﻿832 
  Geological 
  Society. 
  

  

  -wholly 
  of 
  ashes 
  and 
  tuffs 
  (no 
  lava-flows 
  having 
  been 
  detected). 
  

   Movements 
  have 
  often 
  taken 
  place 
  along 
  the 
  two 
  surfaces 
  of 
  the 
  

   Craig-y-Pandy 
  Ash, 
  and 
  the 
  rock 
  is 
  commonly 
  foliated, 
  and 
  locally 
  

   converted 
  into 
  a 
  white 
  or 
  blue 
  china-stone, 
  in 
  which 
  good 
  columnar 
  

   jointing 
  is 
  sometimes 
  shown. 
  The 
  authors 
  have 
  failed 
  to 
  detect 
  

   any 
  traces 
  of 
  thermometamorphic 
  action 
  of 
  these 
  bands 
  on 
  the 
  

   adjoining 
  slates, 
  such 
  as 
  has 
  been 
  recently 
  asserted. 
  It 
  is 
  other- 
  

   wise 
  with 
  an 
  intrusive 
  sill, 
  which 
  has 
  been 
  commonly 
  mistaken 
  

   for 
  the 
  uppermost 
  ash. 
  

  

  The 
  Teirw 
  Beds 
  are 
  compared 
  with 
  the 
  Koman 
  Pell 
  Group 
  of 
  the 
  

   Lake 
  District. 
  The 
  Bryn 
  Beds 
  agree 
  most 
  nearly 
  with 
  the 
  Sleddale 
  

   Beds 
  of 
  the 
  same 
  district. 
  

  

  The 
  Graptolite-Slates 
  are 
  separated 
  from 
  the 
  underlying 
  Pandy 
  

   Series 
  by 
  the 
  Ddolhir 
  Fault, 
  which 
  has 
  cut 
  out 
  a 
  considerable 
  part 
  

   of 
  the 
  succession. 
  

  

  The 
  Ddolhir 
  Beds 
  are 
  extremely 
  rich 
  in 
  fossils, 
  which 
  prove 
  

   their 
  equivalence 
  with 
  the 
  Ashgillian 
  formation 
  of 
  other 
  districts. 
  

   They 
  appear 
  to 
  pass 
  up 
  into 
  the 
  Glyn 
  Grit 
  and 
  Limestone, 
  which 
  

   is 
  essentially 
  a 
  sandy 
  and 
  calcareous 
  facies 
  of 
  the 
  series. 
  The 
  

   Glyn 
  Grit 
  corresponds 
  in 
  position 
  with 
  the 
  Corwen 
  Grit; 
  but, 
  

   recognizing 
  that 
  it 
  belongs 
  to 
  the 
  Bala 
  Series, 
  the 
  authors 
  are 
  

   no 
  longer 
  prepared, 
  without 
  further 
  evidence, 
  to 
  assume 
  the 
  

   equivalence 
  of 
  the 
  two 
  grits. 
  

  

  The 
  Fron-Frys 
  Slates 
  show 
  the 
  lithological 
  characters 
  of 
  the 
  

   Grey 
  Slates 
  of 
  Corwen, 
  but 
  belong 
  rather 
  to 
  the 
  shelly 
  than 
  to 
  the 
  

   graptolitic 
  facies 
  of 
  the 
  Llandovery 
  Series. 
  

  

  The 
  Ty-Draw 
  Slates 
  resemble 
  the 
  pale 
  Tarannon 
  Slates 
  of 
  other 
  

   districts. 
  They 
  appear 
  to 
  pass 
  up 
  into 
  the 
  Denbighshire 
  Slates. 
  

  

  The 
  authors 
  find 
  no 
  indication 
  of 
  the 
  overlap 
  or 
  overstep 
  of 
  the 
  

   Wenlock, 
  Tarannon, 
  and 
  Llandovery 
  Beds 
  mapped 
  by 
  the 
  officers 
  

   of 
  the 
  Geological 
  Survey, 
  or 
  described 
  by 
  previous 
  observers, 
  

   although 
  there 
  is 
  probably 
  an 
  unconformity 
  at 
  the 
  base 
  of 
  the 
  

   Fron-Frys 
  Slates. 
  

  

  The 
  beds 
  of 
  the 
  district 
  dip 
  northwards 
  at 
  an 
  almost 
  uniformly 
  

   low 
  angle, 
  but 
  the 
  structure 
  is 
  considerably 
  complicated 
  by 
  a 
  series 
  

   of 
  faults, 
  most 
  of 
  which 
  have 
  hitherto 
  escaped 
  notice, 
  some 
  being 
  

   very 
  elusive. 
  They 
  include 
  : 
  — 
  (1) 
  east-and-west 
  faults, 
  (2) 
  north- 
  

   north-west 
  and 
  south-south-east, 
  or 
  north-and-south 
  faults, 
  and 
  

   (3) 
  north-north-west 
  and 
  east-south-east 
  faults. 
  

  

  Of 
  the 
  first 
  series 
  the 
  most 
  important 
  is 
  the 
  Ddolhir 
  Fault, 
  which 
  

   dips 
  at 
  an 
  angle 
  of 
  20°, 
  nearly, 
  with 
  the 
  bedding, 
  and 
  may 
  be 
  

   either 
  a 
  thrust-plane 
  or 
  a 
  lag-fault. 
  Of 
  the 
  second 
  series, 
  the 
  most 
  

   remarkable 
  is 
  the 
  Caemor 
  Fault, 
  on 
  the 
  east 
  side 
  of 
  which 
  the 
  rocks 
  

   have 
  been 
  raised 
  nearly 
  a 
  mile, 
  and 
  shifted 
  horizontally 
  to 
  the 
  

   south 
  for 
  nearly 
  3 
  miles. 
  Most 
  of 
  the 
  remaining 
  faults 
  have 
  a 
  

   simple 
  downthrow 
  on 
  the 
  east. 
  

  

  