﻿ROCKS 
  OF 
  THE 
  LEICESTERSHIRE 
  COAL-FIELD. 
  25 
  

  

  Felspathic 
  Grits 
  or 
  Quartzites. 
  — 
  I 
  have 
  had 
  slices 
  cut 
  for 
  micro- 
  

   scopical 
  examination 
  from 
  eight 
  specimens 
  in 
  every 
  way 
  typical 
  of 
  

   those 
  which 
  are, 
  numerically 
  speaking, 
  by 
  far 
  the 
  most 
  important 
  of 
  

   the 
  brecciated 
  fragments. 
  Pour 
  of 
  these 
  are 
  from 
  the 
  Boothorpe 
  

   section 
  (Series 
  1), 
  one 
  from 
  Packington 
  (Series 
  2), 
  one 
  from 
  Measham 
  

   (Series 
  4), 
  and 
  two 
  from 
  Polesworth 
  (Series 
  6). 
  Microscopically 
  

   as 
  well 
  as 
  macroscopically 
  there 
  is 
  a 
  very 
  striking 
  similarity 
  in 
  all 
  

   these 
  specimens. 
  

  

  The 
  quartz-grains 
  are, 
  as 
  a 
  rule, 
  fairly 
  well 
  rolled, 
  and 
  occasionally 
  

   secondary 
  cementing-quartz 
  occurs 
  in 
  optical 
  continuity 
  with 
  the 
  

   original 
  grains. 
  The 
  true 
  nature 
  of 
  the 
  brownish 
  felspathic 
  grains 
  

   it 
  is 
  almost 
  impossible 
  in 
  most 
  cases 
  to 
  determine, 
  as 
  they 
  are 
  much 
  

   decomposed. 
  In 
  one 
  or 
  two 
  cases, 
  however, 
  where 
  decomposition 
  

   has 
  not 
  proceeded 
  so 
  far, 
  they 
  may 
  certainly 
  be 
  described 
  as 
  rotten 
  

   glassy 
  lavas 
  of 
  a 
  rock 
  which 
  we 
  may 
  call 
  trachyte, 
  i. 
  e. 
  one 
  moderately 
  

   rich 
  in 
  silica, 
  certainly 
  not 
  a 
  basalt. 
  Here 
  and 
  there 
  a 
  grain 
  of 
  

   felspar 
  or 
  of 
  tourmaline 
  may 
  be 
  detected. 
  In 
  one 
  specimen, 
  a 
  

   beautifully 
  crystalline 
  grit, 
  both 
  the 
  quartz- 
  and 
  lava- 
  fragments 
  are 
  

   fringed 
  with 
  bristling 
  flakes 
  of 
  a 
  chlorite. 
  

  

  Professor 
  Lap 
  worth 
  recognizes 
  these 
  quartzites 
  as 
  belonging 
  to 
  

   the 
  lower 
  part 
  of 
  the 
  Hartshill 
  quartzite 
  series 
  ; 
  but 
  there 
  is 
  one 
  

   important 
  point 
  of 
  difference 
  which 
  indicates 
  that 
  the 
  Permian 
  

   fragments 
  cannot 
  have 
  been 
  derived 
  from 
  the 
  exact 
  locality 
  in 
  which 
  

   the 
  rocks 
  are 
  now 
  exposed. 
  In 
  most 
  of 
  my 
  specimens 
  the 
  twin 
  

   lamellae 
  of 
  the 
  felspar 
  crystals 
  are 
  somewhat 
  curved, 
  and 
  dull 
  shadows 
  

   play 
  over 
  the 
  quartz-fragments 
  as 
  the 
  stage 
  is 
  rotated. 
  These 
  

   " 
  strain 
  shadows," 
  from 
  which 
  the 
  Hartshill 
  quartzite 
  is 
  free, 
  indi- 
  

   cate 
  that 
  the 
  rock 
  must 
  at 
  one 
  time 
  have 
  been 
  in 
  a 
  state 
  of 
  consi- 
  

   derable 
  tension 
  or 
  compression. 
  This 
  is 
  further 
  confirmed 
  by 
  the 
  

   fact 
  that 
  some 
  of 
  the 
  felspathic 
  grits 
  show 
  distinct 
  signs 
  of 
  cleavage. 
  

  

  Gritty 
  Slates 
  and 
  Flinty 
  Slate. 
  — 
  I 
  have 
  not 
  examined 
  any 
  of 
  these 
  

   microscopically, 
  but 
  it 
  is 
  clearly 
  seen, 
  when 
  a 
  very 
  large 
  number 
  of 
  

   specimens 
  are 
  examined, 
  that 
  they 
  belong 
  to 
  the 
  same 
  series 
  as 
  the 
  

   felspathic 
  grits 
  and 
  quartzites, 
  for 
  every 
  stage 
  of 
  transition 
  can 
  be 
  

   observed. 
  Some 
  of 
  the 
  closer 
  grained 
  and 
  more 
  flinty 
  slates 
  resem- 
  

   ble 
  some 
  of 
  the 
  Charnwood 
  rocks. 
  

  

  Argillite. 
  — 
  Prom 
  Series 
  1, 
  I 
  have 
  obtained 
  a 
  few 
  angular 
  frag- 
  

   ments 
  of 
  a 
  very 
  fine 
  compact 
  greenish 
  argillite, 
  which 
  exhibits 
  a 
  

   curious 
  wrinkled 
  weathering. 
  Under 
  the 
  microscope 
  it 
  is 
  seen 
  to 
  

   be 
  composed 
  of 
  rather 
  uniform 
  and 
  fine-grained 
  materials, 
  which 
  

   have 
  undergone 
  a 
  considerable 
  amount 
  of 
  micro-mineralogical 
  change, 
  

   developing 
  ferrite 
  and 
  chlorite 
  or 
  viridite. 
  In 
  parts 
  of 
  the 
  slide 
  

   granular 
  clusters 
  of 
  a 
  light 
  brown 
  anisotropic 
  mineral 
  are 
  developing, 
  

   but 
  generally 
  there 
  is 
  a 
  want 
  of 
  definite 
  structure. 
  Professor 
  Bonney 
  

   believes 
  that 
  this 
  rock 
  is 
  not 
  newer 
  than 
  Ordovician. 
  

  

  Stratified 
  Volcanic 
  ~Rock 
  a 
  — 
  From 
  each 
  of 
  the 
  Series 
  (1) 
  and 
  (2) 
  

   I 
  have 
  obtained 
  a 
  few 
  fragments 
  of 
  a 
  rock 
  which, 
  macroscopically, 
  

   may 
  be 
  described 
  as 
  a 
  grit, 
  but 
  which, 
  when 
  sliced, 
  is 
  seen 
  to 
  be 
  

   made 
  up 
  mainly 
  of 
  volcanic 
  igneous 
  rock 
  of 
  a 
  distinctly 
  basic 
  cha- 
  

   racter 
  which 
  has 
  been 
  much 
  affected 
  by 
  micro-mineralogical 
  change, 
  

  

  