﻿93 
  PKOF. 
  T. 
  G. 
  BOSKET 
  OX 
  TWO 
  TRAVERSES 
  OF 
  THE 
  

  

  older 
  rock, 
  on 
  which 
  the 
  Upper 
  Schists 
  have 
  been 
  deposited. 
  I 
  think 
  

   that 
  the 
  author 
  of 
  this 
  hypothesis 
  cannot 
  have 
  examined 
  with 
  any 
  

   care 
  the 
  masses 
  described 
  in 
  this 
  paper, 
  or 
  he 
  would 
  have 
  seen 
  that 
  

   these 
  forerunners 
  of 
  the 
  existing- 
  Alps 
  must 
  have 
  been 
  the 
  most 
  

   singular 
  mountains 
  which 
  the 
  world 
  has 
  yet 
  known. 
  

  

  Even 
  less 
  reasonable, 
  as 
  it 
  seems 
  to 
  me, 
  is 
  the 
  proposed 
  correla- 
  

   tion 
  of 
  this 
  " 
  Pietra 
  Yerde 
  " 
  group 
  of 
  the 
  Alps 
  with 
  the 
  Huronian 
  

   of 
  Canada. 
  If 
  the 
  former 
  exists 
  at 
  all, 
  it 
  consists 
  of 
  highly 
  crystal- 
  

   line 
  rocks, 
  and 
  is 
  overlain 
  by 
  a 
  vast 
  series 
  of 
  gneisses 
  and 
  crystalline 
  

   schists. 
  The 
  latter 
  is 
  a 
  great 
  group 
  of 
  fragmental 
  rocks 
  *. 
  In 
  the 
  

   most 
  typical 
  locality 
  they 
  are 
  very 
  slightly 
  altered, 
  and 
  their 
  origin 
  is 
  

   indubitable. 
  Here 
  and 
  there 
  it 
  is 
  true 
  some 
  schists 
  occur, 
  but 
  under 
  

   circumstances 
  and 
  with 
  characters 
  which 
  justify 
  the 
  supposition 
  that 
  

   they 
  are 
  modified 
  basic 
  igneous 
  rocks 
  ; 
  but 
  the 
  rocks 
  which 
  occur 
  in 
  the 
  

   Alps, 
  at 
  the 
  so-called 
  Pietra-Yerde 
  horizon, 
  are 
  highly 
  crystalline, 
  

   and 
  their 
  origin, 
  if 
  it 
  be 
  not 
  igneous, 
  is, 
  at 
  present, 
  undeterminable. 
  

  

  To 
  conclude 
  : 
  rocks 
  of 
  igneous 
  origin, 
  sometimes 
  retaining 
  their 
  

   characteristics, 
  sometimes 
  subsequently 
  modified, 
  probably 
  in 
  con- 
  

   sequence 
  of 
  pressure, 
  occur 
  indubitably 
  at 
  all 
  horizons 
  among 
  the 
  

   crystalline 
  series 
  of 
  the 
  Alps, 
  — 
  serpentine, 
  pyroxenic, 
  and 
  granitoid 
  

   rocks. 
  But 
  I 
  cannot 
  explain, 
  and 
  I 
  believe 
  that 
  anyone 
  who 
  will 
  

   read 
  my 
  description 
  attentively 
  will 
  find 
  it 
  difficult 
  to 
  account 
  for 
  

   the 
  principal 
  members 
  of 
  the 
  Upper-Schist 
  group, 
  if 
  they 
  are 
  not 
  

   altered 
  stratified 
  rocks. 
  Mica-schist, 
  quartz-schist, 
  chloritic 
  schist, 
  

   calc-schist, 
  and 
  limestone 
  pass 
  one 
  into 
  the 
  other, 
  and 
  are 
  associated 
  

   exactly 
  as 
  sandstone, 
  shale, 
  and 
  limestone 
  in 
  an 
  ordinary 
  clastic 
  series. 
  

   The 
  minor 
  mineral 
  bands 
  — 
  the 
  stratification-foliation 
  — 
  are 
  parallel 
  

   with 
  the 
  apparent 
  direction 
  of 
  bedding 
  thus 
  indicated, 
  and 
  I 
  can 
  only 
  

   explain 
  them 
  as 
  resulting 
  from 
  the 
  deposition 
  of 
  materials 
  of 
  variable 
  

   chemical 
  composition, 
  subsequently 
  altered 
  into 
  different 
  minerals. 
  

   I 
  have 
  elsewhere 
  shown 
  f 
  that 
  from 
  indubitably 
  stratified 
  rocks, 
  

   banded 
  schists, 
  which, 
  in 
  their 
  mineral 
  association, 
  though 
  not 
  in 
  

   their 
  minuter 
  structure, 
  exactly 
  resemble 
  the 
  schists 
  of 
  this 
  group, 
  

   have 
  been 
  produced 
  by 
  the 
  action 
  of 
  intrusive 
  igneous 
  masses. 
  If, 
  

   then, 
  we 
  prefer 
  to 
  attribute 
  these 
  structures 
  to 
  some 
  modification 
  

   produced 
  by 
  pressure, 
  such 
  as 
  shearing 
  or 
  rolling-out 
  of 
  a 
  complex 
  

   of 
  igneous 
  rocks, 
  we 
  desert, 
  as 
  it 
  seems 
  to 
  me, 
  inductive 
  reasoning 
  

   for 
  imaginative 
  and 
  gratuitous 
  hypotheses. 
  

  

  In 
  the 
  middle 
  group, 
  or 
  the 
  Lepontine 
  type, 
  the 
  origin 
  of 
  the 
  various 
  

   members 
  is 
  less 
  readily 
  determined 
  ; 
  nevertheless, 
  rocks 
  of 
  variable 
  

   mineral 
  characters 
  are 
  associated 
  in 
  a 
  way 
  which, 
  at 
  any 
  rate, 
  resembles 
  

   that 
  of 
  stratified 
  masses. 
  Many 
  of 
  the 
  gneises 
  exhibit 
  bands 
  

   characterized 
  by 
  different 
  minerals, 
  especially 
  quartz, 
  felspar, 
  and 
  

   mica, 
  which 
  closely 
  resemble 
  those 
  in 
  members 
  of 
  the 
  upper 
  series, 
  

   though 
  all 
  the 
  constituents 
  are 
  more 
  coarsely 
  crystalline. 
  Hence, 
  

  

  * 
  The 
  typical 
  district 
  north-east 
  of 
  Lake 
  Huron 
  is 
  now 
  well 
  known, 
  having 
  

   been 
  admirably 
  described 
  by 
  tbe 
  late 
  Professor 
  Irving 
  (Amer. 
  Journ. 
  Sci. 
  

   vol. 
  xxxiv. 
  pp. 
  204-, 
  249. 
  365). 
  I 
  have 
  also 
  seen 
  soinelbing 
  of 
  it 
  myself 
  (Quart. 
  

   Journ. 
  Greol. 
  Soc. 
  vol. 
  xliv. 
  p. 
  32). 
  

  

  t 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xliv. 
  p. 
  11. 
  

  

  