﻿GABBRO 
  OF 
  CARROCK 
  FELL 
  IN" 
  THE 
  LAKE 
  DISTRICT. 
  299 
  

  

  The 
  percentage 
  of 
  silica 
  places 
  the 
  rock 
  high 
  up 
  in 
  the 
  basic 
  

   series. 
  It 
  appears 
  to 
  have 
  most 
  affinity 
  with 
  the 
  more 
  acid 
  basalts 
  

   and 
  with 
  the 
  augite-andesites 
  ; 
  this 
  agrees 
  with 
  the 
  conclusions 
  formed 
  

   from 
  microscopical 
  and 
  other 
  evidence, 
  as 
  well 
  as 
  with 
  the 
  fact 
  that 
  the 
  

   rock 
  is 
  associated 
  with 
  the 
  Carrock-Fell 
  gabbro, 
  which, 
  according 
  to 
  

   Mr. 
  Teall 
  *, 
  is 
  the 
  plutonic 
  representative 
  of 
  the 
  andesitic 
  dolerites. 
  

   The 
  percentage 
  of 
  iron 
  is 
  unusually 
  high, 
  but 
  is 
  parallelled 
  by 
  some 
  

   continental 
  basalts, 
  analyses 
  of 
  which 
  (taken 
  from 
  Roth's 
  ' 
  Gesteins- 
  

   analysen 
  ') 
  are 
  given 
  above. 
  

  

  Microscopic 
  Characters. 
  

  

  Under 
  the 
  microscope 
  the 
  rock 
  resolves 
  itself 
  into 
  a 
  ground-mass 
  

   of 
  varied 
  constitution, 
  in 
  which 
  are 
  imbedded 
  a 
  limited 
  number 
  of 
  

   small 
  porphyritic 
  crystals 
  or 
  crystal 
  groups. 
  These 
  consist 
  mainly 
  

   of 
  felspar 
  ; 
  augite 
  and 
  quartz 
  are 
  less 
  common, 
  and 
  magnetite 
  (?) 
  

   is 
  rare. 
  

  

  The 
  Felspars 
  are 
  present 
  both 
  as 
  skeleton-crystals 
  and 
  well- 
  

   developed 
  individuals 
  showing 
  the 
  usual 
  form 
  of 
  plagioclase 
  f. 
  All 
  

   stages 
  between 
  these 
  two 
  forms 
  are 
  observable. 
  Polysynthetic 
  

   twinning 
  is 
  common. 
  Occasionally 
  well-formed 
  crystals 
  are 
  honey- 
  

   combed 
  by 
  a 
  network 
  of 
  glass-inclusions. 
  

  

  The 
  Augite 
  is 
  almost 
  colourless, 
  and 
  gives 
  the 
  usual 
  eight-sided 
  

   sections. 
  Prismatic, 
  ortho- 
  and 
  clinopinacoidal 
  cleavages 
  are 
  present. 
  

   The 
  angle 
  of 
  extinction 
  is 
  about 
  39°. 
  Twinning 
  parallel 
  to 
  the 
  ortho- 
  

   pinacoid 
  occurs. 
  

  

  The 
  Quartz 
  is 
  bounded 
  in 
  part 
  by 
  definite 
  crystalline 
  faces, 
  and 
  

   includes 
  trains 
  of 
  vesicles 
  with 
  moving 
  bubbles. 
  Wo 
  indications 
  of 
  

   corrosion 
  by 
  the 
  magma 
  are 
  observable. 
  In 
  one 
  case 
  triangular 
  

   wedges, 
  apparently 
  of 
  quartz, 
  and 
  closely 
  similar 
  to 
  those 
  of 
  the 
  

   micro-pegmatite 
  of 
  the 
  neighbouring 
  granophyre, 
  occur 
  in 
  the 
  

   spherulitic 
  ground-mass 
  in 
  the 
  midst 
  of 
  a 
  group 
  of 
  porphyritic 
  

   felspars. 
  Such 
  wedges 
  do 
  not 
  appear 
  to 
  have 
  been 
  observed 
  

   previously 
  in 
  a 
  glass-basis. 
  

  

  The 
  ground-mass 
  consists 
  of 
  a 
  glass-basis 
  containing 
  globulites, 
  

   crystallites, 
  minute 
  crystals, 
  and 
  granules. 
  The 
  glass 
  is 
  of 
  an 
  olive- 
  

   green 
  colour, 
  and 
  for 
  the 
  most 
  part 
  unde 
  vitrified. 
  Distributed 
  

   through 
  the 
  glass 
  is 
  a 
  series 
  of 
  greenish-brown 
  or 
  reddish 
  granules 
  

   and 
  minute 
  crystals, 
  evidently 
  of 
  augite. 
  In 
  the 
  smallest 
  of 
  these 
  

   the 
  double-refraction 
  is 
  very 
  feeble, 
  while 
  the 
  larger 
  polarize 
  

   brilliantly. 
  The 
  granules 
  may 
  be 
  diffused 
  or 
  collected 
  into 
  grano- 
  

   spherites 
  and 
  belonospherites, 
  or 
  they 
  may 
  be 
  united 
  into 
  small 
  

   polysomatic 
  grains, 
  which, 
  for 
  the 
  sake 
  of 
  brevity, 
  may 
  be 
  termed 
  

   glomerites 
  ; 
  and 
  these, 
  like 
  the 
  simple 
  granules, 
  may 
  enter 
  into 
  the 
  

   composition 
  of 
  spherulitic 
  bodies, 
  and 
  when 
  they 
  predominate, 
  as 
  is 
  

   very 
  commonly 
  the 
  case, 
  the 
  term 
  glomerospherite 
  may 
  be 
  applied 
  to 
  

   the 
  aggregate. 
  These 
  forms 
  pass 
  imperceptibly 
  into 
  one 
  another. 
  

  

  * 
  Op. 
  cit. 
  p. 
  178. 
  

  

  t 
  Unaltered 
  crystals 
  are 
  not 
  sufficiently 
  common 
  to 
  determine 
  the 
  true 
  

   extinction-angles 
  ; 
  but 
  the 
  few 
  observations 
  that 
  were 
  possible 
  pointed 
  to 
  

   anorthite 
  as 
  the 
  probable 
  species. 
  

  

  