﻿J-. 
  W. 
  JUDD 
  05T 
  THE 
  SECOXDAItY 
  B0CE.S 
  0E 
  SCOTLAND. 
  243 
  

  

  explain 
  the 
  cause 
  of 
  the 
  differences 
  presented 
  by 
  the 
  mountains 
  of 
  

   the 
  central 
  and 
  outer 
  portions 
  of 
  the 
  island 
  respectively. 
  While 
  

   thf 
  latter 
  are 
  made 
  up 
  of 
  nearly 
  horizontally 
  disposed 
  lava 
  streams, 
  

   the 
  former 
  consist 
  of 
  a 
  number 
  of 
  intrusive 
  masses 
  composed 
  of 
  

   rocks 
  of 
  very 
  various 
  degrees 
  of 
  hardness, 
  and 
  associated 
  with 
  great 
  

   deposits 
  of 
  volcanic 
  agglomerates 
  and 
  breccias 
  — 
  the 
  several 
  rocks 
  

   yielding, 
  as 
  might 
  be 
  anticipated, 
  to 
  denuding 
  forces 
  in 
  very 
  un- 
  

   equal 
  degrees 
  and 
  in 
  very 
  various 
  manners. 
  

  

  The 
  positions 
  and 
  relations 
  of 
  the 
  rocks 
  of 
  Mull 
  are 
  illustrated 
  in 
  the 
  

   accompanying 
  plan 
  and 
  sections 
  (Pis. 
  XXII. 
  & 
  XXIII.). 
  In 
  these 
  an 
  

   attempt 
  has 
  been 
  made 
  to 
  exhibit 
  the 
  relations 
  of 
  the 
  different 
  rock* 
  

   masses 
  by 
  employing 
  different 
  colours 
  for 
  the 
  two 
  great 
  varieties 
  of 
  

   igneous 
  rocks, 
  while 
  the 
  variations 
  in 
  the 
  depth 
  of 
  tint 
  in 
  either 
  case 
  

   serve 
  to 
  indicate 
  their 
  crystalline, 
  stony, 
  or 
  glassy 
  character. 
  

  

  .Forming 
  the 
  basis 
  of 
  the 
  great 
  central 
  mountain-group 
  of 
  Mull 
  

   are 
  masses 
  of 
  highly 
  siliceous 
  intrusive 
  rocks. 
  AVbere 
  great 
  valleys 
  

   have 
  deeply 
  intersected 
  these 
  masses, 
  they 
  are 
  seen 
  to 
  be 
  composed 
  

   of 
  granite, 
  usually 
  of 
  the 
  hornblendic 
  variety, 
  but 
  passing, 
  in 
  the 
  

   deepest 
  and 
  most 
  central 
  portions 
  of 
  the 
  masses, 
  into 
  ordinary 
  gra- 
  

   nite 
  with 
  mica. 
  But 
  in 
  proportion 
  as 
  we 
  trace 
  the 
  rocks 
  of 
  the 
  

   mass 
  outwards 
  and 
  upwards, 
  Ave 
  find 
  the 
  granite 
  passing 
  bv 
  insen- 
  

   sible 
  gradations 
  into 
  felsite. 
  Lying 
  upon 
  the 
  skirts 
  of 
  these 
  central 
  

   bosses 
  of 
  granitic 
  and 
  felsitic 
  rock 
  are 
  thick 
  masses 
  of 
  felstones, 
  

   disposed 
  in 
  regular 
  sheets 
  and 
  of 
  amygdaloidal 
  structure, 
  which 
  

   alternate 
  with 
  beds 
  of 
  scoria?, 
  lapilli, 
  and 
  ashes, 
  containing 
  many 
  

   included 
  blocks 
  of 
  the 
  stratified 
  rocks 
  of 
  the 
  island. 
  While 
  these 
  

   latter 
  undoubtedly 
  represent 
  the 
  lavas 
  and 
  fragmentary 
  materials 
  

   ejected 
  from 
  a 
  volcanic 
  vent, 
  the 
  central 
  hills 
  of 
  granite 
  and 
  fel- 
  

   site 
  are 
  no 
  less 
  certainly 
  great 
  eruptive 
  masses 
  ; 
  and 
  that 
  this 
  is 
  the 
  

   case 
  is 
  proved 
  by 
  the 
  fact 
  that 
  they 
  give 
  off 
  numerous 
  veins 
  both 
  

   into 
  the 
  overlying 
  fektone 
  lavas 
  &c. 
  and 
  into 
  the 
  surrounding 
  older 
  

   strata, 
  which 
  veins 
  are 
  often 
  seen 
  to 
  have 
  entangled 
  fragments 
  of 
  

   the 
  various 
  rocks 
  which 
  they 
  traverse. 
  

  

  Ptising 
  through 
  the 
  midst 
  of 
  these 
  masses 
  of 
  granitic 
  and 
  felsitic 
  

   rocks, 
  and 
  constituting 
  the 
  bulk 
  of 
  Benin 
  Buy 
  with 
  parts 
  of 
  the 
  

   surrounding 
  mountains 
  of 
  Creach 
  Beinn, 
  Beinn 
  Varrach, 
  and 
  Behm 
  

   Tscdla, 
  Ave 
  find 
  a 
  great 
  mass 
  of 
  gabbro 
  or 
  bas:c 
  rocks 
  from 
  Avhich 
  

   there 
  proceed 
  in 
  every 
  direction 
  innumerable 
  veins, 
  sheets, 
  and 
  

   intrusive 
  masses 
  of 
  irregular 
  form, 
  that 
  traverse 
  the 
  whole 
  of 
  the 
  

   highly 
  siliceous 
  rocks 
  in 
  every 
  direction. 
  In 
  these 
  great 
  intrusive 
  

   masses 
  of 
  gabbro 
  the 
  pyroxenic 
  ingredient 
  in 
  the 
  deeper 
  and 
  more 
  

   central 
  parts 
  appears 
  to 
  be 
  diaUage 
  and 
  hypersthene, 
  which 
  mine- 
  

   rals 
  are 
  replaced, 
  in 
  the 
  peripheral 
  portions 
  by 
  various 
  forms 
  of 
  

   augite. 
  As 
  Ave 
  trace 
  the 
  gabbros 
  into 
  the 
  numerous 
  veins 
  winch 
  

   proceed 
  from 
  the 
  central 
  mass, 
  they 
  are 
  found 
  to 
  graduate 
  into 
  

   dolerites, 
  exhibiting 
  every 
  degree 
  of 
  coarseness 
  of 
  grain, 
  and 
  through 
  

   these 
  into 
  ordinary 
  basalt. 
  In 
  the 
  latter 
  condition 
  the 
  veins 
  and 
  

   dykes 
  often 
  proceed 
  to 
  great 
  distances 
  from 
  the 
  central 
  mass, 
  inter- 
  

   secting 
  alike 
  the 
  lavas 
  of 
  the 
  great 
  plateaux 
  and 
  the 
  various 
  Primary 
  

   and 
  Secondary 
  strata 
  which 
  underlie 
  them 
  (see 
  PI. 
  XX 
  HI. 
  fig. 
  1), 
  

  

  