﻿16 
  T. 
  M'SENNY 
  HUGHES 
  ON 
  THE 
  GEOLOGY 
  OE 
  ANGLESEY 
  

  

  2. 
  On 
  the 
  Geology 
  of 
  Anglesey. 
  — 
  No. 
  2. 
  By 
  T. 
  M 
  c 
  Kenny 
  

   Hughes, 
  M.A., 
  F.G.S., 
  Woodwardian 
  Professor 
  of 
  Geology, 
  

   Cambridge. 
  (Read 
  June 
  8, 
  1881.) 
  

  

  In 
  a 
  former 
  paper 
  on 
  the 
  Geology 
  of 
  Anglesey, 
  which 
  I 
  had 
  the 
  

   honour 
  of 
  reading 
  before 
  the 
  Society 
  on 
  February 
  25th, 
  1880 
  

   (Q. 
  J. 
  G. 
  S. 
  vol. 
  xxxvi. 
  p. 
  237), 
  I 
  endeavoured 
  to 
  fix 
  the 
  geo- 
  

   logical 
  position 
  of 
  some 
  of 
  the 
  sandstones 
  and 
  shales 
  which 
  overlie 
  

   the 
  granitoid, 
  gneissic, 
  and 
  schistose 
  rocks 
  of 
  the 
  central 
  axis. 
  I 
  

   gave 
  lists 
  of 
  fossils 
  and 
  brought 
  forward 
  stratigraphical 
  evidence 
  to 
  

   show 
  that 
  we 
  had 
  at 
  the 
  base 
  of 
  the 
  series 
  a 
  conglomerate 
  and 
  grit 
  

   succeeded 
  by 
  sandstones 
  containing 
  fossils 
  which, 
  in 
  the 
  present 
  

   state 
  of 
  the 
  grouping 
  of 
  those 
  variable 
  zones, 
  would 
  be 
  referred 
  to 
  

   Tremadoc, 
  and 
  that 
  these 
  were 
  followed 
  in 
  ascending 
  order 
  by 
  black 
  

   slates 
  and 
  shales, 
  in 
  which 
  I 
  found 
  one 
  Graptolite 
  zone, 
  which 
  was 
  

   referred 
  to 
  a 
  known 
  horizon 
  in 
  the 
  Arenig. 
  

  

  I 
  have 
  since 
  carried 
  on 
  this 
  investigation, 
  and 
  have 
  discovered 
  

   other 
  fossiliferous 
  localities 
  on 
  different 
  horizons 
  ; 
  and 
  having 
  worked 
  

   out 
  the 
  details 
  of 
  several 
  of 
  the 
  sections, 
  I 
  think 
  I 
  have 
  established 
  

   a 
  sequence 
  amongst 
  the 
  lowest 
  Cambrian 
  rocks, 
  the 
  constant 
  recur- 
  

   rence 
  of 
  which 
  seems 
  to 
  be 
  a 
  strong 
  argument 
  in 
  favour 
  of 
  its 
  being 
  

   the 
  true 
  order 
  of 
  succession 
  of 
  the 
  beds 
  ; 
  and, 
  as 
  a 
  special 
  point 
  of 
  

   interest, 
  I 
  can 
  now 
  offer 
  conclusive 
  evidence 
  to 
  prove 
  that 
  the 
  quartz- 
  

   jasper 
  conglomerates 
  are 
  in 
  the 
  basement 
  beds 
  of 
  the 
  Cambrian, 
  and 
  

   not 
  in 
  the 
  Pre-Cambrian 
  series. 
  I 
  have 
  had 
  the 
  advantage 
  of 
  being 
  

   accompanied 
  during 
  part 
  of 
  two 
  excursions 
  by 
  Dr. 
  R. 
  D. 
  Roberts, 
  

   and 
  also 
  for 
  a 
  few 
  days 
  by 
  Mr. 
  J. 
  J. 
  Harris 
  Teall, 
  and 
  more 
  recently 
  

   have 
  gone 
  over 
  the 
  principal 
  sections 
  with 
  Dr. 
  Hicks. 
  

  

  Of 
  Mr. 
  Tawney's 
  encyclopedic 
  knowledge 
  I 
  have 
  availed 
  myself 
  

   freely 
  in 
  every 
  difficulty 
  in 
  the 
  field 
  and 
  in 
  the 
  museum. 
  

  

  I 
  use 
  Professor 
  Sedgwick's 
  classification 
  and 
  nomenclature. 
  

  

  It 
  may 
  not 
  be 
  out 
  of 
  place 
  to 
  remark 
  here 
  that 
  the 
  basement 
  

   beds 
  of 
  systems 
  have 
  always 
  offered 
  difficulties 
  and 
  proved 
  a 
  fruitful 
  

   source 
  of 
  controversy. 
  If 
  part 
  of 
  a 
  continental 
  area 
  with 
  mountain- 
  

   ranges 
  10,000 
  or 
  20,000 
  or 
  30,000 
  feet 
  high 
  goes 
  down 
  tolerably 
  

   evenly, 
  it 
  is 
  obvious 
  that, 
  before 
  any 
  marine 
  beds 
  can 
  be 
  formed 
  on 
  

   the 
  highest 
  ground, 
  the 
  lowest 
  may 
  have 
  10,000 
  or 
  20,000 
  or 
  30,000 
  

   feet 
  of 
  strata 
  deposited 
  over 
  it 
  ; 
  and 
  the 
  basement 
  beds 
  will 
  probably 
  

   vary 
  considerably 
  over 
  any 
  large 
  area 
  ; 
  yet 
  there 
  will 
  be 
  much 
  in 
  

   common 
  between 
  the 
  shore-deposits 
  as 
  they 
  creep 
  through 
  long 
  ages 
  

   up 
  the 
  slopes 
  of 
  the 
  gradually 
  submerged 
  land. 
  Over 
  limited 
  areas, 
  

   however, 
  we 
  may 
  expect, 
  with 
  very 
  unequal 
  thicknesses, 
  a 
  somewhat 
  

   similar 
  sequence 
  in 
  the 
  early 
  deposits 
  ; 
  and 
  if 
  the 
  material 
  of 
  which 
  

   the 
  old 
  land 
  is 
  made 
  up 
  is 
  of 
  a 
  marked 
  character, 
  we 
  may 
  find 
  our 
  

   correlation 
  not 
  so 
  difficult. 
  

  

  That 
  is 
  precisely 
  what 
  I 
  have 
  to 
  bring 
  before 
  the 
  Society 
  in 
  this 
  

   communication. 
  We 
  have 
  the 
  ridges 
  of 
  an 
  old 
  mountain 
  land 
  ex- 
  

  

  