﻿and 
  its 
  Significance 
  as 
  to 
  their 
  Origin. 
  41 
  

  

  The 
  simultaneous 
  concentration 
  of 
  the 
  rare 
  elements 
  and 
  the 
  

   alkalies 
  can 
  hardly 
  be 
  fortuitous, 
  and 
  if 
  the 
  mineralizers 
  accom- 
  

   plished 
  the 
  former 
  there 
  can 
  be 
  little 
  doubt 
  that 
  they 
  did 
  the 
  

   latter 
  as 
  well, 
  but 
  to 
  a 
  smaller 
  degree. 
  

  

  While 
  the 
  essential 
  features 
  of 
  the 
  production 
  of 
  pegmatites 
  

   are 
  generally 
  accepted 
  as 
  established, 
  many 
  details 
  remain 
  to 
  be 
  

   worked 
  out, 
  particularly 
  on 
  the 
  mechanical 
  side. 
  If 
  this 
  is 
  true 
  

   of, 
  relatively, 
  so 
  small 
  an 
  operation, 
  it 
  is 
  not 
  surprising 
  that 
  one 
  

   of 
  the 
  magnitude 
  here 
  under 
  consideration 
  should 
  be 
  wrapt 
  

   in 
  obscurity. 
  Even 
  if 
  the 
  probability 
  of 
  the 
  hypothesis 
  advanced 
  

   be 
  admitted, 
  it 
  constitutes 
  only 
  one 
  step 
  in 
  a 
  long 
  series 
  of 
  

   most 
  complex 
  problems. 
  

  

  One 
  of 
  these 
  problems 
  may 
  be 
  referred 
  to 
  on 
  account 
  

   of 
  the 
  importance 
  that 
  has 
  been 
  ascribed 
  to 
  it 
  from 
  the 
  

   genetic 
  standpoint. 
  First 
  emphasized 
  by 
  Iddings* 
  in 
  his 
  

   classic 
  paper 
  on 
  the 
  origin 
  of 
  igneous 
  rocks, 
  the 
  distribu- 
  

   tion 
  of 
  the 
  alkaline 
  and 
  subalkaline 
  rocks 
  has 
  since 
  been 
  made 
  

   the 
  basis 
  of 
  the 
  broad 
  generalization 
  that, 
  so 
  far 
  as 
  may 
  be 
  judged 
  

   by 
  the 
  conditions 
  of 
  Tertiary 
  and 
  recent 
  time, 
  alkaline 
  rocks 
  

   occur 
  in 
  regions 
  of 
  radial 
  dislocation, 
  the 
  "Atlantic" 
  regions 
  

   of 
  Suess, 
  while 
  subalkaline 
  rocks- 
  occur 
  in 
  regions 
  of 
  tangential 
  

   dislocation, 
  " 
  Pacific" 
  regions 
  of 
  Suess. 
  

  

  This 
  generalization, 
  first 
  made 
  by 
  Harkerf 
  and 
  later, 
  inde- 
  

   pendently, 
  by 
  Becke^ 
  and 
  by 
  Prior,§ 
  has, 
  as 
  already 
  indicated, 
  

   been 
  taken 
  by 
  the 
  former 
  as 
  the 
  basis 
  of 
  a 
  mechanical 
  hypothesis 
  

   to 
  account 
  for 
  the 
  distribution 
  of 
  alkaline 
  and 
  subalkaline 
  rocks. 
  

   Without 
  trying 
  to 
  determine 
  details, 
  he 
  concludes 
  that 
  distinct 
  

   types 
  of 
  differentiation 
  are 
  effected 
  by 
  the 
  two 
  types 
  of 
  crustal 
  

   disturbance, 
  the 
  one, 
  radial, 
  giving 
  alkaline 
  rocks, 
  the 
  other, 
  

   tangential, 
  giving 
  subalkaline 
  rocks. 
  The 
  differentiation 
  takes 
  

   place 
  chiefly 
  in 
  a 
  horizontal 
  direction 
  and, 
  as 
  a 
  result, 
  very 
  

   extensive 
  regions 
  are 
  underlaid, 
  during 
  a 
  single 
  igneous 
  epoch, 
  

   by 
  alkaline, 
  "or 
  subalkaline, 
  magmas, 
  as 
  the 
  case 
  may 
  be, 
  which 
  

   determine 
  the 
  type 
  of 
  igneous 
  rocks 
  for 
  the 
  region 
  and 
  epoch. 
  

   As 
  a 
  result 
  of 
  changing 
  type 
  of 
  crustal 
  disturbance, 
  a 
  given 
  region 
  

   may 
  be 
  underlaid 
  by 
  alkaline 
  magmas 
  at 
  one 
  time 
  and 
  by 
  sub- 
  

   alkaline 
  magmas 
  at 
  another, 
  with 
  corresponding 
  changes 
  in 
  its 
  

   igneous 
  rocks. 
  

  

  The 
  writer's 
  conception 
  is 
  quite 
  different, 
  since 
  it 
  regards 
  

   the 
  subalkaline 
  magmas 
  as 
  of 
  world-wide 
  extent, 
  and 
  the 
  alka- 
  

   line 
  magmas 
  as 
  derived 
  from 
  these, 
  when 
  local 
  conditions 
  are 
  

  

  * 
  Iddings, 
  J. 
  P. 
  : 
  The 
  Origin 
  of 
  Igneous 
  Rocks, 
  Bull. 
  Phil. 
  Soc. 
  Washing- 
  

   ton, 
  xii, 
  pp. 
  89-202, 
  1892. 
  

  

  f 
  Harker, 
  A. 
  : 
  The 
  Natural 
  History 
  of 
  Igneous 
  Rocks 
  : 
  I. 
  Their 
  Geographi- 
  

   cal 
  and 
  Chronological 
  Distribution, 
  Science 
  Progress, 
  vi, 
  pp. 
  12-33, 
  1896. 
  

  

  X 
  Loc. 
  cit. 
  

  

  § 
  Prior, 
  G. 
  T. 
  : 
  Contribution 
  to 
  the 
  Petrology 
  of 
  British 
  East 
  Africa. 
  Min- 
  

   eralogical 
  Magazine, 
  xiii, 
  pp. 
  228-263, 
  1903. 
  

  

  