﻿302 
  ME. 
  T. 
  T. 
  GEOOil 
  OX 
  A 
  TACHTLTTE 
  ASSOCIATED 
  WITH 
  THE 
  

  

  the 
  point 
  at 
  which 
  the 
  rock 
  analyzed 
  was 
  taken 
  is 
  at 
  a 
  considerable 
  

   distance 
  from 
  that 
  at 
  which 
  the 
  taehyh^te 
  occurs, 
  and 
  that 
  the 
  rock 
  

   must 
  vary 
  greatly 
  in 
  composition 
  is 
  shown 
  by 
  the 
  fact 
  that 
  all 
  

   transitional 
  stages 
  can 
  be 
  observed 
  between 
  the 
  acid 
  granophyre 
  

   and 
  the 
  basic 
  gabbro 
  at 
  this 
  point. 
  

  

  It 
  seems 
  probable 
  that 
  the 
  tachylyte 
  represents 
  a 
  portion 
  of 
  the 
  

   gabbro 
  which 
  remained 
  liquid 
  after 
  the 
  rest 
  had 
  consolidated, 
  and, 
  

   upon 
  fracture 
  of 
  the 
  latter, 
  was 
  injected 
  as 
  a 
  thin 
  sheet 
  into 
  it. 
  

  

  Age. 
  

  

  The 
  evidence 
  as 
  to 
  the 
  age 
  of 
  these 
  associated 
  rocks 
  is 
  unfor- 
  

   tunately 
  unsatisfactory, 
  but 
  the 
  following 
  considerations 
  bear 
  on 
  the 
  

   point 
  : 
  — 
  

  

  The 
  gabbro, 
  in 
  places, 
  shows 
  distinct 
  foliation-banding 
  (flaser 
  

   structure) 
  * 
  and 
  appears 
  to 
  have 
  undergone 
  regional 
  metamorphism. 
  

   The 
  strike 
  of 
  the 
  foliation 
  agrees 
  with 
  that 
  of 
  the 
  neighbouring 
  

   interbedded 
  volcanic 
  series, 
  and 
  it 
  is 
  probable 
  that 
  the 
  movements 
  

   which 
  resulted 
  in 
  giving 
  the 
  rocks 
  of 
  the 
  Lake 
  district 
  a 
  "W.S.'W. 
  and 
  

   E.N.E. 
  strike 
  caused 
  the 
  foliation 
  of 
  the 
  gabbro. 
  The 
  latter, 
  accord- 
  

   ingly, 
  already 
  existed 
  in 
  Devonian 
  times, 
  and 
  if 
  really 
  intrusive 
  in 
  the 
  

   Skiddaw 
  slates 
  must 
  be 
  of 
  post-Cambrian 
  age 
  ; 
  it 
  is 
  therefore 
  probably 
  

   connected 
  with 
  the 
  eruption 
  of 
  the 
  volcanic 
  rocks 
  of 
  the 
  district. 
  Mr. 
  

   Teall 
  has, 
  indeed, 
  pointed 
  out 
  the 
  resemblance 
  of 
  some 
  of 
  the 
  Carrock- 
  

   JFell 
  rocks 
  to 
  the 
  Eycott-Hill 
  lavas, 
  and 
  has 
  suggested 
  that 
  the 
  

   former 
  may 
  be 
  the 
  plutonic 
  representatives 
  of 
  the 
  latter 
  f 
  . 
  The 
  

   tachylyte 
  is, 
  then, 
  probably 
  of 
  Ordovician 
  age, 
  and, 
  with 
  the 
  exception 
  

   of 
  the 
  tachylyte 
  observed 
  by 
  the 
  Officers 
  of 
  the 
  Geological 
  Survey 
  % 
  

   in 
  connexion 
  with 
  the 
  Archaean 
  basic 
  rocks 
  of 
  the 
  west 
  of 
  Sutherland, 
  

   represents 
  the 
  oldest 
  known 
  British 
  glassy 
  rock 
  of 
  basic 
  composition. 
  

  

  Comparisons 
  iviili 
  other 
  Basic 
  Glassy 
  Rocks. 
  

  

  The 
  low 
  percentage 
  of 
  silica 
  and 
  the 
  general 
  agreement 
  of 
  the 
  com- 
  

   position 
  of 
  the 
  Carrock-Eell 
  rock 
  with 
  that 
  of 
  certain 
  basalts 
  ; 
  the 
  

   great 
  abundance 
  of 
  magnetite, 
  of 
  skeleton-crystals, 
  and 
  other 
  develop- 
  

   mental 
  forms 
  ; 
  the 
  minutely 
  granular 
  character 
  of 
  the 
  pyroxene 
  ; 
  the 
  

   tendency 
  of 
  the 
  minerals 
  of 
  the 
  effusion-period 
  to 
  group 
  themselves 
  

   round 
  those 
  of 
  the 
  intratelluric 
  ; 
  the 
  high 
  specific 
  gravity, 
  easy 
  fusi- 
  

   bility, 
  and 
  opacity, 
  combine 
  to 
  place 
  it 
  among 
  the 
  basic 
  glassy 
  rocks. 
  

  

  Among 
  these 
  it 
  has 
  affinities 
  with 
  the 
  variolites 
  of 
  Jalguba 
  § 
  

   described 
  by 
  Loewinson-Lessing 
  ||, 
  and 
  with 
  other 
  glassy 
  forms 
  

   associated 
  with 
  the 
  more 
  basic 
  augite-porphyrites 
  and 
  augite-an- 
  

   desites, 
  and 
  the 
  more 
  acid 
  basalts. 
  Points 
  of 
  resemblance 
  to 
  these 
  

  

  * 
  This 
  was 
  regarded 
  by 
  Mr. 
  Clifton 
  Ward 
  as 
  bedding. 
  

  

  t 
  Op.cit. 
  p. 
  228. 
  

  

  + 
  Quart, 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xliv. 
  1888, 
  p. 
  390. 
  

  

  § 
  Rosenbusch 
  points 
  out 
  tbat 
  this 
  rock 
  appears 
  to 
  differ 
  from 
  the 
  typical 
  

   variolites 
  of 
  Durance, 
  and 
  appends 
  it 
  to 
  the 
  augite-porphyrites. 
  See 
  ' 
  Mikro- 
  

   skopische 
  Physiographie 
  der 
  massigen 
  Gesteine,' 
  p. 
  234. 
  

  

  I 
  Tschermak, 
  ' 
  Mineralog. 
  und 
  petrogr. 
  Mittheilungen,' 
  vi. 
  p. 
  281. 
  

  

  