﻿LAURENTIAN 
  LIMESTONES 
  OF 
  NORTH 
  AMERICA. 
  67 
  

  

  vicinity 
  of 
  Passau 
  are 
  found, 
  under 
  conditions 
  closely 
  similar 
  to 
  

   those 
  of 
  Canada 
  and 
  New 
  York, 
  in 
  the 
  same 
  geological 
  system. 
  

   The 
  crystalline 
  limestone 
  band 
  near 
  Passau, 
  which 
  occurs 
  in 
  horn- 
  

   blendic 
  gneiss, 
  is 
  from 
  fifty 
  to 
  seventy 
  feet 
  in 
  thickness, 
  and 
  is 
  

   directly 
  overlaid 
  by 
  a 
  bed 
  of 
  several 
  feet 
  of 
  hornblende 
  slate, 
  

   between 
  which 
  and 
  the 
  limestone, 
  a 
  bed 
  of 
  three 
  or 
  four 
  feet 
  of 
  

   serpentine 
  is 
  interposed, 
  and 
  in 
  other 
  parts 
  a 
  layer 
  of 
  nearly 
  com- 
  

   pact 
  scapolite, 
  mingled 
  with 
  hornblende 
  and 
  chlorite. 
  The 
  strati- 
  

   fied 
  granular 
  limestone 
  beneath 
  contains, 
  among 
  other 
  minerals, 
  

   serpentine, 
  chondrodite, 
  hornblende, 
  mica, 
  scapolite, 
  garnet 
  and 
  

   graphite 
  ; 
  the 
  disseminated 
  serpentine 
  here, 
  as 
  in 
  Canada, 
  replac- 
  

   ing 
  the 
  Eozoon 
  Canadense. 
  

  

  The 
  occurrence 
  of 
  iolite, 
  as 
  a 
  frequent 
  element 
  in 
  the 
  Laurentian 
  

   gneiss 
  of 
  Bavaria, 
  is 
  a 
  fact 
  of 
  interest, 
  inasmuch 
  as 
  it 
  is 
  also 
  one 
  

   of 
  the 
  minerals 
  of 
  the 
  same 
  ancient 
  gneiss 
  in 
  Scandinavia, 
  and 
  may 
  

   be 
  looked 
  for 
  in 
  this 
  country, 
  although 
  it 
  has 
  not 
  yet 
  been 
  detected 
  

   in 
  the 
  undoubted 
  Laurentian 
  rocks 
  of 
  North 
  America.* 
  

  

  The 
  Hercynian 
  clay-slate 
  series 
  of 
  Bavaria, 
  already 
  referred 
  to, 
  

   and 
  supposed 
  by 
  Gtimbel 
  to 
  correspond 
  to 
  our 
  Huronian 
  series, 
  

   includes 
  a 
  formation 
  of 
  crystalline 
  limestones 
  more 
  than 
  300 
  feet 
  

   in 
  thickness, 
  containing, 
  like 
  the 
  older 
  limestone 
  of 
  the 
  Lauren- 
  

   tian 
  system, 
  graphite, 
  chondrodite, 
  hornblende 
  and 
  serpentine, 
  the 
  

   latter 
  two 
  minerals 
  replacing 
  a 
  peculiar 
  and 
  distinct 
  species 
  of 
  

   Eozoon, 
  named 
  Eozoon 
  Bavaricwn. 
  

  

  Allusion 
  has 
  been 
  made 
  to 
  the 
  crystalline 
  limestones 
  which 
  

   occur 
  in 
  Bolton 
  and 
  the 
  adjoining 
  towns 
  in 
  Eastern 
  Massachusetts, 
  

   and 
  resemble 
  in 
  geognostic 
  and 
  mineralogical 
  characters 
  those 
  

   of 
  the 
  Laurentian 
  system. 
  There 
  are, 
  however, 
  not 
  wanting 
  rea- 
  

   sons 
  for 
  supposing 
  them 
  to 
  belong 
  to 
  a 
  more 
  recent 
  geologic 
  

   period, 
  and 
  the 
  facts 
  recently 
  observed 
  in 
  Bavaria, 
  and 
  detailed 
  

   above, 
  show 
  what 
  was 
  antecedently 
  probable, 
  that 
  similar 
  mine- 
  

   ralogical 
  characteristics 
  may 
  be 
  found 
  in 
  crystalline 
  limestones 
  of 
  

   very 
  different 
  ages. 
  

  

  In 
  this 
  connection, 
  it 
  is 
  not 
  without 
  interest 
  to 
  recall 
  the 
  mineral 
  

   characters 
  of 
  the 
  rocks 
  of 
  Ceylon, 
  which 
  present 
  many 
  striking 
  

   resemblances 
  to 
  the 
  Laurentian 
  strata 
  of 
  North 
  America, 
  and 
  may 
  

   perhaps 
  be 
  found 
  to 
  belong 
  to 
  the 
  same 
  system. 
  The 
  island 
  was, 
  

  

  * 
  Iolite, 
  as 
  I 
  have 
  elsewhere 
  remarked, 
  is 
  related 
  to 
  the 
  feldspars, 
  of 
  which 
  it 
  has 
  the 
  

   atomic 
  \olume, 
  and 
  may 
  be 
  looked 
  upon, 
  chemically, 
  as 
  a 
  feldspar, 
  with 
  the 
  oxygen 
  ratios, 
  

   5 
  : 
  3 
  : 
  1 
  (intermediate 
  between 
  labradorite 
  and 
  anorthite, 
  and 
  corresponding 
  to 
  barsowite 
  

   and 
  bytownite), 
  in 
  which 
  magnesia, 
  sometimes 
  with 
  protoxide 
  of 
  iron 
  takes 
  the 
  place 
  of 
  

   lime 
  and 
  soda. 
  

  

  