﻿578 
  Washington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia. 
  

  

  15 
  kilometers 
  from 
  the 
  sea. 
  The 
  railroad 
  from 
  Cagliari 
  to 
  Mac- 
  

   omer 
  skirts 
  its 
  western 
  edge. 
  It 
  stands 
  near 
  the 
  northwest 
  end 
  

   of 
  the 
  broad 
  belt 
  of 
  Miocene 
  limestone, 
  known 
  as 
  the 
  Cam 
  pi- 
  

   dan 
  o, 
  which 
  extends 
  from 
  Cagliari 
  to 
  Oristano. 
  Its 
  lavas 
  overlie 
  

   this, 
  and 
  beyond 
  it 
  are 
  found 
  the 
  Quaternary 
  deposits 
  of 
  the 
  

   Gulf 
  of 
  Oristano. 
  Like 
  Monte 
  Ferru* 
  it 
  probably 
  began 
  in 
  

   late 
  or 
  post-Miocene 
  time, 
  but 
  I 
  obtained 
  no 
  data 
  to 
  decide 
  its 
  

   exact 
  age. 
  Delia 
  Marmora 
  mentions 
  Tertiary 
  deposits 
  inter- 
  

   calated 
  with 
  the 
  Arci 
  lavas 
  in 
  several 
  places. 
  

  

  As 
  the 
  flows 
  of 
  Arci 
  and 
  Ferru 
  do 
  not 
  meet, 
  and 
  the 
  strati- 
  

   graphy 
  is 
  as 
  yet 
  imperfectly 
  known, 
  it 
  is 
  impossible 
  to 
  decide 
  

   now 
  as 
  to 
  the 
  relative 
  age 
  of 
  the 
  two 
  volcanoes, 
  or 
  whether 
  

   they 
  were 
  active 
  simultaneously. 
  Judging 
  from 
  the 
  much 
  

   greater 
  erosion 
  undergone 
  by 
  Arci, 
  it 
  would 
  seem 
  to 
  be 
  prob- 
  

   able 
  that 
  this 
  is 
  the 
  earlier 
  of 
  the 
  two, 
  at 
  least 
  as 
  regards 
  the 
  

   time 
  of 
  extinction. 
  The 
  apparent 
  shifting 
  of 
  the 
  post-Miocene 
  

   volcanic 
  activity 
  toward 
  the 
  north, 
  as 
  shown 
  by 
  the 
  small 
  recent 
  

   cones 
  of 
  the 
  Logoduro, 
  favors 
  this 
  hypothesis. 
  

  

  The 
  volcanic 
  cone 
  has 
  been 
  so 
  deeply 
  eroded 
  as 
  to 
  have 
  lost 
  

   nearly 
  all 
  trace 
  of 
  its 
  original 
  outlines. 
  The 
  highest 
  point 
  is 
  

   reached 
  in 
  the 
  twin 
  peaks 
  of 
  Trebina 
  Longa 
  (812 
  m.) 
  and 
  

   Trebina 
  Lada 
  (795 
  m.), 
  which 
  form 
  a 
  conspicuous 
  landmark. 
  

   These 
  are 
  situated 
  on 
  the 
  west 
  edge 
  of 
  a 
  sort 
  of 
  platform, 
  with 
  

   a 
  general 
  elevation 
  of 
  about 
  700 
  meters 
  and 
  extending 
  some 
  5 
  

   kilometers 
  from 
  north 
  to 
  south. 
  From 
  this 
  ridge 
  steep 
  and 
  

   narrow 
  valleys 
  radiate 
  irregularly 
  in 
  every 
  direction. 
  Delia 
  

   Marmora 
  considers 
  this 
  ridge 
  to 
  be 
  part 
  of 
  the 
  original 
  crater 
  

   wall, 
  but, 
  as 
  I 
  could 
  only 
  see 
  it 
  from 
  a 
  considerable 
  distance, 
  I 
  

   must 
  reserve 
  my 
  opinion 
  as 
  to 
  the 
  existence 
  of 
  remains 
  of 
  a 
  

   crater. 
  The 
  western 
  part 
  may 
  be 
  visited 
  from 
  the 
  stations 
  of 
  

   Uras 
  and 
  Marrubiu, 
  while 
  the 
  small 
  town 
  of 
  Ales 
  serves 
  as 
  a 
  

   base 
  for 
  study 
  of 
  the 
  eastern 
  portion. 
  

  

  It 
  is 
  not 
  yet 
  possible 
  to 
  give 
  more 
  than 
  a 
  very 
  rough 
  estimate 
  

   of 
  the 
  area 
  covered 
  by 
  the 
  lavas, 
  but 
  judging 
  from 
  the 
  descrip- 
  

   tions 
  and 
  statements 
  of 
  della 
  Marmora 
  this 
  may 
  be 
  placed 
  pro- 
  

   visionally 
  at 
  some 
  500 
  square 
  kilometers, 
  that 
  of 
  Monte 
  Ferru 
  

   being 
  at 
  least 
  700.f 
  

  

  Petrography. 
  

  

  Della 
  Marmora 
  pointed 
  out 
  that 
  Monte 
  Arci, 
  like 
  Monte 
  

   Ferru, 
  consists 
  of 
  a 
  core 
  of 
  feldspathic 
  lavas, 
  called 
  by 
  him 
  

   trachytes 
  and 
  phonolites, 
  covered 
  by 
  a 
  later 
  mantle 
  of 
  basaltic 
  

   flows, 
  which 
  extend 
  for 
  many 
  miles 
  in 
  all 
  directions 
  beyond 
  

   the 
  limits 
  of 
  the 
  earlier 
  lavas. 
  My 
  observations 
  fully 
  bear 
  out 
  

   his 
  statement 
  of 
  the 
  general 
  structure 
  of 
  Arci, 
  just 
  as 
  those 
  of 
  

  

  *Cf. 
  A. 
  Dannenberg, 
  Sb. 
  Preuas. 
  Akad. 
  Wiss., 
  p. 
  856, 
  1903. 
  

   \ 
  A. 
  Dannenberg, 
  Neues 
  Jahrb., 
  Beil. 
  Bd., 
  xxi, 
  p. 
  7, 
  1905. 
  

  

  