﻿T. 
  M'KENtfY 
  HUGHES 
  OS" 
  THE 
  GEOLOGY 
  OF 
  ANGLESET. 
  19 
  

  

  shale 
  in 
  one 
  place 
  remains 
  in 
  beds 
  parallel 
  to 
  the 
  underlying 
  grit, 
  

   which 
  is 
  crossed 
  by 
  strong 
  joints 
  running 
  in 
  the 
  same 
  direction 
  as 
  

   the 
  fault, 
  so 
  that 
  it 
  appears 
  at 
  first 
  sight 
  as 
  if 
  the 
  shale 
  were 
  lying 
  

   on 
  the 
  upturned 
  edges 
  of 
  the 
  grit. 
  Some 
  of 
  this 
  crushed 
  mass 
  of 
  

   grit 
  dips 
  at 
  a 
  high 
  angle, 
  but 
  not 
  in 
  that 
  part 
  of 
  the 
  section. 
  The 
  

   inference 
  of 
  an 
  unconformity 
  between 
  the 
  slates 
  and 
  grittjr 
  con- 
  

   glomerate 
  near 
  Nebo 
  seems 
  to 
  be 
  founded 
  on 
  a 
  misunderstanding 
  of 
  

   these 
  sections. 
  

  

  Crossing 
  now 
  to 
  the 
  south 
  flank 
  of 
  the 
  Porthlygan 
  axis, 
  we 
  find 
  

   that 
  the 
  Cambrian 
  rocks 
  are 
  brought 
  on 
  by 
  a 
  tremendous 
  fault 
  seen 
  

   in 
  the 
  cliff 
  below 
  Llysdulas, 
  as 
  shown 
  in 
  the 
  section 
  (fig. 
  2). 
  We 
  

   first 
  have 
  the 
  banded 
  flags, 
  probably 
  with 
  a 
  reversed 
  dip 
  ; 
  then 
  a 
  

   small 
  fault 
  throws 
  the 
  soft 
  black 
  slates 
  of 
  the 
  Arenig 
  against 
  them. 
  

   These, 
  after 
  some 
  sharp 
  folds, 
  turn 
  up 
  again, 
  and 
  near 
  a 
  couple 
  of 
  

   dykes 
  are 
  found 
  dipping 
  in 
  a 
  northerly 
  direction 
  and 
  sufficiently 
  

   hard 
  and 
  compact 
  to 
  allow 
  one 
  to 
  break 
  lumps 
  up 
  in 
  search 
  of 
  fossils. 
  

   Prom 
  these 
  beds 
  Dr. 
  Roberts 
  and 
  I 
  obtained 
  a 
  few 
  Grraptolites, 
  which 
  

   give 
  us 
  again 
  our 
  horizon 
  and 
  fix 
  the 
  beds 
  as 
  Arenig. 
  These 
  soft 
  

   black 
  shales 
  are 
  seen 
  here 
  and 
  there 
  along 
  Traeth 
  Dulas, 
  and 
  are 
  

   succeeded 
  near 
  City 
  Dulas 
  by 
  the 
  banded 
  flags 
  with 
  a 
  dip 
  of 
  35° 
  

   N.N.E., 
  while, 
  near 
  the 
  lead-mines, 
  brown 
  sandstones 
  apparently 
  

   crop 
  out 
  from 
  below 
  the 
  banded 
  flags. 
  

  

  Following 
  the 
  strike 
  of 
  the 
  beds 
  about 
  | 
  of 
  a 
  mile 
  to 
  the 
  W.N.W. 
  

   we 
  have 
  the 
  banded 
  flags 
  near 
  Ffrwd 
  Fadoc, 
  and 
  again 
  about 
  ^ 
  of 
  a 
  

   mile 
  east 
  of 
  Tynyffordd, 
  and 
  now 
  find 
  ourselves 
  upon 
  the 
  northern 
  

   flank 
  of 
  the 
  Dulas 
  Pre-Cambrian 
  axis. 
  The 
  banded 
  flags 
  get 
  more 
  

   sandy 
  as 
  we 
  descend 
  into 
  the 
  series, 
  and 
  small 
  openings 
  show 
  sand- 
  

   stone 
  and 
  grit, 
  here 
  and 
  there, 
  for 
  a 
  considerable 
  distance. 
  As 
  

   they 
  are 
  suitable 
  material 
  for 
  walling 
  and 
  other 
  building, 
  there 
  are 
  

   many 
  places 
  where 
  they 
  have 
  been 
  quarried; 
  and 
  being 
  rocks 
  

   which 
  resist 
  the 
  ordinary 
  chemical 
  and 
  mechanical 
  denuding 
  agents, 
  

   they 
  often 
  project 
  in 
  bosses 
  and 
  ridges 
  through 
  the 
  soil, 
  showing 
  

   daiiedd 
  y 
  graig 
  (the 
  teeth 
  of 
  the 
  rock), 
  as 
  they 
  say 
  in 
  Wales. 
  These 
  

   sandstones 
  pass 
  down 
  into 
  grits 
  and 
  conglomerates. 
  Near 
  Penlon 
  

   a 
  larger 
  quarry 
  than 
  usual 
  gave 
  us 
  an 
  opportunity 
  of 
  examining 
  the 
  

   base 
  of 
  the 
  Cambrian 
  ; 
  and, 
  when 
  Dr. 
  Roberts 
  and 
  I 
  visited 
  it, 
  the 
  

   face 
  of 
  the 
  rock 
  had 
  been 
  newly 
  cleared, 
  just 
  at 
  the 
  junction 
  between 
  

   the 
  conglomerate 
  and 
  the 
  Pre-Cambrian. 
  They 
  seemed 
  to 
  be 
  un- 
  

   conformable 
  (as 
  shown 
  in 
  the 
  sketch, 
  fig. 
  2) 
  ; 
  but 
  of 
  course 
  in 
  grits 
  

   and 
  conglomerates 
  which 
  are, 
  from 
  the 
  nature 
  of 
  the 
  case, 
  very 
  

   irregular 
  deposits, 
  it 
  would 
  not 
  be 
  safe 
  to 
  infer 
  an 
  unconformity 
  

   from 
  one 
  small 
  section. 
  However, 
  it 
  is 
  satisfactory 
  to 
  have 
  what 
  is 
  

   clear 
  from 
  other 
  evidence 
  confirmed 
  by 
  the 
  minor 
  details, 
  so 
  far 
  as 
  

   they 
  go. 
  The 
  conglomerate 
  consists 
  chiefly 
  of 
  white 
  quartz 
  pebbles 
  

   with 
  some 
  quartzite 
  and 
  jasper 
  and 
  occasional 
  fragments 
  of 
  schist, 
  

   and 
  is 
  exactly 
  like 
  that 
  seen 
  in 
  a 
  similar 
  position 
  near 
  Caernarvon. 
  

  

  Let 
  us 
  now 
  cross 
  the 
  country 
  about 
  a 
  mile 
  and 
  a 
  half 
  to 
  the 
  

   W.S.W., 
  so 
  as 
  to 
  get 
  on 
  to 
  the 
  black 
  slates 
  again, 
  and 
  make 
  another 
  

   traverse 
  due 
  S. 
  across 
  the 
  lower 
  beds 
  of 
  the 
  Cambrian 
  from 
  an 
  ho- 
  

   rizon 
  determined 
  by 
  fossils 
  near 
  Caemawr, 
  past 
  Llanerchymedd 
  on 
  

   to 
  the 
  Pre-Cambrian 
  near 
  Ty'nmaen 
  (fig. 
  3). 
  

  

  c2 
  

  

  