﻿48 
  Washington 
  — 
  Analyses 
  of 
  Italian 
  Volcanic 
  Mocks. 
  

  

  blance 
  to 
  some 
  analyses 
  of 
  minettes 
  are 
  at 
  once 
  evident, 
  though 
  

   these 
  last 
  usually 
  show 
  less 
  silica. 
  It 
  may 
  be 
  noted 
  in 
  this 
  con- 
  

   nection 
  that 
  Rosen 
  bnsch* 
  has 
  already 
  spoken 
  of 
  it 
  as 
  " 
  a 
  

   younger 
  equivalent 
  of 
  certain 
  minettes." 
  The 
  nearest 
  approach 
  

   to 
  it 
  among 
  the 
  various 
  minette 
  analyses 
  which 
  I 
  have 
  exam- 
  

   ined 
  is 
  that 
  of 
  Lepsius 
  of 
  a 
  dike 
  in 
  the 
  Odenwald, 
  as 
  given 
  in 
  

   VI, 
  which, 
  although 
  somewhat 
  decomposed 
  and 
  with 
  notably 
  

   lower 
  potash, 
  is 
  of 
  essentially 
  the 
  same 
  character. 
  Another 
  

   rock 
  with 
  which 
  this 
  may 
  be 
  comparedf 
  is 
  the 
  verite 
  of 
  Osann 
  

   (VIII), 
  in 
  which, 
  however, 
  the 
  alkalies 
  are 
  in 
  inverse 
  ratio. 
  

  

  But 
  it 
  is 
  of 
  special 
  interest 
  to 
  compare 
  the 
  rock 
  at 
  present 
  

   under 
  discussion 
  with 
  others 
  from 
  its 
  own 
  region. 
  That 
  with 
  

   which 
  it 
  presents 
  the 
  closest 
  analogies 
  are 
  the 
  ciminites,J 
  the 
  

   analyses 
  of 
  which 
  have 
  just 
  been 
  given. 
  Although 
  they 
  differ 
  

   to 
  a 
  small 
  extent 
  in 
  certain 
  particulars, 
  as 
  in 
  Ti0 
  2 
  , 
  A1 
  2 
  3 
  , 
  CaO 
  

   and 
  Na 
  2 
  0, 
  yet 
  the 
  general 
  correspondence 
  is 
  remarkably 
  close. 
  

   This 
  is 
  of 
  interest 
  since 
  they 
  differ 
  considerably 
  in 
  mineralog- 
  

   ical 
  composition, 
  the 
  abundant 
  and 
  prominent 
  biotites 
  of 
  the 
  

   Tuscan 
  rock 
  being 
  entirely 
  absent 
  in 
  the 
  ciminite, 
  while 
  the 
  

   latter 
  carries 
  a 
  plagioclase 
  richer 
  in 
  lime, 
  more 
  abundant 
  oli- 
  

   vine 
  and 
  orthoclase, 
  and 
  less 
  augite. 
  They 
  form 
  another 
  addi- 
  

   tion 
  to 
  the 
  growing 
  list 
  of 
  cases 
  of 
  magmas 
  of 
  similar 
  chemical 
  

   composition 
  solidifying 
  under 
  diverse 
  conditions 
  to 
  form 
  quite 
  

   different 
  mineral 
  aggregates. 
  

  

  With 
  this 
  may 
  also 
  be 
  compared 
  the 
  so-called 
  biotite-vulsin- 
  

   ite 
  of 
  Rocca 
  Monfina, 
  the 
  analysis 
  of 
  which 
  is 
  given 
  in 
  II. 
  

   This 
  has 
  been 
  described 
  elsewhere,§ 
  but 
  it 
  may 
  be 
  recalled 
  

   that 
  it 
  is 
  composed 
  of 
  alkali-feldspar 
  and 
  labradorite, 
  biotite 
  

   and 
  diopside, 
  with 
  no 
  olivine. 
  The 
  analysis, 
  as 
  compared 
  with 
  

   I, 
  shows 
  higher 
  A1 
  2 
  3 
  and 
  CaO, 
  and 
  lower 
  MgO 
  and 
  K 
  2 
  0, 
  

   standing 
  between 
  those 
  of 
  the 
  Monte 
  Catini 
  rock 
  or 
  the 
  cimi- 
  

   nites 
  and 
  those 
  of 
  the 
  vnlsinites, 
  one 
  of 
  which 
  is 
  given 
  for 
  

   comparison 
  in 
  IV. 
  While 
  in 
  II, 
  A1 
  2 
  3 
  may 
  be 
  a 
  trifle 
  high 
  

   and 
  MgO 
  a 
  trifle 
  low, 
  yet 
  the 
  analysis 
  corresponds 
  so 
  well 
  with 
  

   the 
  appearance 
  of 
  the 
  rock 
  in 
  thin 
  section 
  that 
  it 
  may 
  be 
  

   regarded 
  as 
  quite 
  trustworthy. 
  The 
  absence 
  of 
  olivine 
  corre- 
  

   lated 
  with 
  lower 
  MgO 
  is 
  of 
  especial 
  importance. 
  There 
  was 
  

   sufficient 
  MgO 
  in 
  the 
  selagite 
  magma 
  to 
  allow 
  the 
  formation 
  

   of 
  olivine 
  after 
  the 
  crystallization 
  of 
  the 
  biotite, 
  while 
  in 
  the 
  

   biotite-vulsinite 
  the 
  early 
  formation 
  of 
  this 
  mineral 
  and 
  augite 
  

   used 
  up 
  all 
  the 
  available 
  supply. 
  As 
  regards 
  the 
  olivine-bear- 
  

   ing 
  ciminites, 
  attention 
  has 
  already 
  been 
  called 
  to 
  the 
  fact 
  that 
  

  

  *Rosenbusch, 
  Neues 
  Jahrb., 
  1880, 
  ii, 
  p. 
  206. 
  

  

  f 
  Kosenbusch 
  (Elem. 
  Gesteinslchre, 
  1898, 
  p. 
  301) 
  has 
  already 
  noted 
  the 
  resem- 
  

   blance 
  between 
  the 
  two 
  rocks. 
  

  

  \ 
  The 
  probability 
  of 
  this 
  close 
  resemblance 
  has 
  been 
  already 
  suggested. 
  (Jour. 
  

   Geol., 
  v, 
  p. 
  354, 
  1897.) 
  

  

  § 
  Jour. 
  Geol., 
  v, 
  p. 
  251, 
  1891. 
  

  

  