﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  1 
  5 
  

  

  granite 
  dikes 
  such 
  as 
  are 
  found 
  nearly 
  everywhere 
  in 
  the 
  Grenville 
  

   Hmestones, 
  sometimes 
  containing 
  only 
  sand. 
  There 
  is 
  comparatively 
  

   little 
  basal 
  conglomerate 
  in 
  the 
  district 
  back 
  from 
  the 
  river, 
  but 
  

   there, 
  both 
  on 
  the 
  mainland 
  of 
  the 
  Alexandria 
  quadrangle 
  and 
  on 
  

   AVellesley 
  and 
  Grindstone 
  islands 
  is 
  an 
  exceedingly 
  coarse 
  conglom- 
  

   erate, 
  from 
  10 
  to 
  20 
  feet 
  thick, 
  full 
  of 
  coarse 
  cobbles 
  derived 
  from 
  

   the 
  ponderous 
  and 
  resistant 
  Grenville 
  quartzite 
  of 
  the 
  vicinity. 
  

  

  Except 
  for 
  these 
  conglomerates 
  the 
  formation 
  is 
  everywhere, 
  a 
  

   sandstone 
  and 
  mostly 
  pretty 
  thoroughly 
  cemented, 
  the 
  cement 
  being 
  

   chiefly 
  of 
  silica. 
  Its 
  colors 
  are 
  red, 
  brown, 
  yellow, 
  white, 
  and 
  rarely 
  

   black. 
  Its 
  thickness 
  over 
  the 
  immediate 
  district 
  will 
  scarcely 
  exceed 
  

   lOo 
  feet, 
  and 
  it 
  thins 
  out 
  toward 
  the 
  west 
  and 
  south. 
  The 
  deposits 
  

   of 
  sand 
  began 
  forming 
  first 
  in 
  the 
  Champlain 
  region 
  and 
  gradually 
  

   worked 
  their 
  way 
  westward, 
  being 
  deposited 
  in 
  a 
  shallow 
  trough 
  or 
  

   basin 
  whose 
  axis 
  roughly 
  coincided 
  with 
  the 
  modern 
  St 
  Lawrence 
  

   axis, 
  so 
  that 
  hereabouts 
  we 
  find 
  simply 
  the 
  thinned 
  western 
  edge 
  of 
  

   the 
  formation. 
  As 
  its 
  thickness 
  here 
  is 
  substantially 
  equal 
  to 
  the 
  

   difference 
  in 
  altitude 
  between 
  the 
  ridge 
  crests 
  and 
  valley 
  bottoms 
  

   of 
  the 
  old 
  erosion 
  surface 
  upon 
  which 
  it 
  was 
  deposited, 
  it 
  follows 
  

   that 
  it 
  varies 
  rapidly 
  in 
  thickness 
  from 
  place 
  to 
  place 
  and 
  was 
  but 
  

   scantily 
  deposited 
  upon 
  the 
  elevations, 
  some 
  of 
  which 
  it 
  utterly 
  

   failed 
  to 
  overtop. 
  

  

  It 
  is 
  not 
  known 
  whether 
  or 
  not 
  the 
  formation 
  in 
  its 
  entirety 
  is 
  a 
  

   marine 
  formation. 
  The 
  sparse 
  fossils 
  indicate 
  such 
  origin 
  for 
  the 
  

   upper 
  beds 
  with 
  comparative 
  certainty, 
  but 
  many 
  things 
  about 
  the 
  

   remainder 
  of 
  the 
  formation 
  suggest 
  a 
  land 
  surface 
  and 
  an 
  arid 
  cH- 
  

   mate 
  as 
  the 
  conditions 
  under 
  which 
  the 
  accumulation 
  took 
  place. 
  

  

  Theresa 
  dolomite. 
  A 
  change 
  in 
  conditions 
  ensued 
  and 
  de- 
  

   posit 
  of 
  dolomite 
  began. 
  Some 
  sand 
  was 
  still 
  supplied 
  from 
  the 
  

   neighboring 
  land 
  however, 
  as 
  the 
  dolomite 
  is 
  everywhere 
  sandy, 
  

   and 
  at 
  first 
  the 
  supply 
  was 
  from 
  time 
  to 
  time 
  in 
  excess, 
  so 
  that 
  layers 
  

   of 
  coarse 
  weak 
  sandstone 
  alternate 
  with 
  those 
  of 
  dolomite. 
  Hence 
  

   there 
  is 
  a 
  gradation 
  from 
  one 
  rock 
  to 
  the 
  other 
  instead 
  of 
  a 
  sharp 
  

   boundary 
  between 
  the 
  two. 
  The 
  greatest 
  thickness 
  of 
  the 
  

   formation 
  within 
  the 
  area 
  mapped 
  does 
  not 
  exceed 
  35 
  feet, 
  

   though 
  its 
  original 
  thickness 
  may 
  have 
  been 
  somewhat 
  greater. 
  

   The 
  thickness 
  increases 
  eastward 
  and 
  diminishes 
  to 
  the 
  west 
  and 
  

   south 
  as 
  was 
  the 
  case 
  with 
  the 
  underlying 
  sandstone. 
  The 
  

   waters 
  were 
  more 
  fitted 
  for 
  the 
  existence 
  of 
  life 
  and 
  the 
  fossils 
  

   are 
  more 
  abundant 
  than 
  in 
  the 
  sandstone, 
  but 
  unfortunately 
  

   conditions 
  for 
  their 
  preservation 
  have 
  not 
  been 
  favorable. 
  

  

  