﻿50 
  Washington 
  — 
  Analyses 
  of 
  Italian 
  Volcanic 
  Rocks. 
  

  

  The 
  close 
  correspondence, 
  chemically, 
  of 
  venanzite 
  with 
  

   the 
  madupite 
  of 
  the 
  Leucite 
  Hills 
  in 
  Wyoming 
  was 
  pointed 
  

   out 
  by 
  Rosenbusch, 
  and 
  it 
  is 
  very 
  interesting 
  to 
  observe 
  that 
  

   the 
  analysis 
  of 
  selagite 
  is 
  very 
  similar 
  to 
  those 
  of 
  wyomingite 
  

   and 
  orendite 
  from 
  the 
  same 
  region. 
  An 
  analysis 
  of 
  wyomingite 
  

   is 
  given 
  in 
  YI, 
  and, 
  although 
  a 
  little 
  lower 
  in 
  Si0 
  2 
  and 
  A1 
  2 
  3 
  , 
  

   and 
  considerably 
  higher 
  in 
  K 
  2 
  0, 
  yet 
  the 
  general 
  chemical 
  

   resemblance 
  between 
  the 
  two 
  magmas 
  is 
  obvious. 
  Wyomingite, 
  

   it 
  will 
  be 
  recalled,* 
  is 
  composed 
  of 
  leucite, 
  phlogopite 
  and 
  

   diopside, 
  and 
  hence 
  differs 
  considerably 
  from 
  the 
  selagite 
  in 
  

   mineralogical 
  composition. 
  

  

  It 
  may 
  seem 
  at 
  first 
  sight 
  that, 
  since 
  the 
  wyomingite 
  is 
  at 
  

   the 
  same 
  time 
  a 
  flow 
  and 
  rich 
  in 
  phlogopite, 
  this 
  analogy 
  in 
  

   chemical 
  composition 
  militates 
  against 
  the 
  views 
  on 
  the 
  split- 
  

   ting 
  up 
  of 
  the 
  biotite 
  molecule 
  which 
  have 
  just 
  been 
  expressed. 
  

   Examination 
  of 
  the 
  analyses 
  involved, 
  however, 
  shows 
  that, 
  

   while 
  that 
  of 
  wyomingite 
  is 
  similar 
  in 
  general 
  character, 
  it 
  

   differs 
  from 
  the 
  others 
  in 
  important 
  particulars. 
  It 
  is, 
  espe- 
  

   cially 
  as 
  compared 
  with 
  the 
  ciminite 
  analysis 
  given 
  on 
  page 
  44, 
  

   lower 
  in 
  Si0 
  2 
  , 
  A1 
  2 
  3 
  , 
  and 
  at 
  the 
  same 
  time 
  very 
  much 
  higher 
  in 
  

   K 
  2 
  0. 
  The 
  consequence 
  of 
  this 
  would 
  be 
  that 
  as 
  there 
  was 
  not 
  

   enough 
  AJ 
  2 
  3 
  to 
  satisfy 
  all 
  the 
  K 
  2 
  0, 
  the 
  surplus 
  of 
  this 
  condi- 
  

   tioned 
  the 
  formation 
  of 
  the 
  potash-mica, 
  phlogopite. 
  

  

  As 
  an 
  instance 
  of 
  the 
  complexity 
  of 
  these 
  problems 
  the 
  case 
  

   of 
  the 
  biotite-augite-latites 
  may 
  be 
  mentioned, 
  which 
  have 
  

   been 
  described 
  by 
  Ransome 
  in 
  the 
  paper 
  cited 
  above. 
  While 
  

   these 
  also 
  resemble 
  in 
  general 
  features 
  the 
  other 
  analyses, 
  yet 
  

   they 
  differ 
  in 
  the 
  much 
  higher 
  content 
  of 
  Si0 
  2 
  , 
  this 
  being 
  61 
  

   to 
  66 
  in 
  analyses 
  quoted 
  by 
  him.f 
  Here 
  we 
  have, 
  apparently, 
  

   the 
  additional 
  factor 
  introduced 
  of 
  the 
  tendency 
  of 
  biotite 
  to 
  

   form 
  in 
  the 
  more 
  acid 
  magmas, 
  while 
  augite 
  forms 
  in 
  the 
  more 
  

   basic, 
  but 
  otherwise 
  analogous, 
  magmas. 
  

  

  It 
  is 
  evident 
  from 
  the 
  foregoing 
  that 
  the 
  name 
  of 
  mica-tra- 
  

   chyte 
  is 
  a 
  misnomer, 
  both 
  chemically 
  and 
  mineralogically, 
  and 
  

   that 
  andesite 
  is 
  equally 
  inappropriate. 
  While 
  the 
  rock 
  is 
  

   chemically 
  equivalent 
  to 
  the 
  ciminites, 
  yet 
  the 
  abundant 
  bio- 
  

   tite, 
  which 
  gives 
  it 
  a 
  characteristic 
  lamprophyric 
  appearance, 
  

   serves 
  to 
  distinguish 
  it 
  sharply 
  from 
  these, 
  and 
  at 
  the 
  same 
  

   time 
  its 
  more 
  basic 
  composition 
  and 
  content 
  of 
  olivine 
  separate 
  

   it 
  from 
  the 
  vulsinites. 
  It 
  would 
  seem 
  then 
  advisable 
  to 
  revive 
  

   for 
  it 
  Haiiy's 
  old 
  name 
  of 
  selagite, 
  meaning 
  thereby 
  a 
  latitic 
  

   rock, 
  of 
  lamprophyric 
  appearance 
  or 
  structure, 
  composed 
  essen- 
  

   tially 
  of 
  alkali 
  and 
  soda-lime 
  feldspars, 
  biotite, 
  diopside 
  and 
  

   olivine, 
  and 
  with 
  a 
  chemical 
  composition 
  approximating 
  to 
  that 
  

  

  *Cf. 
  Cross, 
  this 
  Journal, 
  iv, 
  p. 
  138, 
  1897. 
  He 
  remarks 
  on 
  its 
  lamprophyric 
  

   character. 
  

  

  f 
  Ransome, 
  op. 
  cit., 
  p. 
  363. 
  

  

  