﻿298 
  ME. 
  T. 
  T. 
  GE00M 
  ON" 
  A 
  TACHILYTE 
  ASSOCIATED 
  WITH 
  THE 
  

  

  17. 
  On 
  a 
  Tachylyte 
  associated 
  ivith 
  the 
  Gabbeo 
  of 
  Caeeock 
  Fell 
  

   in 
  the 
  Lake 
  Disteict. 
  By 
  Theo. 
  T. 
  Geoom, 
  Esq., 
  B.Sc, 
  Scholar 
  

   of 
  St. 
  John's 
  College, 
  Cambridge. 
  (Bead 
  December 
  5, 
  1888.) 
  

  

  (Communicated 
  by 
  Prof. 
  T. 
  M 
  C 
  K 
  Hughes, 
  M.A., 
  F.G.S.) 
  

   [Plate 
  XII.] 
  

  

  Whilst 
  engaged 
  in 
  studying 
  the 
  igneous 
  rocks 
  of 
  Carrock 
  Fell 
  I 
  

   observed 
  a 
  peculiar 
  vein 
  of 
  tachylyte 
  traversing 
  the 
  gabbro 
  close 
  to 
  

   the 
  point 
  at 
  which 
  the 
  latter 
  passes 
  into 
  the 
  granophyre 
  *. 
  The 
  vein 
  

   is 
  about 
  an 
  inch 
  thick, 
  and 
  shows 
  well-marked 
  flow-structure 
  parallel 
  

   to 
  the 
  sides. 
  The 
  centre 
  of 
  the 
  vein 
  is 
  of 
  a 
  pale 
  greenish 
  colour, 
  

   and 
  shows 
  spherulitic 
  structure, 
  while 
  on 
  each 
  side 
  is 
  a 
  purplish- 
  

   grey 
  zone. 
  The 
  rock 
  shows 
  a 
  horny 
  or 
  subvitreous 
  lustre, 
  weathers 
  

   to 
  a 
  yellowish-brown 
  tint, 
  and 
  is 
  slightly 
  magnetic; 
  under 
  the 
  

   blow-pipe 
  splinters 
  fuse 
  readily 
  to 
  a 
  black 
  enamel 
  ; 
  the 
  hardness 
  is 
  

   about 
  6^. 
  The 
  specific 
  gravity 
  of 
  the 
  whole 
  rock 
  is 
  2*99, 
  while 
  

   that 
  of 
  the 
  central 
  zone 
  is 
  2-95. 
  The 
  average 
  density 
  of 
  basalt- 
  

   glasses 
  is, 
  according 
  to 
  Messrs. 
  Judd 
  and 
  Cole, 
  about 
  2*7, 
  that 
  of 
  the 
  

   Scotch 
  varieties 
  varies 
  from 
  2*72 
  to 
  2-89. 
  The 
  specific 
  gravity 
  of 
  

   Carrock-Fell 
  rock, 
  it 
  will 
  be 
  seen, 
  considerably 
  exceeds 
  the 
  heaviest 
  

   of 
  them, 
  but 
  appears 
  to 
  approach 
  that 
  of 
  the 
  continental 
  variolites, 
  

   such 
  as 
  those 
  of 
  Durance 
  t 
  or 
  Jalguba 
  t. 
  

  

  Chemical 
  Composition. 
  

  

  An 
  analysis 
  of 
  the 
  rock 
  has 
  been 
  kindly 
  made 
  for 
  me 
  by 
  Mr. 
  A. 
  

   B. 
  H. 
  Adie, 
  B.A., 
  of 
  Trinity 
  College, 
  Cambridge. 
  The 
  composition 
  is 
  

   given 
  below 
  : 
  — 
  

  

  I. 
  II. 
  III. 
  IV. 
  

  

  Si0 
  2 
  53-63 
  51-66 
  53-2 
  51-42 
  

  

  TiO 
  trace. 
  

  

  A1 
  2 
  3 
  ..]... 
  15-93 
  10-34 
  8-7 
  15-39 
  

  

  Fe 
  2 
  0, 
  20-00 
  23-33 
  21-8 
  

  

  FeO 
  . 
  . 
  . 
  . 
  21-04 
  

  

  MnO 
  trace. 
  0'34 
  

  

  CaO 
  7-88 
  6-6Q 
  10-3 
  4-09 
  

  

  MgO 
  0-78 
  1-67 
  . 
  . 
  3-68 
  

  

  K 
  2 
  0-50 
  . 
  . 
  1 
  -, 
  9 
  1-07 
  

  

  Na 
  2 
  4-48 
  . 
  . 
  J 
  2-37 
  

  

  H 
  o 
  &c 
  0-56 
  3-00 
  4-1 
  0-55 
  

  

  103-76 
  97-00 
  99-3 
  99-61 
  

  

  I. 
  Carrock-Fell 
  tachylyte. 
  III. 
  Basalt 
  of 
  Mt. 
  Gravenaire 
  in 
  Auvergne. 
  

   II. 
  Basalt 
  of 
  Beaulieu. 
  IV. 
  Basalt 
  of 
  Steinsberg, 
  near 
  Sinsheim, 
  in 
  Baden. 
  

  

  * 
  Clifton 
  Ward, 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxii. 
  ; 
  and 
  Teall, 
  ' 
  British 
  

   Petrography,' 
  p. 
  178. 
  

  

  t 
  Michel 
  Levy, 
  BuU. 
  Soc. 
  Geol. 
  Fr. 
  (3) 
  v. 
  p. 
  248. 
  

  

  I 
  Tscherrnak, 
  Mineralog. 
  und 
  petrogr. 
  Mittheilungen, 
  vi. 
  pp. 
  294, 
  295. 
  

  

  