﻿GABBRO 
  OF 
  CARROCK 
  FELL 
  IN 
  THE 
  LAKE 
  DISTRICT. 
  301 
  

  

  these, 
  together 
  with 
  the 
  skeleton-crystals 
  of 
  felspar, 
  belong 
  to 
  the 
  

   intratelluric 
  period 
  of 
  Rosen 
  bnsch. 
  The 
  later 
  ^Won-period 
  appears 
  

   to 
  have 
  commenced 
  with 
  the 
  development 
  of 
  numerous 
  granules 
  and 
  

   glomerites 
  of 
  augite, 
  which 
  were 
  scattered 
  everywhere 
  through 
  the 
  

   ground-mass. 
  Tbe 
  concentration 
  of 
  these 
  into 
  granospherites 
  &c. 
  

   in 
  the 
  peripheral 
  part 
  of 
  the 
  vein 
  was 
  accompanied 
  or 
  even 
  preceded 
  

   in 
  some 
  cases 
  by 
  the 
  separation 
  of 
  iron-ore 
  ; 
  but 
  the 
  deposition 
  of 
  

   the 
  latter 
  mineral 
  was 
  mainly 
  associated 
  with 
  the 
  development 
  of 
  a 
  

   second 
  generation 
  of 
  felspar 
  of 
  a 
  more 
  acid 
  type 
  than 
  the 
  first 
  : 
  

   these 
  either 
  formed 
  spherocrystals 
  or 
  united 
  into 
  a 
  complicated 
  

   network 
  of 
  fibres. 
  Simultaneously, 
  or 
  perhaps 
  rather 
  later, 
  the 
  

   pyroxene 
  granules, 
  glomerites, 
  and 
  glomerospberites 
  collected 
  along 
  

   the 
  margins 
  of 
  the 
  vein 
  and 
  around 
  crystals 
  and 
  other 
  bodies. 
  

   The 
  rock 
  was 
  still 
  fluid, 
  for 
  all 
  the 
  above 
  structures 
  have 
  been 
  

   modified 
  by 
  flowing 
  of 
  the 
  mass 
  : 
  when 
  the 
  vein 
  began 
  to 
  consolidate, 
  

   cracks 
  starting 
  from 
  the 
  sides 
  traversed 
  all 
  the 
  zones 
  of 
  the 
  rock, 
  

   and 
  were 
  filled 
  up 
  by 
  the 
  most 
  acid 
  portions 
  of 
  the 
  magma, 
  which 
  

   were 
  still 
  in 
  a 
  liquid 
  (or 
  potentially 
  liquid) 
  condition; 
  the 
  last 
  

   event 
  was 
  the 
  development 
  of 
  a 
  second 
  generation 
  of 
  quartz 
  in 
  the 
  

   form 
  of 
  clear 
  rounded 
  spherules 
  devoid, 
  of 
  vesicles. 
  With 
  the 
  

   exception 
  of 
  secondary 
  products 
  in 
  some 
  of 
  the 
  felspars 
  and 
  augites, 
  

   and 
  the 
  slight 
  devitrification 
  (?) 
  of 
  portions 
  of 
  the 
  glass 
  — 
  " 
  meta- 
  

   somatic" 
  changes 
  which 
  were 
  perhaps 
  effected 
  during 
  the 
  period 
  

   when 
  the 
  gabbro 
  and 
  felsite 
  were 
  regionally 
  metamorphosed 
  — 
  no 
  

   appreciable 
  change 
  in 
  the 
  rock 
  has 
  taken 
  place 
  since 
  Silurian 
  or, 
  

   probably, 
  since 
  Ordovician 
  times. 
  

  

  Relation 
  to 
  the 
  Gabbro. 
  

  

  Under 
  the 
  microscope 
  the 
  junction 
  is 
  seen 
  to 
  be 
  somewhat 
  ir- 
  

   regular, 
  the 
  glassy 
  magma 
  of 
  the 
  tachylyte 
  penetrating 
  into 
  bays 
  

   and 
  fjords 
  in 
  the 
  gabbro 
  ; 
  the 
  line 
  of 
  separation 
  is 
  nevertheless 
  

   perfectly 
  sharp. 
  Where 
  the 
  glass 
  penetrates 
  deeply 
  into 
  the 
  gabbro 
  

   it 
  frequently 
  follows 
  the 
  boundaries 
  of 
  the 
  crystals 
  of 
  the 
  latter, 
  

   having 
  apparently 
  in 
  such 
  cases 
  simply 
  occupied 
  the 
  space 
  of 
  dis- 
  

   lodged 
  portions. 
  Some 
  of 
  the 
  angular 
  fragments 
  of 
  felspar 
  and 
  

   augite 
  in 
  the 
  glass 
  may 
  represent 
  such 
  pieces 
  broken 
  off 
  and 
  floated 
  

   away 
  ; 
  they 
  never 
  merge 
  into 
  the 
  glass, 
  and 
  discountenance 
  the 
  idea 
  

   of 
  any 
  assimilation 
  having 
  taken 
  place. 
  This 
  is 
  in 
  harmony 
  with 
  

   the 
  absence 
  of 
  any 
  corrosive 
  effect 
  on 
  the 
  crystals 
  of 
  the 
  glass, 
  for 
  

   these 
  seem 
  to 
  be 
  to 
  a 
  large 
  extent 
  the 
  same 
  as 
  those 
  of 
  the 
  gabbro. 
  

  

  At 
  first 
  I 
  supposed 
  the 
  tachylyte 
  to 
  be 
  of 
  later 
  date 
  than 
  the 
  

   gabbro 
  ; 
  but 
  microscopical 
  examination 
  shows 
  such 
  close 
  minera- 
  

   logical 
  relations 
  between 
  the 
  two 
  rocks 
  that 
  I 
  can 
  only 
  suppose 
  them 
  

   connected 
  at 
  some 
  point 
  *. 
  An 
  analysis 
  of 
  the 
  gabbro 
  given 
  by 
  

   Mr. 
  Clifton 
  W 
  T 
  ard 
  f 
  would 
  seem 
  not 
  to 
  support 
  this 
  conclusion 
  ; 
  but 
  

  

  * 
  The 
  gabbro 
  contains 
  quartz, 
  plagioclase, 
  augite 
  (with 
  prismatic 
  and 
  pina- 
  

   coidal 
  cleavages), 
  and 
  magnetite 
  or 
  ilmenite, 
  and, 
  in 
  addition 
  to 
  these, 
  diallage, 
  

   which 
  seems 
  to 
  he 
  the 
  only 
  important 
  mineral 
  not 
  found 
  in 
  the 
  tachylyte. 
  

  

  t 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxii. 
  p. 
  24. 
  

  

  