﻿Washington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia. 
  579 
  

  

  Doelter, 
  Pannenberg, 
  Deprat, 
  and 
  myself 
  confirm 
  his 
  views 
  as 
  

   to 
  the 
  broad 
  features 
  of 
  Ferru. 
  

  

  The 
  specimens 
  collected 
  by 
  me 
  are 
  of 
  the 
  following 
  rocks 
  : 
  

  

  Rhyolite, 
  liparose, 
  (1.4.1.3) 
  

   Trachyte, 
  phlegrose, 
  (1.5.1.3) 
  

  

  Andesite 
  \ 
  dacose 
  ' 
  ( 
  IL4 
  - 
  2 
  - 
  4 
  ) 
  

   Andesite, 
  -j 
  tonaloge? 
  (H.4.3.4) 
  

  

  Basalt, 
  andose, 
  (II.5.3.4) 
  

  

  Delia 
  Marmora 
  mentions 
  phonolites, 
  apparently 
  like 
  those 
  

   of 
  Ferru, 
  as 
  being 
  rather 
  common, 
  but 
  I 
  could 
  find 
  no 
  occur- 
  

   rence 
  of 
  such 
  rocks. 
  They 
  would 
  seem, 
  however, 
  to 
  occur 
  

   most 
  frequently 
  in 
  the 
  eastern 
  part 
  of 
  the 
  volcano, 
  which 
  I 
  did 
  

   not 
  visit. 
  As, 
  however, 
  della 
  Marmora's 
  petrography 
  was 
  that 
  

   of 
  the 
  first 
  half 
  of 
  the 
  nineteenth 
  century, 
  it 
  is 
  possible 
  that 
  

   his 
  "phonolites" 
  would 
  not 
  be 
  called 
  so 
  now. 
  His 
  "trachytes" 
  

   are 
  in 
  reality 
  rhyolites. 
  

  

  Rhyolite 
  (Liparose, 
  L.J/..1.3) 
  

  

  Rhyolites 
  of 
  various 
  types 
  are 
  the 
  most 
  abundant 
  rocks 
  in 
  

   the 
  portions 
  of 
  the 
  volcano 
  visited 
  by 
  me. 
  Their 
  massive 
  flows 
  

   form 
  the 
  bulk 
  of 
  the 
  spurs 
  along 
  the 
  southwest 
  edge 
  of 
  the 
  

   mass 
  and 
  are 
  well 
  exposed 
  in 
  the 
  narrow 
  valleys 
  (so-called 
  

   concas) 
  which 
  cut 
  into 
  it. 
  Judging 
  from 
  the 
  descriptions 
  of 
  

   della 
  Marmora* 
  these 
  rhyolites 
  unquestionably 
  constitute 
  in 
  

   great 
  part 
  the 
  internal 
  domal 
  core 
  of 
  the 
  volcano, 
  reaching 
  up 
  

   to 
  the 
  basalt-crowned 
  summits 
  of 
  the 
  Trebinas. 
  

  

  Lithoidal 
  Rhyolite. 
  — 
  What 
  may 
  be 
  regarded 
  as 
  the 
  typical 
  

   rhyolite 
  (the 
  trachyte 
  ancienne 
  of 
  della 
  Marmora) 
  is 
  dense 
  

   and 
  lithoidal, 
  of 
  a 
  dull 
  luster 
  and 
  even 
  fracture. 
  It 
  is 
  very 
  

   light 
  in 
  color, 
  usually 
  an 
  ash-gray, 
  but 
  sometimes 
  pinkish 
  

   or 
  bluish 
  or 
  even 
  white. 
  It 
  is 
  often 
  somewhat 
  banded 
  with 
  

   narrow 
  streaks 
  of 
  lighter 
  color 
  which 
  show 
  well-marked 
  flow 
  

   structure. 
  The 
  flows 
  are 
  generally 
  very 
  massive 
  and 
  com- 
  

   pact, 
  but 
  in 
  the 
  Canale 
  Perdiera, 
  north 
  of 
  Uras, 
  the 
  rhyolite 
  is 
  

   platy, 
  splitting 
  readily 
  into 
  thin 
  (horizontal) 
  slabs, 
  which 
  may 
  

   be 
  not 
  more 
  than 
  a 
  centimeter 
  or 
  so 
  thick. 
  Rhyolitic 
  tuffs 
  

   seem 
  to 
  be 
  very 
  rare. 
  

  

  The 
  type 
  is 
  practically 
  aphyric, 
  the 
  only 
  phenocrysts 
  visible 
  

   being 
  very 
  rare 
  and 
  small 
  crystals 
  of 
  feldspar 
  and 
  quartz, 
  

   with 
  an 
  occasional 
  small 
  biotite 
  table. 
  Some 
  specimens 
  also 
  

   show 
  some 
  small 
  specks 
  of 
  a 
  rose-colored 
  mineral, 
  none 
  of 
  

   which 
  could 
  be 
  identified 
  in 
  the 
  thin 
  sections. 
  

  

  At 
  a 
  spring 
  called 
  Oapanna, 
  about 
  5 
  kilometers 
  eastnortheast 
  

   of 
  Marrubiu, 
  narrow 
  crevices 
  and 
  lithophysal 
  cavities 
  of 
  the 
  

   rhyolite 
  are 
  drusy 
  with 
  very 
  small 
  quartz 
  crystals. 
  On 
  these 
  

   *Op. 
  cit., 
  pp. 
  499-501. 
  

  

  