﻿42 
  Smyth, 
  Jr. 
  — 
  Composition 
  of 
  the 
  Alkaline 
  Hocks 
  

  

  favorable, 
  without 
  any 
  regional 
  transfer 
  of 
  magma 
  in 
  a 
  hori- 
  

   zontal 
  direction. 
  If 
  this 
  conception 
  is 
  right, 
  it 
  may 
  still 
  be 
  true 
  

   that 
  the 
  final 
  determining 
  factor 
  for 
  the 
  genesis 
  of 
  alkaline 
  

   rocks 
  is 
  a 
  mechanical 
  one, 
  and, 
  perhaps, 
  as 
  suggested 
  by 
  many 
  

   occurrences, 
  the 
  necessary 
  local 
  conditions 
  for 
  the 
  derivation 
  

   of 
  alkaline 
  from 
  subalkaline 
  magmas 
  most 
  often 
  exist 
  in 
  regions 
  

   of 
  radial 
  dislocation. 
  As 
  to 
  the 
  precise 
  character 
  of 
  such 
  

   mechanical 
  control, 
  it 
  is, 
  of 
  course, 
  impossible 
  to 
  speak 
  with 
  

   even 
  an 
  approach 
  to 
  certainty, 
  for 
  we 
  are 
  here 
  brought 
  face 
  to 
  

   face 
  with 
  the 
  still 
  unsolved 
  fundamental 
  problems 
  of 
  vulcanism. 
  

   It 
  may 
  be 
  that, 
  in 
  very 
  unstable 
  regions, 
  with 
  belts 
  of 
  intense 
  

   lateral 
  thrust 
  in 
  which 
  vulcanism 
  is 
  greatly 
  developed, 
  subalka- 
  

   line 
  rocks 
  are 
  intruded 
  and 
  extruded 
  without 
  opportunity 
  for 
  

   the 
  associated 
  mineralizers 
  to 
  effect 
  the 
  differentiation 
  neces- 
  

   sary 
  to 
  produce 
  alkaline 
  magmas. 
  Whatever 
  the 
  mechanical 
  

   conditions 
  needful 
  to 
  cause 
  solid, 
  but 
  potentially 
  molten, 
  rocks 
  

   to 
  become 
  liquid 
  (a 
  much 
  discussed 
  problem 
  wholly 
  transcend- 
  

   ing 
  the 
  limits 
  of 
  this 
  paper), 
  in 
  regions 
  of 
  such 
  instability, 
  

   intrusion 
  and 
  eruption 
  may 
  follow 
  so 
  rapidly 
  upon 
  melting 
  

   that 
  no 
  opportunity 
  is 
  afforded 
  for 
  the 
  separation 
  of 
  alkaline 
  

   fractions. 
  Instead, 
  over 
  vast 
  areas, 
  we 
  find 
  a 
  monotonous 
  

   assemblage 
  of 
  basalts, 
  andesites, 
  etc., 
  typical 
  subalkaline 
  rocks. 
  

  

  In 
  the 
  more 
  stable 
  regions, 
  on 
  the 
  other 
  hand, 
  where, 
  instead 
  

   of 
  typical 
  mountain-making 
  due 
  to 
  lateral 
  thrust,. 
  radial 
  dis- 
  

   placement 
  prevails, 
  magma 
  basins 
  may 
  remain, 
  for 
  long 
  

   periods, 
  undisturbed 
  by 
  external 
  agencies, 
  and, 
  if 
  the 
  proper 
  

   conditions 
  of 
  pressure, 
  temperature 
  and 
  viscosity 
  exist, 
  the 
  

   mineralizers 
  have 
  the 
  opportunity 
  to 
  exert 
  their 
  selective 
  influ- 
  

   ence 
  and 
  extract 
  from 
  the 
  mass 
  those 
  constituents 
  with 
  which 
  

   they 
  readily 
  form 
  mobile 
  compounds. 
  In 
  other 
  words, 
  alka- 
  

   line 
  magmas 
  may 
  be 
  formed 
  under 
  relatively 
  stable 
  conditions 
  

   which 
  admit 
  of 
  a 
  long 
  continued 
  delicate 
  adjustment 
  of 
  equi- 
  

   librium. 
  Such 
  conditions 
  must 
  necessarily 
  be 
  rare, 
  a 
  fact 
  with 
  

   which 
  the 
  comparative 
  rarity 
  of 
  alkaline 
  rocks 
  is 
  in 
  harmony. 
  

  

  Iddings* 
  holds 
  that 
  stable 
  conditions 
  may 
  prevent 
  differen- 
  

   tiation, 
  and 
  this 
  is 
  doubtless 
  true 
  for 
  some 
  types 
  of 
  differentia- 
  

   tion, 
  but 
  where 
  it 
  is 
  a 
  process 
  of 
  magmatic 
  extraction, 
  such 
  as 
  

   is 
  here 
  considered, 
  stability 
  of 
  conditions 
  may 
  be 
  more 
  favor- 
  

   able. 
  

  

  The 
  magmas 
  formed 
  under 
  these 
  conditions 
  are 
  tapped 
  off 
  

   by 
  vertical 
  fissures 
  and 
  give 
  rise 
  to 
  intrusions 
  and 
  extrusions 
  

   of 
  alkaline 
  rocks. 
  Presumably, 
  too, 
  the 
  vertical 
  fissures 
  some- 
  

   times 
  play 
  an 
  important 
  role 
  in 
  permitting 
  the 
  active 
  circula- 
  

   tion 
  of 
  mineralizers 
  through 
  relatively 
  rigid 
  and, 
  thus, 
  but 
  for 
  

   the 
  mineralizers, 
  stagnant, 
  bodies 
  of 
  magma. 
  

  

  * 
  J. 
  P. 
  Iddings 
  : 
  Igneous 
  Rocks, 
  i, 
  p. 
  292, 
  1909. 
  

  

  