﻿46 
  Washington 
  — 
  Analyses 
  of 
  Italian 
  Volcanic 
  Rocks. 
  

  

  to 
  others, 
  it 
  is 
  to 
  be 
  hoped, 
  in 
  emphasizing 
  the 
  necessity 
  for 
  

   extreme 
  care 
  in 
  the 
  determination 
  of 
  MgO, 
  the 
  neglect 
  of 
  

   which 
  has 
  rendered 
  unreliable, 
  as 
  regards 
  A1 
  2 
  3 
  and 
  MgO, 
  

   many 
  an 
  otherwise 
  good 
  analysis. 
  

  

  " 
  Mica-trachyte 
  " 
  {Selagite), 
  Monte 
  Catini, 
  Tuscany. 
  — 
  The 
  

   occurrence 
  of 
  this 
  interesting 
  rock 
  forms 
  part 
  of 
  a 
  small 
  hill, 
  

   southwest 
  of 
  Volterra, 
  with 
  an 
  area 
  of 
  about 
  half 
  a 
  square 
  

   kilometer. 
  A 
  rough 
  columnar 
  structure 
  is 
  quite 
  evident. 
  

   According 
  to 
  Reyer* 
  the 
  rock 
  forms 
  a 
  surface 
  flow, 
  while 
  

   according 
  to 
  Lotti,f 
  with 
  whose 
  conclusions 
  my 
  own 
  observa- 
  

   tions 
  agree 
  in 
  the 
  main, 
  it 
  forms 
  a 
  sort 
  of 
  stock 
  or 
  volcanic 
  

   neck, 
  which 
  has 
  come 
  up 
  through 
  Tertiary 
  marls. 
  The 
  age 
  of 
  

   the 
  intrusion 
  is 
  undoubtedly 
  Post-Eocene, 
  and 
  very 
  probably 
  

   Post-Pliocene, 
  since 
  marls 
  apparently 
  of 
  this 
  age 
  have 
  been 
  

   altered 
  at 
  the 
  contact.:): 
  Copper 
  ores 
  (erubescite, 
  chalcopyrite, 
  

   chalcocite, 
  etc.) 
  are 
  mined 
  in 
  the 
  vicinity, 
  they 
  occurring 
  in 
  

   connection 
  with 
  dikes 
  of 
  diabase 
  or 
  gabbro. 
  The 
  most 
  

   abundant 
  salt 
  springs 
  of 
  Italy, 
  from 
  which 
  the 
  government 
  

   obtains 
  the 
  greater 
  portion 
  of 
  the 
  salt 
  used 
  in 
  the 
  kingdom, 
  

   are 
  in 
  the 
  Yal 
  di 
  Cecina 
  near 
  by. 
  

  

  The 
  rock 
  has 
  long 
  been 
  known, 
  having 
  been 
  described 
  by 
  

   Hauy,§ 
  who 
  named 
  it 
  " 
  selagite," 
  by 
  which 
  name 
  it 
  has 
  been 
  

   called 
  by 
  Italian 
  geologists. 
  Briefly 
  noticed 
  by 
  various 
  writers, 
  

   it 
  was 
  first 
  accurately 
  described 
  by 
  Rosenbusch 
  || 
  in 
  1880, 
  to 
  

   whose 
  description 
  but 
  little 
  can 
  be 
  added. 
  

  

  The 
  rock 
  specimens 
  show 
  very 
  many 
  brown 
  biotite 
  flakes 
  

   scattered 
  through 
  a 
  dull, 
  somewhat 
  earthy, 
  fine-grained, 
  green- 
  

   ish-gray 
  groundmass, 
  giving 
  it 
  much 
  the 
  appearance 
  of 
  a 
  

   minette. 
  Under 
  the 
  microscope, 
  sections 
  show 
  phenocrysts 
  of 
  

   biotite 
  in 
  large 
  thin 
  plates. 
  These 
  are 
  of 
  a 
  rather 
  pale 
  pinkish 
  

   or 
  yellowish 
  brown 
  color, 
  and 
  are 
  not 
  very 
  pleochroic. 
  Many 
  

   of 
  them 
  are 
  quite 
  colorless 
  at 
  the 
  center, 
  and 
  occasionally 
  one 
  

   sees 
  what 
  seems 
  to 
  be 
  a 
  skeleton 
  development, 
  at 
  the 
  borders, 
  

   producing, 
  in 
  basal 
  sections, 
  a 
  fringe 
  of 
  small 
  irregular 
  hex- 
  

   agonal 
  forms 
  about 
  the 
  edges 
  of 
  the 
  solid, 
  and 
  generally 
  lighter 
  

   colored, 
  interior. 
  

  

  The 
  groundmass 
  is 
  composed 
  of 
  small 
  colorless 
  crystals 
  and 
  

   anhedra 
  of 
  diopside, 
  some 
  green, 
  fibrous, 
  pilitic 
  pseudomorphs 
  

   after 
  olivine, 
  none 
  of 
  the 
  original 
  mineral 
  being 
  detected, 
  and 
  

   long 
  narrow 
  laths 
  of 
  feldspar, 
  which 
  is 
  unstriated, 
  and 
  gen- 
  

   erally 
  extinguishes 
  parallel 
  to 
  the 
  long 
  axis, 
  but 
  which 
  the 
  

   investigations 
  of 
  Rosenbusch 
  showed 
  to 
  be 
  of 
  both 
  alkali- 
  

  

  * 
  Reyer, 
  Aus 
  Toskana, 
  Wien, 
  1884, 
  p. 
  55. 
  

  

  f 
  Lotti, 
  Boll. 
  R. 
  Com. 
  Geol. 
  Ital 
  , 
  1884, 
  p. 
  36, 
  and 
  fig. 
  1, 
  pi. 
  viii. 
  

   {Lotti. 
  Boll. 
  R. 
  Com. 
  Geol. 
  Ital., 
  1885, 
  p. 
  254. 
  

  

  § 
  Cf. 
  Zirkel, 
  Lehrbuch, 
  ii, 
  p. 
  386, 
  1894, 
  and 
  Loewinson-Lessing, 
  Petr. 
  Lex., 
  

   p. 
  213, 
  1894. 
  

  

  I 
  Rosenbusch, 
  Neues 
  Jabrb 
  , 
  1880, 
  ii, 
  p. 
  206. 
  

  

  