﻿370 
  W. 
  H. 
  Twenhofel 
  — 
  Granite 
  Bowlders 
  in 
  (f 
  ) 
  

  

  There 
  is 
  much 
  variation 
  in 
  texture, 
  but 
  every 
  specimen 
  is 
  

   porphyritic. 
  In 
  some 
  the 
  texture 
  is 
  very 
  coarse 
  with 
  the 
  aver- 
  

   age 
  grain 
  approximating 
  one 
  fourth 
  inch 
  and 
  in 
  no 
  bowlder 
  

   was 
  a 
  microscopic 
  texture 
  observed. 
  Bowlders 
  of 
  coarse 
  and 
  

   fine 
  grain 
  occur 
  together 
  and 
  some 
  are 
  coarse-grained 
  on 
  one 
  

   portion 
  and 
  fine 
  on 
  another 
  with 
  the 
  dividing 
  line 
  between 
  

   the 
  two 
  textures 
  quite 
  sharply 
  defined. 
  This 
  juxtaposition 
  of 
  

   the 
  two 
  textures 
  may 
  have 
  been 
  produced 
  by 
  the 
  recementa- 
  

   tion 
  following 
  the 
  shattering, 
  the 
  fine-grained 
  granite 
  being 
  

   the 
  cement. 
  

  

  In 
  addition 
  to 
  the 
  granite 
  porphyry 
  bowlders, 
  three 
  or 
  four 
  of 
  

   chert 
  were 
  seen 
  and 
  on 
  the 
  most 
  western 
  mound 
  were 
  found 
  two 
  

   of 
  quartzite 
  of 
  which 
  the 
  larger 
  is 
  about 
  two 
  feet 
  in 
  diameter 
  

   and 
  is 
  composed 
  of 
  glassy 
  gray 
  sands 
  of 
  fine 
  grain. 
  Most 
  of 
  

   this 
  bowlder 
  is 
  still 
  on 
  the 
  mound. 
  The 
  smaller 
  of 
  the 
  two 
  

   bowlders 
  weighs 
  about 
  half 
  a 
  pound 
  and, 
  while 
  the 
  quartz 
  is 
  

   of 
  the 
  glassy 
  type, 
  it 
  is 
  of 
  a 
  greenish 
  shade. 
  This 
  bowlder 
  is 
  

   in 
  the 
  writer's 
  collection. 
  The 
  larger 
  quartzite 
  bowlder 
  is 
  so 
  

   deeply 
  corroded 
  as 
  to 
  have 
  a 
  spongy 
  appearance. 
  

  

  Every 
  one 
  of 
  the 
  porphyry 
  bowlders 
  is 
  more 
  or 
  less 
  obscurely 
  

   subangular 
  in 
  shape 
  ; 
  but, 
  as 
  previously 
  stated, 
  the 
  surface 
  of 
  

   every 
  one 
  of 
  them 
  is 
  much 
  weathered 
  and, 
  hence, 
  rough 
  and 
  

   pitted, 
  so 
  that 
  little 
  data 
  relating 
  to 
  the 
  character 
  of 
  the 
  origi- 
  

   nal 
  surface 
  could 
  be 
  obtained. 
  In 
  fact, 
  the 
  original 
  shapes 
  

   may 
  have 
  been 
  anything, 
  while 
  the 
  present 
  shapes 
  may 
  be 
  alto- 
  

   gether 
  due 
  to 
  weathering. 
  The 
  smaller 
  quartzite 
  bowlder 
  

   appears 
  originally 
  to 
  have 
  had 
  one 
  side 
  flat. 
  

  

  Possible 
  Methods 
  of 
  Origin 
  of 
  the 
  Boidders. 
  

  

  Four 
  hypotheses 
  may 
  be 
  considered. 
  They 
  are 
  as 
  follows 
  : 
  

   (1) 
  the 
  bowlders 
  are 
  firmly 
  cemented 
  masses 
  of 
  coarser 
  sedi- 
  

   ments 
  in 
  which 
  large 
  unworn 
  crystals 
  of 
  feldspar 
  were 
  import- 
  

   ant 
  as 
  constituents 
  ; 
  (2) 
  they 
  are 
  bowlders 
  of 
  weathering 
  

   derived 
  from 
  a 
  sheet, 
  dike, 
  or 
  flow 
  of 
  igneous 
  rock 
  which 
  is 
  

   there 
  enclosed 
  in 
  the 
  sediments 
  ; 
  (3) 
  they 
  were 
  carried 
  to 
  their 
  

   present 
  positions 
  by 
  streams; 
  (4) 
  ice 
  was 
  the 
  transporting 
  agent. 
  

  

  The 
  disposition 
  of 
  three 
  of 
  these 
  hypotheses 
  is 
  readily 
  accom- 
  

   plished. 
  The 
  bowlders 
  are 
  certainly 
  not 
  firmly 
  cemented 
  

   coarse 
  sediments, 
  for 
  such 
  a 
  possibility 
  is 
  precluded 
  by 
  the 
  

   absence 
  of 
  rounded 
  fragments, 
  the 
  presence 
  of 
  well-formed 
  

   crystals 
  of 
  feldspar, 
  the 
  intimate 
  dovetailing 
  of 
  the 
  minerals 
  

   and 
  the 
  total 
  absence 
  of 
  any 
  signs 
  of 
  deposition 
  in 
  the 
  material 
  

   composing 
  the 
  bowlders. 
  Every 
  one 
  of 
  the 
  granitic 
  bowlders 
  

   is 
  true 
  granite 
  and 
  each 
  was 
  directly 
  derived 
  from 
  a 
  common 
  

   parent 
  igneous 
  mass. 
  It 
  is 
  absolutely 
  impossible 
  to 
  hold 
  to 
  

   the 
  first 
  hypothesis. 
  

  

  