﻿Geology. 
  453 
  

  

  Felch 
  Mountain 
  district 
  Smyth 
  distinguishes 
  the 
  following 
  : 
  "The 
  

   Sturgeon 
  quart 
  zite, 
  with 
  an 
  average 
  thickness 
  of 
  about 
  450 
  ft. 
  ; 
  

   the 
  Randville 
  dolomite, 
  with 
  an 
  average 
  thickness 
  of 
  750 
  ft. 
  ; 
  

   the 
  Mansfield 
  schist, 
  a 
  mica-schist 
  derived 
  from 
  some 
  clastic 
  rock, 
  

   with 
  a 
  thickness 
  of 
  about 
  200 
  ft., 
  and 
  the 
  Groveland 
  iron 
  forma- 
  

   tion, 
  composed 
  of 
  two 
  kinds 
  of 
  rock 
  — 
  one 
  consisting 
  of 
  quartz, 
  

   hematite 
  and 
  magnetite, 
  and 
  the 
  other 
  of 
  an 
  iron-amphibole 
  with 
  

   quartz 
  and 
  the 
  iron 
  oxides 
  as 
  associates 
  (griinerite-schists). 
  The 
  

   quartz-iron 
  oxide 
  rocks 
  are 
  composed 
  partly 
  of 
  clastic 
  quartz 
  and 
  

   partly 
  of 
  crystalline 
  silica. 
  In 
  this 
  respect 
  the 
  ore-bearing 
  mem- 
  

   ber 
  of 
  the 
  formation 
  is 
  intermediate 
  in 
  character 
  between 
  the 
  

   crystalline 
  ore-bearing 
  member 
  of 
  the 
  Negaunee 
  formation 
  in 
  the 
  

   Marquette 
  district 
  and 
  the 
  clastic 
  ore-bearing 
  beds 
  at 
  the 
  base 
  of 
  

   the 
  Ishpeming 
  formation. 
  The 
  iron 
  oxides 
  are 
  thought 
  to 
  have 
  

   been 
  derived 
  from 
  glauconite 
  or 
  iron 
  carbonate 
  deposited 
  with 
  

   the 
  original 
  sediments. 
  

  

  In 
  the 
  western 
  portion 
  of 
  the 
  district 
  the 
  Lower 
  Huronian 
  

   series 
  is 
  represented 
  by 
  the 
  Randville 
  dolomite, 
  the 
  Mansfield 
  

   slate 
  and 
  a 
  great 
  thickness 
  of 
  volcanic 
  deposits 
  known 
  as 
  the 
  

   Hemlock 
  formation. 
  The 
  slate 
  formation 
  comprises 
  graywackes, 
  

   clay 
  slates, 
  phyllites, 
  cherts 
  and 
  iron 
  ores, 
  together 
  with 
  certain 
  

   other 
  rocks 
  derived 
  from 
  these. 
  Its 
  maximum 
  thickness 
  is 
  1900 
  

   ft. 
  and 
  in 
  it 
  is 
  the 
  only 
  productive 
  Bessemer 
  ore-producing 
  mine 
  

   in 
  the 
  district. 
  

  

  The 
  Hemlock 
  formation 
  consists 
  almost 
  exclusively 
  of 
  basic 
  

   and 
  acid 
  volcanic 
  rocks, 
  both 
  lavas 
  and 
  tuffs, 
  and 
  several 
  varieties 
  

   of 
  crystalline 
  schists 
  derived 
  from 
  them. 
  The 
  thickness 
  esti- 
  

   mated 
  from 
  the 
  dip 
  is 
  about 
  23,000 
  ft. 
  but 
  this 
  estimate 
  is 
  regarded 
  

   as 
  excessive. 
  The 
  figures 
  are 
  thought 
  to 
  result 
  from 
  the 
  preseuce 
  

   of 
  undiscovered 
  reduplication 
  of 
  beds 
  due 
  to 
  folding. 
  The 
  acid 
  

   volcanics 
  are 
  rhyolites, 
  the 
  basic 
  ores 
  are 
  basalts. 
  The 
  latter 
  are 
  

   largely 
  pyroclastic, 
  the 
  former 
  principally 
  devitrified 
  glassy 
  lavas. 
  

   The 
  basic 
  lavas 
  are 
  characterized 
  almost 
  everywhere 
  by 
  a 
  well- 
  

   marked 
  ellipsoidal 
  structure. 
  The 
  ellipsoids 
  vary 
  in 
  size 
  from 
  a 
  

   few 
  inches 
  to 
  6 
  or 
  8 
  inches 
  in 
  diameter. 
  They 
  are 
  usually 
  ellip- 
  

   soidal 
  in 
  cross 
  section, 
  and 
  their 
  longer 
  axes 
  are 
  in 
  the 
  supposed 
  

   direction 
  of 
  flow 
  of 
  the 
  rock 
  in 
  which 
  they 
  occur. 
  They 
  may 
  or 
  

   may 
  not 
  be 
  amygdaloidal. 
  When 
  amygdaloidal 
  the 
  amygdules 
  

   are 
  as 
  a 
  rule 
  distributed 
  through 
  the 
  ellipsoids, 
  but 
  on 
  the 
  whole 
  

   the 
  masses 
  are 
  more 
  scoriaceous 
  on 
  the 
  periphery 
  than 
  near 
  the 
  

   center. 
  In 
  exceptional 
  cases 
  the 
  amygdules 
  are 
  on 
  the 
  upper 
  sides 
  

   of 
  the 
  ellipsoids. 
  The 
  matrix 
  between 
  the 
  ellipsoids 
  is 
  often 
  schis- 
  

   tose, 
  but 
  at 
  times 
  it 
  appears 
  almost 
  massive. 
  Sometimes 
  it 
  is 
  brec- 
  

   ciated. 
  This 
  structure 
  together 
  with 
  much 
  of 
  the 
  spheroidal 
  struc- 
  

   ture 
  described 
  in 
  basic 
  lavas 
  from 
  other 
  localities, 
  is 
  explained 
  by 
  

   Clements 
  as 
  due 
  to 
  the 
  fact 
  that 
  the 
  lava 
  was 
  originally 
  of 
  the 
  

   aa 
  form 
  of 
  the 
  Hawaian 
  volcanoes, 
  and 
  that 
  it 
  has 
  since 
  been 
  

   metamorphosed 
  under 
  the 
  influence 
  of 
  pressure. 
  

  

  All 
  of 
  the 
  lavas 
  have 
  been 
  much 
  altered 
  since 
  their 
  eruption. 
  

   In 
  the 
  zone 
  of 
  weathering 
  the 
  alteration 
  has 
  consisted 
  largely 
  of 
  

  

  