﻿LAURENTIAN 
  LIMESTONES 
  OF 
  NORTH 
  AMERICA. 
  81 
  

  

  to 
  several 
  other 
  hydrous 
  alumino-magnesian 
  silicates 
  approaching 
  

   to 
  loganite 
  in 
  composition, 
  and 
  resembling 
  serpentine 
  or 
  pyral- 
  

   lolite 
  in 
  sensible 
  characters. 
  Two 
  of 
  these, 
  described 
  and 
  analyzed 
  

   by 
  Dr. 
  Beck, 
  occur 
  in 
  the 
  Laurentian 
  limestones 
  of 
  Orange 
  county, 
  

   New 
  York. 
  These 
  form 
  altogether 
  an 
  interesting 
  and 
  but 
  little 
  

   understood 
  group 
  of 
  minerals, 
  which 
  are 
  perhaps 
  most 
  important 
  

   in 
  the 
  history 
  of 
  the 
  crystalline 
  limestones 
  than 
  has 
  hitherto 
  been 
  

   supposed. 
  

  

  Ortpioclase. 
  — 
  This 
  species 
  is 
  common 
  in 
  Laurentian 
  limestones, 
  

   generally 
  with 
  pyroxene 
  and 
  sphene, 
  and 
  sometimes 
  accompanied 
  

   by 
  scapolite 
  or 
  a 
  triclinic 
  feldspar. 
  The 
  orthoclase 
  of 
  the 
  veins 
  is, 
  

   as 
  I 
  have 
  shown, 
  sometimes 
  a 
  very 
  pure 
  potash-feldspar, 
  while 
  the 
  

   variety 
  named 
  loxoclase 
  by 
  Breithaupt, 
  which 
  occurs 
  with 
  pyrox- 
  

   ene 
  in 
  a 
  Laurentian 
  vein 
  in 
  Hammond, 
  New 
  York, 
  was 
  found 
  by 
  

   Smith 
  and 
  Brush 
  to 
  have 
  a 
  predominance 
  of 
  soda 
  (Geol 
  Can., 
  p. 
  

   475). 
  Large 
  isolated 
  crystals 
  of 
  white 
  orthoclase 
  are 
  found, 
  with 
  

   spinal, 
  apatite 
  and 
  fluor, 
  in 
  a 
  veinstone 
  of 
  lamellar 
  pink 
  carbonate 
  

   of 
  lime 
  in 
  Ross. 
  The 
  perthite 
  of 
  Burgess, 
  which 
  probably 
  belongs 
  

   to 
  a 
  granitic 
  veinstone, 
  is 
  also 
  an 
  example 
  of 
  an 
  orthoclase 
  with 
  a 
  

   large 
  amount 
  of 
  soda.* 
  An 
  orthoclase, 
  reddish-brown 
  in 
  color, 
  like 
  

   the 
  perthite, 
  but 
  without 
  its 
  aventurine-like 
  reflections, 
  also 
  occurs 
  

   in 
  Burgess, 
  mixed 
  with 
  a 
  little 
  quartz, 
  and 
  sometimes 
  with 
  green 
  

   apatite, 
  in 
  the 
  midst 
  of 
  a 
  large 
  vein 
  chiefly 
  of 
  apatite 
  and 
  calcite, 
  

   forming 
  a 
  granite-like 
  vertical 
  layer, 
  equidistant 
  from 
  the 
  two 
  

   walls 
  of 
  the 
  vein. 
  Orthoclase 
  is 
  sometimes 
  disseminated 
  in 
  the 
  

   beds 
  of 
  pyroxenite 
  which 
  accompany 
  the 
  Laurentian 
  limestones, 
  

   and 
  are 
  in 
  that 
  case 
  interstratified 
  with 
  beds 
  of 
  an 
  aggregate 
  of 
  

   orthoclase 
  and 
  quartz, 
  forming 
  a 
  granitic 
  gneiss, 
  into 
  which 
  the 
  

   pyroxenite 
  graduates. 
  

  

  Oligoclase. 
  — 
  To 
  this 
  species 
  I 
  refer 
  provisionally, 
  a 
  white 
  

   translucent 
  triclinic 
  feldspar, 
  which 
  occurs 
  in 
  small 
  masses 
  with 
  

   orthoclase, 
  pyroxene 
  and 
  sphene 
  in 
  a 
  vein 
  at 
  Grenville, 
  already 
  

   noticed. 
  A 
  similar 
  white 
  feldspar, 
  recognizable 
  by 
  the 
  beautiful 
  

   striation 
  of 
  its 
  cleavage 
  planes, 
  occurs 
  with 
  pyroxene 
  and 
  sphene 
  

   in 
  Willsborough, 
  New 
  York, 
  and 
  a 
  dark 
  green 
  triclinic 
  feldspar 
  is 
  

   found 
  with 
  apatite, 
  pyrites 
  and 
  magnetite 
  near 
  Dover, 
  New 
  Jersey, 
  

   and 
  with 
  magnetite 
  and 
  allanite 
  at 
  Port 
  Henry, 
  New 
  York. 
  None 
  

  

  * 
  It 
  has 
  since 
  been 
  shown 
  by 
  Gerhard 
  to 
  be 
  made 
  up 
  of 
  thin 
  layers 
  of 
  reddish 
  orthoclase 
  

   and 
  whitish 
  albite. 
  See 
  Dana's 
  Mineralogy, 
  fifth 
  edition, 
  page 
  356. 
  T. 
  S. 
  H. 
  

  

  