﻿588 
  Washington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia. 
  

  

  Di 
  7-91 
  4-91 
  

  

  Hy 
  14-71 
  2T04 
  

  

  Mt 
  0-93 
  2-78 
  

  

  II 
  . 
  3-34 
  4-26 
  

  

  Ap 
  0-34 
  1-01 
  

  

  According 
  to 
  these 
  A 
  falls 
  in 
  dacosc 
  with 
  the 
  full 
  symbol 
  

   IL4 
  // 
  .2(3).4, 
  l 
  and 
  B 
  in 
  tonalose 
  with 
  the 
  symbol 
  II(III).4".3". 
  

   4(5). 
  These 
  norms 
  are 
  of 
  interest 
  in 
  showing 
  the 
  presence 
  of 
  

   about 
  12 
  per 
  cent 
  of 
  excess 
  silica. 
  This 
  is 
  much 
  more 
  than 
  is 
  

   visible 
  as 
  quartz 
  in 
  the 
  hoi 
  o 
  crystal 
  line 
  type 
  (B), 
  and 
  must 
  be 
  

   present 
  in 
  the 
  glass 
  base 
  in 
  the 
  other. 
  Such 
  an 
  excess 
  of 
  silica 
  

   is 
  very 
  commonly 
  observed 
  among 
  the 
  analyses 
  of 
  andesites, 
  

   and 
  these 
  lavas 
  generally 
  show 
  an 
  excess 
  of 
  some 
  10 
  to 
  20 
  per 
  

   cent. 
  This 
  silica 
  is 
  either 
  not 
  present 
  at 
  all 
  or 
  only 
  in 
  small 
  part 
  

   in 
  the 
  mode, 
  and 
  is 
  to 
  be 
  regarded 
  as 
  "occult," 
  to 
  use 
  a 
  

   term 
  recently 
  proposed.* 
  

  

  Basalt 
  (Andose, 
  11.5.3.^). 
  

  

  Basalts 
  formed 
  the 
  last 
  lavas 
  poured 
  out 
  by 
  the 
  volcano 
  and 
  

   they 
  cover 
  large 
  extents 
  of 
  country 
  around 
  it, 
  very 
  much 
  as 
  

   they 
  do 
  at 
  Monte 
  Ferru. 
  These 
  flows 
  are 
  noted 
  by 
  della 
  

   Marmora 
  at 
  many 
  localities 
  and 
  were 
  seen 
  by 
  me 
  near 
  Uras 
  and 
  

   Marrubiu 
  along 
  the 
  west 
  flank. 
  They 
  were 
  always 
  observed 
  

   covering 
  the 
  rhyolites 
  and 
  other 
  salic 
  rocks, 
  and 
  were 
  never 
  

   covered 
  by 
  them. 
  Della 
  Marmora 
  also 
  mentions 
  their 
  occur- 
  

   rence 
  as 
  overlying 
  the 
  " 
  trachytes," 
  and 
  hef 
  describes 
  the 
  sheet 
  

   which 
  caps 
  the 
  rhyolites 
  at 
  the 
  culminating 
  points 
  of 
  the 
  

   Trebinas. 
  According 
  to 
  him;}; 
  basaltic 
  rocks 
  also 
  form 
  dikes 
  

   which 
  cut 
  Pliocene 
  marls 
  near 
  Ales. 
  These 
  I 
  did 
  not 
  see. 
  

  

  These 
  basalts 
  are 
  very 
  dark 
  gray, 
  almost 
  or 
  quite 
  black, 
  and 
  

   very 
  dense, 
  either 
  perfectly 
  compact 
  or 
  showing 
  only 
  a 
  few 
  

   vesicles. 
  Very 
  small 
  and 
  rare 
  feldspars 
  are 
  the 
  only 
  pheno- 
  

   crysts 
  visible 
  to 
  the 
  naked 
  eye. 
  The 
  groundmass 
  is 
  quite 
  

   aphanitic. 
  

  

  In 
  thin 
  section 
  the 
  specimens 
  collected 
  by 
  me 
  vary 
  somewhat 
  

   in 
  texture 
  as 
  well 
  as 
  in 
  mode. 
  

  

  That 
  from 
  the 
  thick 
  flow 
  at 
  Uras, 
  an 
  analysis 
  of 
  which 
  is 
  

   given 
  beyond, 
  is 
  ophitic, 
  composed 
  of 
  tables 
  of 
  labradorite 
  

   (Ab 
  2 
  An 
  3 
  ) 
  with 
  the 
  usual 
  twinning, 
  and 
  anhedral 
  to 
  subhedral 
  

   grains 
  of 
  colorless 
  pyroxene. 
  Part 
  of 
  this 
  is 
  hypersthene 
  and 
  

   part 
  diopside, 
  the 
  two 
  being 
  distinguishable 
  by 
  the 
  extinction 
  

   angles 
  and 
  birefringence, 
  as 
  in 
  the 
  andesites. 
  Much 
  of 
  the 
  

   pyroxene 
  is 
  apparently 
  slightly 
  altered, 
  being 
  of 
  a 
  yellow- 
  

  

  * 
  J. 
  P. 
  Iddings, 
  Igneous 
  Rocks, 
  ii, 
  p. 
  19, 
  1913. 
  

   \ 
  Della 
  Marmora, 
  op. 
  cit., 
  pp. 
  500 
  and 
  622. 
  

   % 
  Delia 
  Marmora, 
  op. 
  cit., 
  p. 
  620. 
  

  

  