﻿CRYSTALLINE 
  ROCKS 
  OP 
  THE 
  ALPS. 
  95 
  

  

  described, 
  but 
  the 
  tunnel 
  at 
  the 
  eastern 
  entrance 
  seems 
  to 
  be 
  made 
  

   in 
  a 
  rather 
  harder 
  schist 
  or 
  in 
  gneiss. 
  

  

  We 
  were 
  thus 
  able 
  to 
  trace 
  this 
  peculiar 
  lead-coloured 
  schist 
  — 
  

   the 
  typical 
  " 
  Thonglimmerschiefer 
  " 
  — 
  in 
  accordance 
  with 
  Yon 
  Hauer's 
  

   mapping, 
  along 
  its 
  strike, 
  roughly 
  speaking, 
  for 
  a 
  distance 
  of 
  over 
  

   60 
  miles 
  south 
  of 
  the 
  central 
  range, 
  and 
  for 
  a 
  distance 
  of 
  over 
  90 
  

   miles 
  north 
  of 
  it, 
  and 
  to 
  find 
  it 
  associated 
  with 
  similar 
  chloritic 
  

   schists, 
  quartzose 
  schists, 
  calc-schists 
  and 
  crystalline 
  limestones 
  in 
  

   several 
  localities, 
  On 
  both 
  sides 
  of 
  the 
  central 
  range 
  we 
  found 
  

   gneisses 
  and 
  mica-schists 
  of 
  like 
  lithological 
  character, 
  and 
  in 
  the 
  

   heart 
  of 
  it, 
  apparently 
  underlying 
  the 
  " 
  Thonglimmerschiefer 
  " 
  series, 
  

   a 
  group 
  of 
  gneisses, 
  of 
  hornblendic 
  and 
  granitoid 
  rocks 
  of 
  more 
  ancient 
  

   aspect, 
  which 
  not 
  seldom 
  recalled 
  the 
  oldest 
  rocks 
  with 
  which 
  we 
  

   are 
  acquainted, 
  

  

  III. 
  Conclusions. 
  

  

  From 
  the 
  investigations 
  above 
  described, 
  aided 
  by 
  a 
  study 
  of 
  the 
  

   collection 
  in 
  the 
  Innsbruck 
  Museum, 
  I 
  do 
  not 
  see 
  how 
  to 
  escape 
  the 
  

   following 
  conclusions 
  : 
  — 
  

  

  (1) 
  The 
  " 
  Thonglimmerschiefer 
  " 
  series, 
  consisting 
  of 
  lead-coloured 
  

   micaceous 
  schists, 
  with 
  more 
  or 
  less 
  quartzose 
  and 
  calcareous 
  

   varieties, 
  sometimes 
  becoming 
  marbles 
  and 
  occasionally 
  chloritic 
  

   schists, 
  forms 
  a 
  group 
  of 
  rocks, 
  well 
  defined 
  lithologically 
  and 
  strati- 
  

   graphically. 
  I 
  have 
  traced 
  it 
  myself, 
  as 
  mentioned 
  above 
  ; 
  but 
  accor- 
  

   ding 
  to 
  Yon 
  Hauer's 
  map 
  (and 
  his 
  types 
  are 
  so 
  well-marked 
  that 
  I 
  

   feel 
  confidence 
  in 
  its 
  accuracy) 
  this 
  group 
  extends 
  eastwards 
  to 
  

   within 
  a 
  few 
  leagues 
  of 
  Gratz, 
  over 
  full 
  5 
  degrees 
  of 
  longitude 
  in 
  all. 
  

  

  (2) 
  Apparently 
  underlying 
  this 
  comes 
  another 
  group, 
  consisting 
  

   of 
  mica- 
  schists 
  and 
  gneisses, 
  bearing 
  a 
  general 
  resemblance 
  to 
  the 
  

   " 
  Lepontine 
  " 
  group 
  which 
  is 
  so 
  well 
  exhibited 
  on 
  the 
  St.-Gothard 
  

   Pass 
  and 
  in 
  the 
  adjacent 
  parts 
  of 
  North 
  Italy. 
  While 
  it 
  is 
  difficult 
  in 
  

   some 
  districts 
  to 
  separate 
  this 
  lithological 
  group 
  from 
  the 
  former, 
  

   the 
  typical 
  members 
  of 
  each 
  are 
  perfectly 
  distinct. 
  

  

  (3) 
  Yet 
  lower, 
  and 
  more 
  rarely 
  exposed, 
  comes 
  a 
  group 
  chiefly 
  

   consisting 
  of 
  banded 
  gneisses, 
  and 
  granitoid 
  gneisses 
  or 
  gneissoid 
  

   granites. 
  The 
  latter 
  may 
  be 
  igneous 
  in 
  origin, 
  but 
  they 
  have 
  a 
  

   curiously 
  ancient 
  fades, 
  while 
  the 
  banded 
  gneisses 
  are 
  very 
  like 
  

   some 
  of 
  the 
  Laurentian 
  and 
  Hebridean 
  rocks. 
  These 
  appear 
  to 
  pass 
  

   up 
  gradually 
  into 
  the 
  last-named 
  group, 
  so 
  that 
  it 
  is 
  difficult 
  to 
  draw 
  

   a 
  sharp 
  line 
  between 
  them, 
  though, 
  as 
  before, 
  type 
  specimens 
  of 
  

   each 
  are 
  very 
  distinct, 
  the 
  one 
  being 
  very 
  hard, 
  tough, 
  often 
  break- 
  

   ing 
  readily 
  across 
  the 
  banding, 
  the 
  other, 
  whiter, 
  commonly 
  more 
  

   micaceous, 
  friable 
  and 
  fissile. 
  

  

  Each 
  of 
  these 
  groups 
  is 
  characterized 
  by 
  a 
  general 
  constancy 
  in 
  

   the 
  size 
  of 
  the 
  constituents 
  in 
  typical 
  examples, 
  so 
  that 
  there 
  is 
  a 
  

   marked 
  increase 
  in 
  coarseness 
  from 
  (1) 
  to 
  (8). 
  This 
  may 
  be 
  roughly 
  

   represented 
  by 
  saying 
  that 
  the 
  diameter 
  of 
  grains 
  of 
  the 
  same 
  mineral 
  

   will 
  be 
  as 
  the 
  numbers 
  1, 
  2, 
  3. 
  

  

  (4) 
  A 
  study 
  of 
  the 
  geological 
  map 
  of 
  the 
  Austrian 
  Alps 
  suggests 
  

  

  