﻿192 
  PEOI". 
  J. 
  W. 
  JUED 
  OX 
  THE 
  TEETIAEY 
  VOECAXOE3 
  

  

  Messrs. 
  Arnold 
  Hague 
  and 
  J. 
  P. 
  Iddings 
  in 
  regard 
  to 
  the 
  rocks 
  of 
  

   the 
  Washoe 
  district 
  * 
  ; 
  with 
  Signor 
  B. 
  Lotti 
  in 
  respect 
  to 
  the 
  Elba 
  

   rocks 
  f 
  ; 
  and 
  with 
  Prof. 
  A. 
  Stelzner 
  in 
  respect 
  to 
  the 
  rocks 
  of 
  the 
  

   Andes 
  i, 
  previously 
  studied 
  by 
  Darwin. 
  

  

  It 
  is 
  very 
  gratifying 
  to 
  me 
  to 
  find 
  that 
  in 
  the 
  memoir 
  recently 
  

   published 
  there 
  is 
  the 
  most 
  unreserved 
  and 
  complete 
  acceptance 
  of 
  

   the 
  conclusions 
  announced 
  by 
  me 
  in 
  1874 
  ; 
  and 
  this 
  acceptance 
  is 
  of 
  

   the 
  greater 
  significance 
  when 
  we 
  bear 
  in 
  mind 
  the 
  great 
  difference 
  

   in 
  the 
  views 
  previously 
  maintained 
  by 
  the 
  author 
  of 
  the 
  memoir. 
  

   The 
  studies 
  of 
  the 
  Director-General 
  of 
  the 
  Geological 
  Survey 
  and 
  of 
  

   different 
  members 
  of 
  his 
  staff 
  have 
  afforded 
  evidence 
  of 
  the 
  most 
  

   unmistakable 
  character 
  that 
  gabbros 
  graduate 
  insensibly 
  into 
  dole- 
  

   rites, 
  and 
  dolerites 
  into 
  basalts, 
  while 
  the 
  last-mentioned 
  rocks 
  pass 
  

   into 
  tachylytes 
  (Trans. 
  Koy. 
  Soc. 
  Edinb. 
  1SSS, 
  pp. 
  122-12-1) 
  ; 
  they 
  are 
  

   equally 
  satisfied 
  with 
  the 
  proofs 
  that 
  true 
  granites 
  merge 
  inseusibly 
  

   through 
  various 
  intermediate 
  forms, 
  called 
  " 
  granophyric 
  " 
  by 
  Eosen- 
  

   busch, 
  into 
  the 
  acid 
  lavas 
  and 
  pitchstones 
  (foe. 
  cit. 
  pp. 
  145-150) 
  ; 
  

   not 
  less 
  unhesitating 
  is 
  the 
  testimony 
  of 
  the 
  recently 
  published 
  

   memoir 
  to 
  the 
  relation 
  between 
  the 
  textures 
  of 
  the 
  several 
  types 
  of 
  

   igneous 
  rocks 
  in 
  the 
  Hebrides 
  and 
  their 
  geological 
  positions. 
  It 
  is 
  

   maintained 
  that 
  the 
  finer-grained 
  varieties 
  occur 
  in 
  the 
  smaller 
  in- 
  

   trusive 
  masses 
  and 
  in 
  the 
  peripheral 
  portions 
  of 
  larger 
  ones 
  (T. 
  R. 
  S. 
  E. 
  

   1888, 
  pp. 
  120-147, 
  &c), 
  and 
  that 
  in 
  many 
  massive 
  lava-streams 
  we 
  

   find 
  rocks 
  of 
  more 
  highly 
  crystalline 
  character 
  than 
  in 
  some 
  of 
  the 
  

   smaller 
  intrusive 
  dykes 
  and 
  sheets 
  (loc. 
  cit. 
  pp. 
  77, 
  117, 
  &c). 
  Einally 
  

   the 
  conclusion 
  is 
  stated 
  that 
  the 
  differences 
  between 
  the 
  gabbros 
  

   and 
  the 
  basic 
  lavas, 
  and 
  between 
  the 
  granites 
  and 
  the 
  more 
  acid 
  

   ones, 
  must 
  be 
  ascribed 
  to 
  the 
  circumstance 
  that 
  the 
  Plutonic 
  rocks 
  

   have 
  consolidated 
  at 
  great 
  depths 
  beneath 
  the 
  surface, 
  and 
  there- 
  

   fore 
  slowly 
  and 
  under 
  great 
  pressure, 
  while 
  the 
  lavas 
  cooling 
  at 
  

   the 
  surface 
  have 
  not 
  been 
  subjected 
  to 
  these 
  conditions 
  (loc. 
  cit. 
  

   p. 
  140 
  &c). 
  These 
  passages 
  may 
  be 
  compared 
  with 
  the 
  general 
  

   account 
  of 
  the 
  relations 
  of 
  the 
  igneous 
  rocks 
  of 
  the 
  district 
  (Q. 
  J. 
  G. 
  S. 
  

   1874, 
  pp. 
  233-248). 
  

  

  B. 
  Tlie 
  Existence 
  of 
  Jive 
  Great 
  Centres 
  of 
  Eruption, 
  with 
  many 
  

   minor 
  and 
  scattered 
  Vents 
  in 
  the 
  Western 
  Isles 
  of 
  Scotland. 
  

  

  My 
  conclusions 
  on 
  this 
  subject, 
  as 
  given 
  in 
  1874, 
  may 
  be 
  stated 
  

   briefly 
  as 
  follows 
  : 
  — 
  

  

  There 
  exist 
  at 
  five 
  well-marked 
  centres 
  — 
  namely, 
  Mull, 
  Ardna- 
  

   murchan, 
  Hum, 
  Skye, 
  and 
  St. 
  Kilda 
  — 
  clear 
  evidence 
  that 
  eruptive 
  

   action 
  on 
  a 
  grand 
  scale 
  took 
  place. 
  This 
  evidence 
  is 
  as 
  follows 
  : 
  — 
  

  

  (1) 
  There 
  occur 
  enormous 
  masses 
  of 
  lava, 
  of 
  both 
  acid 
  and 
  basic 
  

  

  * 
  "On 
  the 
  Development 
  of 
  Crystallization 
  in 
  Igneous 
  Eocks 
  of 
  Washoe, 
  

   Nevada," 
  U. 
  S. 
  Geol. 
  Surv. 
  Bull. 
  >*o. 
  17 
  (18S5). 
  

  

  t 
  ' 
  Memorie 
  descrittive 
  della 
  Carta 
  Geologica 
  dTtalia,' 
  vol. 
  ii. 
  ; 
  " 
  Descrizione 
  

   Geologica 
  dell' 
  Isola 
  d' 
  Elba 
  " 
  di 
  B. 
  Lotti. 
  E. 
  Com. 
  Geol. 
  d' 
  Italia 
  (1886 
  . 
  

  

  \ 
  Beitrage 
  zur 
  Geologie 
  und 
  Palaontologie 
  der 
  Argentiniscken 
  Eepublic 
  

   (1885), 
  pp. 
  198-213. 
  

  

  