﻿T. 
  M'KENNY 
  HUGHES 
  ON 
  THE 
  GEOLOGY 
  OE 
  ANGLESEY. 
  2S 
  

  

  with 
  alternations, 
  many 
  of 
  the 
  varieties 
  of 
  conglomerate 
  and 
  grit 
  

   which, 
  even 
  when 
  occurring 
  separately 
  in 
  other 
  localities, 
  we 
  have 
  

   on 
  independent 
  evidence 
  referred 
  to 
  the 
  same 
  series, 
  though 
  differ- 
  

   ing 
  locally 
  accordiug 
  to 
  the 
  character 
  of 
  the 
  underlying 
  rocks, 
  and 
  

   the 
  drift 
  of 
  the 
  shingle 
  and 
  other 
  circumstances. 
  

  

  We 
  find 
  here 
  in 
  the 
  basement 
  beds 
  a 
  coarse 
  grit, 
  such 
  as 
  is 
  seen 
  

   at 
  Caernarvon, 
  at 
  Carreg 
  goch, 
  at 
  Llanddeiniolen, 
  at 
  Bangor, 
  on 
  the 
  

   coast 
  east 
  of 
  Amlwch, 
  and 
  at 
  Penlon. 
  We 
  have 
  here 
  also 
  with 
  the 
  

   grit 
  a 
  quartz 
  -jasper 
  conglomerate, 
  just 
  like 
  that 
  seen 
  N.E. 
  of 
  Twt 
  

   Hill, 
  west 
  of 
  Llanfaelog, 
  and 
  at 
  Penlon. 
  There 
  is 
  also, 
  associated 
  

   with 
  this 
  conglomerate 
  and 
  grit, 
  a 
  coarse 
  mixed 
  conglomerate 
  made 
  

   up 
  of 
  pebbles 
  of 
  quartzite, 
  schist, 
  rarely 
  felsite, 
  and 
  various 
  other 
  

   Pre-Cambrian 
  rocks, 
  and 
  approaching 
  more 
  closely 
  the 
  Bangor 
  type 
  

   of 
  the 
  Cambrian 
  basement 
  beds. 
  

  

  Eelsites 
  are 
  not 
  largely 
  developed 
  in 
  the 
  Pre-Cambrian 
  of 
  Anglesey, 
  

   so 
  that 
  pebbles 
  of 
  that 
  rock 
  cannot 
  be 
  expected 
  to 
  form 
  a 
  large 
  part 
  

   of 
  the 
  overlying 
  cod 
  glomerate, 
  as 
  they 
  do 
  from 
  Llanddeiniolen 
  to 
  

   Bangor, 
  where 
  the 
  Cambrian 
  rocks 
  occur 
  on, 
  or 
  not 
  far 
  from, 
  enor- 
  

   mous 
  masses 
  of 
  felsite. 
  

  

  The 
  base 
  of 
  the 
  Cambrian, 
  near 
  Llanerchymedd, 
  rests 
  on 
  a 
  green 
  

   chloritic 
  felspathic 
  mudstone, 
  often 
  compact 
  and 
  hornstony, 
  weather- 
  

   ing 
  olive 
  and 
  yellow. 
  The 
  character 
  of 
  the 
  rock 
  may 
  be 
  examined 
  

   in 
  the 
  quarry 
  at 
  the 
  junction 
  of 
  the 
  two 
  roads 
  near 
  the 
  windmill. 
  

   It 
  is 
  more 
  blue 
  where 
  not 
  weathered, 
  with 
  bands 
  of 
  lighter 
  blue 
  or 
  

   apple-green, 
  often 
  showing 
  small 
  faults, 
  and 
  exactly 
  resembling 
  some 
  

   of 
  the 
  banded 
  slates 
  of 
  Brynian 
  Bangor. 
  

  

  The 
  microscope 
  does 
  not 
  add 
  much 
  to 
  our 
  knowledge 
  of 
  it 
  on 
  the 
  

   point 
  under 
  consideration 
  (see 
  Woodwardian 
  Museum 
  slide 
  248), 
  as 
  

   the 
  fragments 
  of 
  this 
  rock 
  in 
  the 
  overlying 
  series 
  are 
  obvious 
  and 
  

   unmistakable. 
  

  

  One 
  large 
  pebble 
  of 
  felsitic 
  rock 
  is 
  exhibited, 
  which 
  I 
  procured 
  

   from 
  the 
  Cambrian 
  conglomerate 
  east 
  of 
  the 
  windmill, 
  and 
  which 
  

   must 
  have 
  drifted 
  some 
  distance 
  ; 
  it 
  resembles 
  exactly 
  a 
  rock 
  com- 
  

   mon 
  among 
  the 
  pebbles 
  of 
  the 
  Cambrian 
  basement 
  bed 
  of 
  Bangor. 
  

  

  Just 
  beyond 
  the 
  depression 
  which 
  crosses 
  the 
  hill 
  south 
  of 
  Pen- 
  

   cefn, 
  the 
  granitoid 
  series 
  comes 
  on. 
  I 
  have 
  not 
  observed 
  any 
  thing 
  

   that 
  could 
  be 
  referred 
  to 
  the 
  Dinorwig 
  beds 
  (Felsitic 
  series) 
  in 
  this 
  

   area. 
  The 
  granitoid 
  rocks 
  are 
  somewhat 
  finer 
  than 
  that 
  seen 
  on 
  

   Twt 
  Hill 
  or 
  in 
  the 
  heart 
  of 
  the 
  Llanfaelog 
  axis. 
  

  

  About 
  3 
  miles 
  N.N.W. 
  of 
  Llanerchymedd 
  the 
  granitoid 
  rocks 
  turn 
  

   up 
  again. 
  They 
  here 
  consist 
  of 
  quartz, 
  felspar, 
  and 
  often 
  mica, 
  are 
  

   sometimes 
  platy 
  and 
  schistose, 
  and 
  contain 
  subordinate 
  greenish 
  

   chloritic 
  schistose 
  rocks 
  and 
  perhaps 
  altered 
  dykes. 
  The 
  Cambrian 
  

   conglomerate 
  is 
  here 
  composed 
  chiefly 
  of 
  quartz 
  with 
  some 
  fragments 
  

   of 
  jasper, 
  mica-schists, 
  Lydian 
  stone, 
  &c, 
  and 
  much 
  resembles 
  the 
  

   conglomerate 
  in 
  the 
  quarry 
  between 
  Ts 
  cuborwen 
  and 
  Tygwyn,N.E. 
  

   of 
  Twt 
  Hill 
  (see 
  Q. 
  J. 
  G. 
  S. 
  vol. 
  xxxv. 
  p. 
  691, 
  f. 
  10), 
  where 
  the 
  

   granitoid 
  rock 
  crops 
  out 
  immediately 
  behind 
  the 
  conglomerate, 
  

   though 
  the 
  actual 
  junction 
  is 
  not 
  seen. 
  In 
  microscopic 
  structure 
  

   also 
  the 
  Cambrian 
  conglomerate 
  from 
  these 
  two 
  localities 
  agrees 
  in- 
  

  

  