﻿Washington 
  — 
  Analyses 
  of 
  Italian 
  Volcanic 
  Rocks. 
  51 
  

  

  of 
  the 
  analysis 
  here 
  given. 
  It 
  might 
  also, 
  with 
  propriety, 
  be 
  

   called 
  a 
  minette, 
  though 
  it 
  is 
  rather 
  acid 
  for 
  this, 
  and 
  the 
  name 
  

   is 
  at 
  present 
  generally 
  applied 
  to 
  rocks 
  occurring 
  in 
  dikes. 
  

  

  Andesite, 
  Radicofani, 
  Tuscany. 
  — 
  Northeast 
  of 
  Lake 
  Bol- 
  

   sena, 
  and 
  east 
  of 
  Moute 
  Aniata, 
  there 
  rises 
  through 
  the 
  Plio- 
  

   cene 
  marls 
  a 
  volcanic 
  plug 
  or 
  neck, 
  which 
  forms 
  a 
  steep 
  hill, 
  

   crowned 
  with 
  a 
  ruined 
  castle, 
  and 
  with 
  the 
  small 
  village 
  of 
  

   Radicofani 
  at 
  its 
  southern 
  foot. 
  It 
  resembles 
  very 
  much 
  the 
  

   volcanic 
  necks 
  of 
  Uvalde 
  County, 
  Texas, 
  as 
  described 
  by 
  Hill 
  

   and 
  Y 
  a 
  ugh 
  an.* 
  

  

  The 
  neck 
  is 
  composed 
  for 
  the 
  most 
  part 
  of 
  a 
  dense 
  black, 
  

   basaltic-looking 
  rock, 
  which 
  is 
  light 
  gray 
  and 
  andesitic 
  in 
  places, 
  

   while 
  at 
  the 
  top 
  of 
  the 
  hill 
  it 
  is 
  reddish 
  and 
  highly 
  vesicular.f 
  

   A 
  columnar 
  structure 
  is 
  shown 
  here 
  and 
  there, 
  but 
  it 
  is 
  not 
  

   highly 
  developed. 
  The 
  accumulation 
  of 
  talus 
  at 
  its 
  base 
  

   absolutely 
  forbids 
  any 
  study 
  of 
  the 
  contact, 
  but 
  no 
  signs 
  of 
  

   metamorphism 
  were 
  visible 
  in 
  the 
  marls 
  outside 
  the 
  talus 
  slope. 
  

  

  The 
  locality 
  and 
  the 
  rock 
  have 
  been 
  described 
  by 
  vom 
  Rath,+ 
  

   Bucca§ 
  and 
  Mercalli,|| 
  but 
  a 
  short 
  account 
  of 
  my 
  specimens 
  

   will 
  not 
  be 
  out 
  of 
  place. 
  Megascopically 
  the 
  predominant 
  

   dark 
  variety 
  is 
  black 
  and 
  aphanitic, 
  with 
  some 
  scattered 
  minute 
  

   phenocrysts 
  of 
  diopside. 
  The 
  lighter 
  kind, 
  which 
  occurs 
  in 
  

   less 
  quantity, 
  is 
  rather 
  pale 
  gray, 
  also 
  dense, 
  and 
  shows 
  small, 
  

   yellowish 
  phenocrysts 
  of 
  diopside 
  and 
  olivine. 
  It 
  has 
  the 
  

   appearance 
  of 
  being 
  a 
  bleached 
  facies 
  of 
  the 
  darker 
  kind, 
  but 
  

   the 
  results 
  of 
  the 
  analyses 
  do 
  not 
  bear 
  out 
  this 
  supposition. 
  

  

  In 
  thin 
  section 
  little 
  difference 
  is 
  observable 
  between 
  the 
  

   two 
  varieties. 
  They 
  are 
  both 
  basaltic 
  in 
  structure, 
  and 
  show 
  

   crystals 
  and 
  fragments 
  of 
  pale 
  green 
  or 
  colorless 
  diopside, 
  

   some 
  labradorite, 
  and 
  rare 
  crystals 
  and 
  fragments 
  of 
  olivine, 
  

   lying 
  in 
  a 
  groundmass 
  of 
  diopside 
  grains, 
  labradorite 
  laths, 
  

   magnetite 
  and 
  apatite, 
  with 
  some 
  leptomorphic 
  orthoclase 
  (?), 
  

   and 
  interstitial, 
  brown 
  globulitic 
  glass. 
  

  

  An 
  analysis 
  was 
  made 
  of 
  specimens 
  of 
  each 
  variety, 
  and 
  the 
  

   results 
  are 
  given 
  in 
  I 
  and 
  II. 
  They 
  are 
  not 
  entirely 
  satisfac- 
  

   tory, 
  both 
  summing 
  up 
  too 
  high, 
  and 
  there 
  seems 
  to 
  be 
  a 
  small 
  

   constant 
  error, 
  which 
  I 
  am 
  not 
  able 
  to 
  place 
  without 
  duplicate 
  

   analyses, 
  for 
  which 
  time 
  is 
  lacking. 
  They 
  must, 
  however, 
  rep- 
  

   resent 
  fairly 
  w 
  T 
  ell 
  the 
  composition 
  of 
  the 
  rocks, 
  and 
  are 
  interest- 
  

   ing 
  in 
  being 
  almost 
  identical, 
  so 
  much 
  so 
  as 
  to 
  pass 
  muster 
  for 
  

   duplicates. 
  This 
  was 
  somewhat 
  surprising 
  in 
  view 
  of 
  their 
  

   quite 
  different 
  megascopic 
  appearance, 
  but 
  the 
  study 
  of 
  thin 
  

  

  * 
  Hill 
  and 
  Yaughan, 
  18th 
  Ann. 
  Rep. 
  U. 
  S. 
  G. 
  S., 
  p. 
  202 
  and 
  pi. 
  xxii, 
  1898. 
  

   f 
  This 
  summit 
  scoria 
  is 
  largely 
  used 
  in 
  the 
  country 
  round 
  about 
  for 
  scraping 
  

   the 
  hair 
  off 
  of 
  pig 
  skins, 
  and 
  is 
  hence 
  rapidly 
  disappearing. 
  

   % 
  Vom 
  Rath, 
  Zeitschr. 
  d. 
  d. 
  geol. 
  Ges.. 
  xvii, 
  p. 
  405, 
  1865. 
  

   § 
  Bucca, 
  Boll. 
  R. 
  Com. 
  Geol. 
  Ital., 
  1887. 
  p. 
  274. 
  

   | 
  Mercalli, 
  Atti 
  Soc. 
  Ital. 
  Sci. 
  Nat., 
  xxx' 
  1887. 
  

  

  