﻿T. 
  M'XENNY 
  HUGHES 
  OF 
  THE 
  GEOLOGY 
  OF 
  ANGLESEY. 
  25 
  

  

  N.E. 
  ; 
  and 
  so, 
  as 
  we 
  have 
  the 
  granitoid 
  rocks 
  at 
  the 
  S."W. 
  end 
  only, 
  

   we 
  find 
  the 
  basement 
  bed 
  there 
  is 
  either 
  a 
  kind 
  of 
  arkose 
  or 
  chiefly 
  

   composed 
  of 
  quartz 
  with 
  a 
  few 
  pieces 
  of 
  mica-schist 
  and 
  jasper. 
  As 
  

   we 
  follow 
  it 
  4 
  or 
  5 
  miles 
  to 
  the 
  N.E., 
  the 
  quartz 
  has 
  got 
  pounded 
  

   into 
  small 
  grains, 
  giving 
  the 
  grit 
  of 
  Carreggoch 
  and 
  Llanddeiniolen, 
  

   where 
  the 
  grit 
  contains 
  much 
  felspathic 
  material 
  and 
  is 
  overlain 
  by 
  a 
  

   coarse 
  conglomerate 
  of 
  pebbles 
  of 
  the 
  felsite 
  which 
  here 
  formed 
  the 
  

   shore 
  ; 
  and 
  further 
  on, 
  towards 
  Bangor, 
  fragments 
  of 
  the 
  still 
  higher 
  

   Bangor 
  volcanic 
  series 
  helped 
  to 
  make 
  up 
  the 
  Cambrian 
  shingle-beach. 
  

  

  Fragments 
  of 
  vein-quartz 
  and 
  quartzite 
  from 
  the 
  Pre-Cambrian 
  

   occur 
  scattered 
  through 
  the 
  basement 
  beds 
  everywhere 
  ; 
  but 
  the 
  grit 
  

   is 
  perhaps 
  derived 
  largely 
  from 
  the 
  imperfect 
  crystals 
  of 
  the 
  

   Dimetian. 
  

  

  On 
  the 
  IST.W. 
  side 
  of 
  the 
  Anglesey 
  central 
  axis, 
  which 
  consists 
  of 
  

   various 
  gneissic 
  rocks 
  (Dimetian 
  in 
  part) 
  and 
  a 
  felspathic 
  mudstone 
  

   (Pebidian), 
  the 
  Cambrian 
  basement 
  bed 
  varies 
  accordingly, 
  and 
  is 
  

   generally 
  more 
  quartzose. 
  Near 
  Penlon, 
  where 
  it 
  rests 
  on 
  quartz- 
  

   felspar 
  rock, 
  it 
  consists 
  chiefly 
  of 
  quartz, 
  and 
  exactly 
  resembles 
  the 
  

   Twt-Hill 
  conglomerate. 
  Near 
  Llanerchymedd, 
  where 
  there 
  is 
  a 
  mass 
  

   of 
  greenish 
  schistose 
  rock 
  succeeding 
  the 
  Dimetian, 
  the 
  Cambrian 
  

   basement 
  bed 
  contains 
  a 
  large 
  quantity 
  of 
  fragments 
  of 
  that 
  rock, 
  

   certain 
  bands 
  being 
  chiefly 
  composed 
  of 
  it. 
  Near 
  Bryngwallen, 
  

   where 
  the 
  underlying 
  Archaean 
  consists 
  of 
  gneissic 
  rocks, 
  the 
  Cam- 
  

   brian 
  basement 
  beds 
  are 
  made 
  up 
  of 
  grit 
  and 
  quartz 
  conglomerate 
  

   in 
  lithological 
  character 
  exactly 
  like 
  that 
  of 
  Twt 
  Hill, 
  but 
  here 
  

   fortunately 
  fossiliferous. 
  

  

  If 
  we 
  follow 
  the 
  basement 
  beds 
  to 
  the 
  S.W. 
  we 
  find 
  a 
  much 
  

   greater 
  mixture 
  of 
  rocks 
  in 
  the 
  conglomerate, 
  until 
  among 
  the 
  bosses 
  

   of 
  rock 
  in 
  the 
  sand 
  dunes 
  north 
  of 
  Cymmeran 
  Bay 
  we 
  see 
  evidence 
  

   of 
  the 
  existence 
  of 
  newer 
  Pre-Cambrian 
  beds 
  than 
  are 
  now 
  exposed 
  

   anywhere 
  in 
  that 
  neighbourhood. 
  

  

  Thus 
  along 
  the 
  N.W. 
  flank 
  of 
  the 
  Anglesey 
  Pre-Cambrian 
  axis 
  

   we 
  have 
  the 
  same 
  variation 
  in 
  the 
  lithological 
  character 
  of 
  the 
  

   Cambrian 
  basement 
  beds 
  as 
  we 
  have 
  along 
  the 
  S.E. 
  flank 
  of 
  the 
  

   Bangor-Caernarvon 
  axis 
  ; 
  here 
  a 
  fine 
  quartz 
  conglomerate 
  or 
  grit, 
  

   and 
  there 
  a 
  coarse 
  shingle, 
  composed 
  of 
  large 
  pebbles 
  of 
  felspathic 
  

   rocks 
  and 
  rough 
  quartzite 
  according 
  to 
  drift 
  of 
  the 
  shore-currents 
  

   and 
  the 
  material 
  of 
  the 
  older 
  rocks 
  from 
  which 
  each 
  particular 
  part 
  

   of 
  the 
  conglomerate 
  was 
  derived. 
  

  

  Palaeontology. 
  

  

  The 
  lowest 
  horizon 
  at 
  which 
  I 
  have 
  yet 
  found 
  fossils 
  in 
  Anglesey 
  

   is 
  in 
  the 
  flaggy 
  grit 
  associated 
  with 
  the 
  basement 
  conglomerate 
  near 
  

   Pen 
  cefn 
  south 
  of 
  Llanerchymedd. 
  Prom 
  these 
  beds 
  I 
  have 
  pro- 
  

   cured 
  slabs 
  with 
  obscure 
  fucoidal 
  markings 
  and 
  very 
  distinct 
  An- 
  

   nelid 
  tracks 
  nearly 
  a 
  foot 
  in 
  length. 
  

  

  Further 
  to 
  the 
  S.W.,near 
  Bryngwallen, 
  in 
  beds 
  immediately 
  suc- 
  

   ceeding 
  the 
  conglomerate, 
  the 
  Orthis, 
  mentioned 
  in 
  my 
  previous 
  

   paper 
  (Q. 
  J. 
  G. 
  S. 
  vol. 
  xxxvi. 
  p. 
  238, 
  May 
  1880), 
  first 
  appears. 
  I 
  

  

  