﻿454 
  Scientific 
  Intelligence. 
  

  

  calcification. 
  At 
  lower 
  depths 
  silicification 
  processes 
  predomi- 
  

   nated.* 
  

  

  The 
  Upper" 
  Huronian 
  beds 
  in 
  the 
  western 
  portions 
  of 
  the 
  area 
  

   consist 
  of 
  graywackes, 
  ferruginous 
  graywackes, 
  micaceous, 
  car- 
  

   bonaceous 
  and 
  ferruginous 
  clay-slates 
  and 
  their 
  crystalline 
  deriva- 
  

   tives, 
  and 
  thinly 
  laminated 
  cherty 
  siderite-slates, 
  ferruginous 
  

   cherts 
  and 
  iron 
  ores. 
  The 
  extensive 
  folding 
  which 
  the 
  series 
  has 
  

   undergone, 
  coupled 
  with 
  the 
  intrusions 
  of 
  igneous 
  rocks, 
  has 
  

   made 
  crystalline 
  schists 
  from 
  some 
  of 
  the 
  slates 
  and 
  graywackes. 
  

   Interlaminated 
  with 
  these 
  schists 
  are 
  also 
  some 
  perfectly 
  crystal- 
  

   line 
  hornblende-gneisses, 
  that 
  are 
  supposed 
  to 
  have 
  resulted 
  from 
  

   the 
  metamorphism 
  of 
  intrusive 
  sheets 
  or 
  interbedded 
  flows 
  of 
  

   basic 
  igneous 
  rocks. 
  Because 
  of 
  the 
  scarcity 
  of 
  exposures 
  the 
  

   authors 
  were 
  not 
  able 
  to 
  separate 
  the 
  Upper 
  Huronian 
  beds 
  into 
  

   formations. 
  

  

  The 
  ores 
  in 
  the 
  Upper 
  Huronian 
  are 
  non-Bessemer. 
  They 
  are 
  

   associated 
  with 
  cherts, 
  and 
  together 
  with 
  these 
  rocks 
  form 
  len- 
  

   ticular 
  bodies 
  lying 
  in 
  the 
  troughs 
  of 
  synclinal 
  folds 
  with 
  

   impervious 
  slate 
  bottoms. 
  

  

  Cutting 
  through 
  the 
  Huronian 
  sediments 
  are 
  various 
  acid 
  and 
  

   basic 
  intrusives 
  which 
  in 
  the 
  main 
  are 
  massive 
  and 
  possess 
  the 
  

   granular 
  texture. 
  Since 
  they 
  nowhere 
  seem 
  to 
  be 
  involved 
  in 
  

   the 
  folding 
  of 
  the 
  district 
  it 
  is 
  concluded 
  that 
  they 
  were 
  intruded 
  

   after 
  the 
  folding 
  took 
  place, 
  and 
  since 
  many 
  of 
  them 
  are 
  very 
  sim- 
  

   ilar 
  petrographically 
  to 
  the 
  lavas 
  of 
  the 
  Keweenawan 
  series 
  

   further 
  north, 
  they 
  are 
  correlated 
  with 
  the 
  latter. 
  The 
  time 
  of 
  

   folding 
  of 
  the 
  district 
  is 
  therefore 
  placed 
  at 
  the 
  beginning 
  of 
  

   Keweenawan 
  time. 
  

  

  Lithologically 
  the 
  intrusives 
  are 
  classed 
  as 
  biotite-granites, 
  

   muscovite-biotite 
  granites, 
  metadolerites, 
  metabasalts 
  and 
  picrite 
  

   porphyries 
  or 
  porphyritic 
  limburgites. 
  The 
  dolerites 
  have 
  meta- 
  

   morphosed 
  the 
  Mansfield 
  slates 
  with 
  which 
  they 
  are 
  in 
  contact, 
  

   producing 
  from 
  them 
  spilosites, 
  desmisites 
  and 
  adinoles.f 
  

  

  Near 
  the 
  town 
  of 
  Crystal 
  Falls 
  there 
  is 
  an 
  extremely 
  interest- 
  

   ing 
  series 
  of 
  intrusions 
  ranging 
  from 
  acid 
  granites, 
  tonalites 
  and 
  

   quartz-mica 
  diorites 
  through 
  diorites 
  of 
  intermediate 
  acidity 
  to 
  

   basic 
  hornblende-gabbros, 
  gabbros, 
  norites 
  and 
  various 
  peridotites. 
  

   It 
  was 
  not 
  possible 
  to 
  determine 
  which 
  of 
  these 
  rocks 
  most 
  nearly 
  

   resembles 
  in 
  composition 
  the 
  original 
  magma 
  of 
  which 
  the 
  various 
  

   types 
  are 
  differentiation 
  products. 
  It 
  is 
  known, 
  however, 
  that 
  

   the 
  hornblende-gabbro 
  first 
  reached 
  its 
  present 
  position, 
  and 
  that 
  

   in 
  general 
  the 
  forces 
  of 
  differentiation 
  were 
  toward 
  increasing 
  

   acidity 
  and 
  increasing 
  basicity. 
  

  

  * 
  Since 
  the 
  publication 
  of 
  this 
  monograph 
  the 
  results 
  of 
  a 
  detailed 
  exami- 
  

   nation 
  of 
  the 
  Menominee 
  district, 
  which 
  is 
  more 
  closely 
  related 
  to 
  the 
  Crystal 
  

   Falls 
  district 
  than 
  is 
  the 
  Marquette 
  area, 
  seems 
  to 
  show 
  that 
  the 
  Mansfield 
  slate, 
  

   the 
  G-roveland 
  formation 
  and 
  the 
  Hemlock 
  formation 
  are 
  all 
  members 
  of 
  the 
  

   Upper 
  Huronian 
  and 
  that 
  the 
  Bessemer 
  Mansfield 
  ore 
  is 
  the 
  equivalent 
  of 
  the 
  

   Upper 
  Huronian 
  Bessemer 
  ores 
  of 
  the 
  Menominee 
  range. 
  The 
  division 
  between 
  

   the 
  Lower 
  and 
  Upper 
  Huronian 
  would, 
  therefore, 
  seem 
  to 
  be 
  between 
  the 
  Rand- 
  

   vine 
  dolomite 
  and 
  the 
  Mansfield 
  slate. 
  

  

  f 
  Cf. 
  this 
  Journal, 
  IV, 
  vii, 
  81, 
  1899. 
  

  

  