  these 
  thicknesses 
  the 
  summits 
  of 
  the 
  outliers 
  

   are 
  often 
  overtopped 
  by 
  neighboring 
  granite 
  knobs. 
  The 
  evidence 
  

   of 
  the 
  occasional 
  inliers 
  of 
  Precambric 
  rocks 
  in 
  the 
  Potsdam 
  is 
  

   even 
  more 
  Obvious. 
  The 
  two 
  small 
  inliers 
  east 
  of 
  Goose 
  bay 
  (Alex- 
  

   andria 
  quadrangle) 
  along 
  the 
  road 
  from 
  Alexandria 
  Bay 
  to 
  Chip- 
  

   pewa 
  Bay, 
  have 
  their 
  tops 
  at 
  the 
  same 
  level 
  as 
  that 
  of 
  the 
  sandstone 
  

   plain 
  in 
  which 
  they 
  lie, 
  yet 
  a 
  20 
  foot 
  thickness 
  of 
  sandstone 
  shows 
  

   at 
  the 
  Potsdam 
  margin, 
  just 
  to 
  the 
  northward. 
  This 
  line 
  of 
  evi- 
  

   dence 
  might 
  be 
  pursued 
  at 
  great 
  length 
  but 
  since 
  it 
  is 
  less 
  conclusive 
  

   than 
  are 
  the 
  other 
  lines 
  the 
  above 
  will 
  suflice. 
  

  

  