﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  45 
  

  

  of 
  the 
  dike 
  numbered 
  i 
  on 
  the 
  map. 
  Two 
  have 
  been 
  found 
  on 
  

   Wellesley 
  island, 
  the 
  wider 
  of 
  which 
  measures 
  30 
  feet. 
  Seven 
  have 
  

   been 
  found 
  on 
  the 
  mainland 
  of 
  the 
  Alexandria 
  sheet, 
  in 
  rather 
  

   widely 
  scattered 
  distribution, 
  and 
  in 
  general 
  much 
  narrower. 
  None 
  

   have 
  been 
  observed 
  on 
  the 
  Theresa 
  sheet. 
  Smyth 
  has 
  described 
  them 
  

   as 
  abundant 
  on 
  the 
  Canadian 
  mainland 
  and 
  islands 
  in 
  the 
  vicinity 
  

   of 
  Gananoque, 
  hence 
  in 
  the 
  near 
  vicinity 
  of 
  Grindstone 
  island, 
  

   which 
  would 
  seem 
  to 
  have 
  been 
  the 
  chief 
  center 
  of 
  activity. 
  For 
  

   petrographic 
  details 
  the 
  reader 
  is 
  referred 
  to 
  his 
  account 
  which, 
  

   though 
  based 
  on 
  Canadian 
  material, 
  also 
  describes 
  these 
  accurately.^ 
  

  

  The 
  dikes 
  trend 
  in 
  various 
  directions, 
  from 
  northwest 
  around 
  

   through 
  north 
  to 
  northeast. 
  Smyth 
  states 
  that 
  those 
  seen 
  around 
  

   Gananoque 
  trend 
  chiefly 
  to 
  the 
  north, 
  and 
  were 
  all 
  cutting 
  granite. 
  

   It 
  is 
  to 
  be 
  noted 
  that 
  all 
  those 
  trending 
  northeast, 
  in 
  our 
  district 
  

   here, 
  are 
  cutting 
  Grenville 
  rocks 
  with 
  general 
  northeast 
  strike, 
  while 
  

   all 
  the 
  dikes 
  cutting 
  the 
  igneous 
  rocks 
  trend 
  north 
  or 
  northwest. 
  

   This 
  is 
  also 
  true 
  of 
  two 
  of 
  the 
  dikes 
  cutting 
  the 
  Grenville, 
  but 
  in 
  

   both 
  cases 
  the 
  Grenville 
  is 
  in 
  comparatively 
  small 
  bulk, 
  and 
  entirely 
  

   inclosed 
  by 
  igneous 
  rocks. 
  The 
  dike 
  directions 
  are 
  therefore 
  appar- 
  

   ently 
  determined 
  by 
  preexisting 
  structures 
  in 
  the 
  rocks, 
  by 
  the 
  strike 
  

   in 
  the 
  Grenville, 
  and 
  by 
  a 
  joint 
  set 
  in 
  the 
  igneous 
  rocks. 
  Small 
  

   masses 
  of 
  Grenville 
  rocks 
  did 
  not 
  suffice 
  to 
  change 
  the 
  direction 
  of 
  

   dikes 
  passing 
  across 
  them, 
  the 
  igneous 
  rocks 
  here 
  being 
  the 
  deter- 
  

   mining 
  factor. 
  

  

  Though 
  the}^ 
  give 
  no 
  evidence 
  of 
  having 
  been 
  severely 
  deformed, 
  

   yet 
  the 
  rock 
  of 
  the 
  larger 
  dikes 
  does 
  show 
  evidences 
  of 
  considerable 
  

   pressure. 
  Many 
  of 
  the 
  feldspar 
  crystals 
  are 
  distinctly 
  bent, 
  and 
  both 
  

   the 
  feldspar 
  and 
  augite 
  of 
  the 
  rock 
  shows 
  evidence 
  of 
  strain 
  by 
  their 
  

   undulatory 
  extinction. 
  In 
  this 
  respect 
  they 
  contrast 
  with 
  the 
  diabases 
  

   of 
  the 
  eastern 
  Adirondacks, 
  which 
  show 
  no 
  such 
  strain 
  effects. 
  

   The 
  eastern 
  dikes 
  also 
  have 
  chiefly 
  east-west 
  trends, 
  differ 
  somewhat 
  

   in 
  mineralogy, 
  and 
  are 
  more 
  numerous 
  and 
  widespread; 
  and 
  are 
  

   also 
  separated 
  from 
  this 
  area 
  by 
  a 
  wide 
  region 
  in 
  which 
  such 
  dikes 
  

   are 
  absent. 
  We 
  seem 
  here 
  therefore 
  to 
  be 
  dealing 
  with 
  a 
  wholly 
  

   different 
  center 
  of 
  igneous 
  activity, 
  and 
  a 
  much 
  less 
  extensive 
  one 
  

   than 
  that 
  farther 
  east. 
  

  

  Owing 
  to 
  their 
  size 
  and 
  comparative 
  freshness 
  these 
  dikes 
  have 
  a 
  

   potential 
  value 
  in 
  the 
  region 
  as 
  a 
  comparatively 
  accessible 
  source 
  of 
  

   good 
  road 
  metal. 
  

  

  Contact 
  rocks. 
  The 
  contact 
  effects 
  of 
  the 
  igneous 
  rocks 
  upon 
  the 
  

   Grenville 
  sediments, 
  and 
  vice 
  versa, 
  may 
  be 
  grouped 
  under 
  three 
  

   1 
  N. 
  Y. 
  Acad. 
  Sci. 
  Trans. 
  13 
  :2ogri4. 
  

  

  