﻿54 
  Washington 
  — 
  Analyses 
  of 
  Italian 
  Volcanic 
  Rocks. 
  

  

  The 
  apparently 
  high 
  A1 
  2 
  3 
  is 
  largely 
  taken 
  up 
  by 
  the 
  

   abundant 
  leucite, 
  while 
  the 
  melilite 
  and 
  especially 
  the 
  nephelite 
  

   also 
  demand 
  quite 
  an 
  amount 
  of 
  it. 
  Owing 
  to 
  the 
  composition 
  

   of 
  the 
  melilite 
  and 
  diopside 
  a 
  calculation 
  of 
  the 
  mineral 
  com- 
  

   position 
  of 
  the 
  rock 
  is 
  not 
  easy 
  or 
  satisfactory. 
  Assuming^ 
  

   however, 
  that 
  by 
  far 
  the 
  greater 
  part 
  of 
  the 
  K 
  2 
  and 
  Na 
  2 
  D 
  

   belongs 
  in 
  the 
  leucite 
  and 
  nephelite, 
  it 
  is 
  calculated 
  that 
  leucite 
  

   makes 
  up 
  about 
  52 
  and 
  nephelite 
  about 
  10 
  per 
  cent, 
  leaving 
  

   some 
  38 
  per 
  cent 
  of 
  melilite, 
  diopside 
  and 
  magnetite. 
  It 
  is 
  

   difficult 
  to 
  say 
  exactly 
  how 
  these 
  are 
  distributed, 
  but 
  a 
  rough 
  

   estimate, 
  combined 
  with 
  the 
  microscopical 
  examination, 
  gives 
  

   melilite 
  and 
  diopside 
  each 
  about 
  17 
  per 
  cent 
  and 
  magnetite 
  

   about 
  5 
  or 
  less. 
  

  

  In 
  the 
  leucitite 
  of 
  Bearpaw 
  Mountain, 
  Montana 
  (III), 
  

   described 
  by 
  Pirsson,* 
  MgO 
  is 
  also 
  low, 
  though 
  A1 
  2 
  8 
  is 
  con- 
  

   siderably 
  lower 
  than 
  in 
  the 
  Alban 
  rock. 
  The 
  Montana 
  rock, 
  

   however, 
  is 
  composed 
  only 
  of 
  leucite 
  (57*1 
  per 
  cent), 
  diopside 
  

   (31*1 
  per 
  cent) 
  and 
  magnetite 
  (11*8 
  per 
  cent), 
  and 
  contains 
  

   neither 
  nephelite 
  nor 
  melilite. 
  

  

  Comparison 
  of 
  these 
  analyses 
  with 
  that 
  of 
  venanzite 
  given 
  

   on 
  a 
  preceding 
  page 
  is 
  of 
  interest. 
  The 
  lower 
  SiO 
  a 
  and 
  Al 
  2 
  O 
  s 
  

   and 
  higher 
  MgO 
  in 
  this 
  latter 
  have 
  conditioned 
  the 
  abundant 
  

   olivine 
  and 
  considerable 
  biotite, 
  both 
  of 
  which 
  are 
  wholly 
  

   absent 
  in 
  the 
  leucitite. 
  The 
  nepheline 
  of 
  the 
  latter 
  at 
  the 
  

   Alban 
  Hills 
  is, 
  conversely, 
  lacking 
  in 
  the 
  venanzite. 
  

  

  In 
  this 
  connection 
  I 
  would 
  express 
  my 
  complete 
  agreement 
  

   with 
  Dr. 
  Crossf 
  in 
  his 
  suggestion 
  that 
  "the 
  term 
  leucitite 
  be 
  

   reserved 
  for 
  the 
  rock 
  which 
  has 
  not 
  yet 
  been 
  discovered, 
  con- 
  

   sisting 
  essentially 
  of 
  leucite, 
  with 
  all 
  other 
  minerals 
  of 
  subordi- 
  

   nate 
  importance. 
  There 
  is 
  good 
  reason 
  to 
  believe 
  that 
  such 
  

   rocks 
  are 
  possible 
  and 
  will 
  be 
  found 
  at 
  no 
  distant 
  day. 
  The 
  

   same 
  suggestion 
  is 
  made 
  for 
  nephelinite, 
  on 
  the 
  same 
  grounds." 
  

  

  Following 
  out 
  this 
  idea 
  it 
  is 
  evident 
  from 
  the 
  above 
  esti- 
  

   ment 
  of 
  the 
  mineralogical 
  composition 
  of 
  the 
  Capo 
  di 
  Bove 
  

   rock, 
  however 
  crude 
  it 
  may 
  be, 
  that 
  the 
  term 
  leucitite 
  is 
  not 
  

   quite 
  appropriate. 
  The 
  same 
  would 
  hold 
  true 
  of 
  the 
  term 
  as 
  

   applied 
  to 
  any 
  of 
  the 
  Italian 
  or 
  other 
  leucitites 
  so 
  far 
  as 
  known. 
  

   The 
  present 
  is 
  not 
  the 
  place 
  for 
  any 
  discussion, 
  nor 
  for 
  the 
  

   bestowal 
  of 
  new 
  names, 
  but 
  the 
  fact 
  is 
  obvious 
  that 
  a 
  critical 
  

   revision 
  and 
  reclassification 
  of 
  all 
  the 
  leucitic 
  rocks 
  is 
  highly 
  

   desirable. 
  

  

  Locust, 
  N. 
  J., 
  November, 
  1899. 
  

  

  * 
  Weed 
  and 
  Pirsson, 
  this 
  Journal, 
  ii, 
  p. 
  143, 
  1896. 
  

   f 
  Cross, 
  this 
  Journal, 
  iv, 
  p. 
  137, 
  1897. 
  

  

  