﻿T. 
  M'KENNY 
  HUGHES 
  ON 
  THE 
  GEOLOGY 
  OE 
  ANGLESEY. 
  1/ 
  

  

  posed 
  below 
  the 
  basement 
  beds 
  of 
  the 
  Cambrian 
  ; 
  and 
  the 
  question 
  is, 
  

   Can 
  we 
  make 
  out 
  the 
  succession 
  among 
  those 
  basement 
  beds 
  so 
  clearly 
  

   that 
  when 
  one 
  or 
  two 
  members 
  have 
  varied 
  considerably 
  from 
  the 
  

   type 
  we 
  start 
  with, 
  we 
  still, 
  from 
  their 
  position 
  and 
  character, 
  can 
  

   feel 
  sure 
  of 
  their 
  identification 
  ? 
  

  

  I 
  propose, 
  first 
  of 
  all, 
  to 
  describe 
  the 
  strata 
  seen 
  in 
  some 
  of 
  the 
  

   more 
  important 
  transverse 
  sections 
  that 
  I 
  have 
  examined, 
  and 
  next 
  

   to 
  offer 
  a 
  few 
  remarks 
  upon 
  the 
  lithological 
  character 
  of 
  those 
  rocks 
  

   which 
  more 
  immediately 
  come 
  within 
  the 
  scope 
  of 
  this 
  inquiry, 
  and 
  

   of 
  which 
  illustrative 
  specimens 
  with 
  microscope 
  slides 
  of 
  some 
  of 
  the 
  

   more 
  important 
  are 
  exhibited* 
  ; 
  and, 
  lastly, 
  to 
  consider 
  the 
  palseon- 
  

   tological 
  evidence, 
  to 
  which 
  I 
  have 
  recently 
  made 
  some 
  important 
  

   additions. 
  

  

  One 
  of 
  the 
  clearest 
  and 
  most 
  continuous 
  sections 
  is 
  that 
  seen 
  in 
  

   the 
  cliff 
  from 
  Porth 
  Corwg 
  to 
  Porthlygan 
  on 
  the 
  extreme 
  N.E. 
  coast 
  

   (fig. 
  1). 
  The 
  higher 
  beds 
  consist 
  chiefly 
  of 
  black 
  slate 
  and 
  flaggy 
  

   shales 
  with 
  subordinate 
  brecciated 
  conglomerates. 
  Of 
  these 
  I 
  shall 
  

   not 
  say 
  much, 
  as 
  I 
  have 
  not 
  yet 
  traced 
  the 
  base 
  of 
  the 
  Silurian. 
  At 
  

   Trwyndu 
  there 
  are 
  two 
  ribs 
  of 
  felsite 
  (2, 
  3 
  in 
  fig. 
  1), 
  and 
  associated 
  

   with 
  them 
  satiny 
  slates, 
  not 
  unlike 
  some 
  of 
  those 
  in 
  the 
  gnarled 
  

   series, 
  and 
  veined 
  quartzose 
  beds, 
  all 
  much 
  resembling 
  the 
  similarly 
  

   associated 
  series 
  of 
  Paris 
  Mountain. 
  

  

  In 
  Porthygwichiad 
  we 
  have 
  black 
  soft 
  slates 
  like 
  those 
  which 
  in 
  

   adjoining 
  areas 
  are 
  proved 
  by 
  fossils 
  to 
  be 
  Upper 
  Arenig. 
  At 
  the 
  

   south 
  end 
  of 
  the 
  bay 
  there 
  are 
  banded 
  sandy 
  flags 
  which 
  pass 
  down 
  

   into 
  the 
  brown 
  sandstones 
  of 
  Penrhynglas. 
  At 
  their 
  base 
  there 
  is 
  

   another 
  mass 
  of 
  black 
  slate 
  with 
  scattered 
  pebbles 
  of 
  quartzite, 
  &c, 
  

   some 
  as 
  much 
  as 
  10 
  inches 
  in 
  diameter, 
  but 
  mostly 
  small. 
  These 
  

   black 
  slates 
  are 
  seen 
  in 
  the 
  little 
  cove 
  south 
  of 
  Penrhynglas 
  to 
  be 
  

   faulted 
  against 
  the 
  gneissic 
  series. 
  

  

  This 
  section 
  is 
  drawn 
  along 
  the 
  line 
  of 
  part 
  of 
  the 
  Survey 
  Section 
  

   (sheet 
  40, 
  section 
  2), 
  and 
  agrees 
  with 
  it 
  in 
  the 
  general 
  lie 
  of 
  the 
  

   rocks. 
  

  

  The 
  Penrhynglas 
  fault 
  probably 
  runs 
  along 
  the 
  road 
  below 
  Rhos- 
  

   mannerchganol. 
  

  

  It 
  is 
  pretty 
  clear 
  that 
  the 
  black 
  slates 
  seen 
  near 
  Penrallt 
  are 
  

   thrown 
  against 
  the 
  gneissic 
  rocks 
  of 
  JNebo 
  on 
  the 
  north 
  by 
  a 
  fault 
  

   which 
  crosses 
  the 
  road 
  to 
  Tycanol 
  south 
  of 
  the 
  P 
  in 
  Penrallt, 
  and 
  

   crosses 
  the 
  road 
  from 
  Tynewydd 
  to 
  Tyddyn 
  Mawr, 
  south 
  of 
  the 
  r 
  in 
  

   Penrallt. 
  In 
  fact 
  all 
  this 
  district 
  seems 
  to 
  be 
  cut 
  up 
  by 
  faults, 
  as 
  

   we 
  might 
  expect 
  from 
  what 
  we 
  see 
  in 
  Porthygwichiad, 
  Porthlygan, 
  

   and 
  in 
  Penrhynglas. 
  In 
  two 
  quarries 
  S.E. 
  of 
  JSTebo, 
  the 
  black 
  slates 
  

   are 
  seen 
  thrown 
  against 
  and 
  mashed 
  into 
  the 
  inequalities 
  of 
  the 
  

   Cambrian 
  sandstone 
  and 
  grit, 
  which 
  is 
  turned 
  up 
  against 
  the 
  southern 
  

   flank 
  of 
  the 
  Nebo 
  gneiss, 
  just 
  as 
  a 
  somewhat 
  higher 
  part 
  of 
  the 
  

   Cambrian 
  series 
  was 
  turned 
  up 
  on 
  the 
  south 
  side 
  of 
  the 
  axis 
  below 
  

   Llysdulas. 
  It 
  may 
  be 
  by 
  the 
  same 
  fault, 
  but 
  I 
  did 
  not 
  trace 
  it 
  

   out. 
  

  

  In 
  the 
  larger 
  quarry 
  south 
  of 
  Dwyllechiad, 
  a 
  portion 
  of 
  the 
  crushed 
  

   * 
  These 
  specimens 
  are 
  now 
  in 
  the 
  Woodwardian 
  Museum, 
  Cambridge. 
  

   Q.J.G.S. 
  No. 
  149. 
  c 
  

  

  