﻿24 
  T. 
  ll'EENNY 
  HUGHES 
  ON 
  THE 
  GEOLOGY 
  OE 
  ANGLESEY. 
  

  

  timately 
  ; 
  while 
  for 
  comparison 
  with 
  the 
  finer 
  and 
  more 
  compact 
  

   quartz-conglomerate 
  of 
  the 
  Field 
  quarry, 
  Twt 
  Hill 
  (Q. 
  J. 
  G. 
  S. 
  

   vol. 
  xxxv. 
  Nov. 
  1879, 
  p. 
  683, 
  f. 
  2), 
  I 
  rather 
  refer 
  to 
  a 
  quartz- 
  con- 
  

   glomerate 
  much 
  higher 
  up 
  in 
  the 
  Cambrian, 
  which 
  occurs 
  on 
  the 
  

   shore 
  S.W. 
  of 
  Garth 
  Point, 
  Bangor, 
  near 
  where 
  the 
  great 
  cloaca 
  

   opens 
  on 
  the 
  Straits 
  (Woodwardian 
  Museum 
  slide 
  217). 
  S.W. 
  of 
  

   Llanerchymedd, 
  near 
  Bryngwallen, 
  the 
  Pre-Cambrian 
  rocks 
  consist 
  

   of 
  a 
  variable 
  gneissic 
  series, 
  and 
  the 
  Cambrian 
  basement 
  beds 
  are 
  

   finer 
  and 
  composed 
  chiefly 
  of 
  quartz. 
  They 
  can 
  be 
  well 
  studied 
  in 
  

   a 
  quarry 
  in 
  the 
  field, 
  on 
  the 
  north 
  side 
  of 
  the 
  road, 
  below 
  the 
  g 
  of 
  

   Bryngwallen, 
  while 
  the 
  gneiss 
  is 
  exposed 
  along 
  the 
  road, 
  and 
  in 
  one 
  

   place 
  a 
  quarry 
  is 
  opened 
  in 
  it. 
  

  

  In 
  the 
  quarry 
  on 
  the 
  north 
  of 
  the 
  road 
  the 
  Cambrian 
  conglomerate 
  

   is 
  thin 
  and 
  irregular. 
  It 
  resembles 
  much 
  the 
  conglomerate 
  of 
  Twt 
  

   Hill, 
  both 
  in 
  the 
  mass 
  and 
  under 
  the 
  microscope 
  (see 
  W. 
  M. 
  slides 
  

   Nos. 
  214, 
  215). 
  It 
  is 
  seen 
  to 
  pass 
  into 
  an 
  altered 
  and 
  veiny 
  grit, 
  

   portions 
  of 
  which 
  are 
  crushed 
  ; 
  but 
  in 
  the 
  weathered 
  portions 
  there 
  

   are 
  traces 
  of 
  fossils. 
  The 
  slice 
  exhibited 
  (W. 
  M. 
  slide 
  215) 
  is 
  cut 
  

   across 
  the 
  cast 
  of 
  an 
  Orthis. 
  

  

  The 
  sandstones 
  and 
  grits 
  of 
  the 
  Cambrian 
  of 
  this 
  district 
  show 
  

   nothing 
  peculiar 
  in 
  their 
  composition, 
  and 
  do 
  not 
  differ 
  much 
  from 
  

   one 
  another, 
  except 
  in 
  the 
  amount 
  of 
  felspathic 
  matter 
  in 
  the 
  matrix. 
  

   Even 
  where 
  the 
  fragments 
  are 
  not 
  large 
  enough 
  to 
  be 
  seen 
  with 
  the 
  

   naked 
  eye, 
  the 
  presence 
  of 
  any 
  considerable 
  quantity 
  can 
  generally 
  

   be 
  detected 
  at 
  once 
  in 
  the 
  field 
  by 
  the 
  dull 
  yellow 
  of 
  the 
  weathered 
  

   portions 
  and 
  the 
  manner 
  in 
  which 
  the 
  whole 
  mass 
  of 
  rock 
  breaks 
  up 
  

   and 
  yields 
  to 
  atmospheric 
  agencies. 
  

  

  Where 
  they 
  are 
  composed 
  entirely 
  of 
  quartz 
  and 
  have 
  been 
  much 
  

   crushed, 
  and 
  veining 
  has 
  supervened, 
  we 
  have, 
  of 
  course, 
  every 
  stage 
  

   from 
  sandstone 
  or 
  grit 
  to 
  white 
  quartz. 
  Examples 
  of 
  this 
  may 
  be 
  

   seen 
  in 
  the 
  three 
  specimens 
  exhibited 
  from 
  the 
  same 
  mass 
  at 
  Bryn- 
  

   gwallen 
  : 
  (19) 
  is 
  much 
  crushed 
  ; 
  (20) 
  is 
  compact, 
  uncrushed, 
  and 
  

   not 
  weathered 
  ; 
  (21 
  and 
  corresponding 
  slides, 
  Nos. 
  214, 
  215, 
  and 
  

   222) 
  is 
  weathered 
  and 
  contains 
  fossils. 
  

  

  As 
  to 
  the 
  shales 
  there 
  is 
  no 
  question, 
  and 
  therefore 
  I 
  need 
  not 
  

   refer 
  to 
  them 
  under 
  this 
  head. 
  

  

  The 
  brecciated 
  conglomerates 
  vary 
  considerably 
  in 
  the 
  upper 
  part 
  

   of 
  the 
  black 
  shales 
  ; 
  they 
  are 
  often 
  very 
  coarse. 
  Their 
  ash-like 
  ap- 
  

   pearance 
  is 
  often 
  increased 
  by 
  the 
  rust-lined 
  cavities 
  from 
  which 
  

   bits 
  of 
  earthy 
  iron-ore 
  or 
  pyrites 
  have 
  perished 
  under 
  the 
  action 
  of 
  

   weathering. 
  The 
  scattered 
  fragments 
  in 
  the 
  highest 
  rocks 
  of 
  this 
  

   character 
  seen 
  in 
  Porth 
  Corwg 
  consist 
  of 
  limestone, 
  quartzite, 
  grit, 
  

   purplish, 
  black, 
  and 
  grey 
  slate, 
  quartz, 
  &c. 
  ; 
  lower 
  down, 
  the 
  slaty 
  

   fragments 
  predominate. 
  

  

  In 
  the 
  brecciated 
  beds 
  near 
  the 
  base 
  of 
  the 
  shales, 
  on 
  both 
  sides 
  of 
  

   the 
  Llecheynfarwy 
  axis, 
  S.W. 
  of 
  Llanerchymedd, 
  the 
  fragments 
  are 
  

   not 
  so 
  much 
  scattered 
  through 
  the 
  mass, 
  but 
  form 
  a 
  compact 
  breccia 
  

   of 
  small 
  pieces, 
  chiefly 
  of 
  slate. 
  

  

  In 
  the 
  Bangor-Caernarvon 
  district 
  the 
  drifting 
  of 
  the 
  shingle 
  which 
  

   formed 
  the 
  shore-deposits 
  of 
  the 
  Cambrian 
  sea 
  was 
  from 
  S.W. 
  to 
  

  

  