﻿22 
  T. 
  M'KESTNY 
  HUGHES 
  ON 
  THE 
  GEOLOGY 
  OF 
  ANGLESEY. 
  

  

  Cilgwyn 
  see 
  sandstones 
  with 
  subordinate 
  shaly 
  beds 
  dipping 
  at 
  

   about 
  30 
  c 
  E.X.E. 
  In 
  a 
  railway-cutting 
  and 
  small 
  quarry 
  near 
  Cae 
  

   scynan 
  there 
  are 
  some 
  ashy-looking 
  beds 
  of 
  brecciated 
  conglomerate 
  

   associated 
  with 
  thin 
  brown 
  sandstones 
  and 
  subordinate 
  shale. 
  

   Similar 
  beds 
  form 
  a 
  ridge 
  in 
  the 
  field 
  on 
  the 
  west 
  of 
  the 
  railway 
  for 
  

   some 
  distance 
  to 
  the 
  south. 
  There 
  ought 
  to 
  be 
  fossils 
  in 
  these 
  beds 
  : 
  

   but 
  we 
  only 
  saw 
  one 
  undeterminable 
  thing, 
  found 
  by 
  Mr. 
  Teall*. 
  

   (See 
  p. 
  26.)" 
  

  

  Cropping 
  out 
  from 
  behind 
  these 
  brecciated 
  beds 
  on 
  the 
  "W. 
  are 
  the 
  

   grits 
  and 
  conglomerates 
  of 
  the 
  base 
  of 
  the 
  Cambrian. 
  In 
  all 
  this 
  

   section 
  the 
  sandstones 
  predominate 
  over 
  the 
  shales. 
  Crossing 
  now 
  

   to 
  the 
  west 
  side 
  of 
  the 
  Llecheynfarwy 
  axis, 
  we 
  find 
  the 
  Cambrian 
  

   rocks 
  dipping 
  away 
  to 
  the 
  N.W., 
  and 
  the 
  sequence 
  can 
  be 
  clearly 
  

   made 
  out 
  in 
  several 
  traverses. 
  The 
  Pre-Cambrian 
  rocks 
  consist 
  of 
  

   gneiss, 
  which 
  is 
  immediately 
  succeeded 
  by 
  grits, 
  often 
  conglomeratic 
  ; 
  

   but 
  there 
  is 
  no 
  conglomerate 
  seen 
  so 
  coarse 
  as 
  that 
  on 
  the 
  other 
  side 
  

   of 
  the 
  axis, 
  or 
  on 
  the 
  same 
  side 
  of 
  the 
  axis 
  further 
  south. 
  The 
  grits 
  

   pass 
  up 
  into 
  fossiliferous 
  sandstones, 
  as 
  seen 
  near 
  Bryngwallen, 
  and 
  

   for 
  miles 
  to 
  the 
  S.W., 
  as 
  indicated 
  upon 
  the 
  survey 
  map. 
  Near 
  the 
  

   top 
  of 
  these 
  sandstones 
  there 
  is 
  sometimes, 
  as 
  near 
  Penrhosrhyfel, 
  a 
  

   brecciated 
  conglomerate 
  containing 
  small 
  fragments 
  of 
  slaty 
  and 
  

   schistose 
  rock. 
  The 
  sandstones 
  are 
  succeeded 
  by 
  black 
  shales, 
  in 
  

   the 
  lower 
  part 
  of 
  which, 
  as 
  may 
  be 
  well 
  seen 
  along 
  the 
  roads 
  by 
  

   Carman 
  and 
  Geir, 
  there 
  are 
  considerable 
  beds 
  of 
  a 
  brecciated 
  con- 
  

   glomerate 
  similar 
  to 
  those 
  noted 
  in 
  the 
  corresponding 
  position 
  east 
  

   of 
  Mynyddmawr 
  (see 
  fig. 
  4). 
  

  

  Thus 
  we 
  establish 
  the 
  following 
  sequence 
  all 
  over 
  the 
  area 
  de- 
  

   scribed 
  : 
  — 
  

  

  1. 
  Black 
  slates, 
  some 
  of 
  which 
  must 
  be 
  referred 
  to 
  Silurian, 
  with 
  

   subordinate 
  ash-like 
  beds 
  and 
  brecciated 
  conglomerates. 
  

  

  2. 
  Black 
  slates 
  and 
  shales 
  with 
  fine 
  breccias 
  and 
  subordinate 
  

   sandstones, 
  especially 
  in 
  the 
  lower 
  part. 
  

  

  3. 
  Banded 
  nags 
  passing 
  down 
  into 
  sandstones, 
  with 
  subordinate 
  

   shales, 
  which 
  become 
  more 
  important 
  in 
  some 
  localities 
  than 
  in 
  others. 
  

  

  4. 
  Tough 
  sandstones 
  and 
  flaggy 
  grits, 
  passing 
  down 
  into 
  conglo- 
  

   merate, 
  which 
  varies 
  according 
  to 
  the 
  character 
  of 
  the 
  underlying 
  

   rocks, 
  but 
  in 
  which 
  we 
  almost 
  always 
  find 
  some 
  bands 
  composed 
  

   chiefly 
  of 
  white 
  quartz 
  with 
  occasional 
  fragments 
  of 
  jasper, 
  quartzite, 
  

   schist, 
  &c, 
  and 
  some 
  beds 
  of 
  large 
  felsite 
  pebbles. 
  

  

  Sometimes 
  the 
  coarse 
  conglomerate 
  is 
  at 
  the 
  bottom, 
  sometimes 
  

   the 
  finer 
  material 
  forms 
  the 
  lower 
  beds, 
  and 
  bands 
  of 
  coarser 
  material 
  

   occur 
  higher 
  up, 
  as, 
  indeed, 
  is 
  a 
  common 
  arrangement 
  in 
  all 
  basement 
  

   beds, 
  e.g. 
  Carboniferous, 
  New 
  Red, 
  &c. 
  

  

  Petrology. 
  

  

  The 
  ridge 
  which 
  culminates 
  in 
  y 
  Foel, 
  east 
  of 
  Llanerchymedd 
  (see 
  

   fig. 
  3), 
  throws 
  great 
  light 
  on 
  the 
  question 
  of 
  the 
  identification 
  of 
  the 
  

   basement 
  beds 
  of 
  the 
  Cambrian 
  ; 
  for 
  here 
  we 
  find 
  in 
  succession, 
  and 
  

  

  * 
  I 
  have 
  since 
  found 
  specimens 
  of 
  Ortkis, 
  probably 
  0. 
  Hicksii, 
  in 
  these 
  beds. 
  

  

  