Yol.  49.]  LLANDOVERY  AND  ASSOCIATED  SOCKS  OF  COR  WEN.  427 
calcareous  grits,  which,  rest  un  conform  ably  on  the  Bala  Beds.  Above 
these  grits  come  blackish  mudstones,  in  which  he  obtained  1  Mono- 
grcijptus  convolutus ,  M.  Sedgwickii  ?,  31.  tenuis,  M.  gregarius,  and 
Climaeograptus  normalise  He  states  that  similar  black  beds  occur 
above  the  Corwen  Grit  of  Hant  Cawrddu,  near  Corwen. 
During  the  summer  of  1892  we  devoted  some  time  to  the  exami¬ 
nation  of  the  rocks  in  the  neighbourhood  of  Corwen,  and  especially 
of  the  Corwen  Grit  and  the  associated  beds.  We  have  found 
graptolite-shales  above  the  grit,  with  graptolites  of  the  gregarius- 
zone  similar  to  those  of  Cerrig-y-druidion  ;  and  these  shales  occur 
both  in  Yant  Cawrddu  and  Yant  Llechog.  This  discovery  confirms 
the  view  put  forward  by  Prof.  Hughes  that  the  grit  and  the 
overlying  beds  are  of  Llandovery  age. 
In  one  point  we  have  a  slight  correction  to  make.  In  the  Survey 
map  of  the  district  a  small  patch  of  Denbighshire  Grits  is  repre¬ 
sented  as  brought  in  by  a  fault  near  the  western  end  of  Corwen. 
It  forms  a  long  low  hill,  on  which  a  part  of  the  town  is  built.  In 
his  paper  Prof.  Hughes  refers  this  to  the  Corwen  Grit ;  but  ac¬ 
cording  to  our  observations  it  is  entirely  different  from  that  rock, 
and  on  the  other  hand  closely  resembles  the  Denbighshire  Grits, 
so  that  we  hold  the  Survey  map  to  be  correct  in  this  particular.  The 
Corwen  Grit,  as  seen  near  the  top  of  the  hill  to  the  south,  is  a 
nearly  pure  siliceous  rock,  very  hard  and  compact,  containing  no  fel- 
spathic  material.  The  Denbighshire  Grits  of  the  neighbourhood  are 
much  less  compact  and  contain  grains  of  decomposed  felspar  ;  and 
the  grits  of  this  small  patch  possess  the  same  characters. 
II.  General  Sequence  or  the  Strata. 
The  rocks  in  the  neighbourhood  of  Corwen  as  far  up  as  theTarannon 
Shales  are  coloured  on  the  Geological  Survey  map  (sheet  74)  as  Bala. 
But,  as  Prof.  Hughes  has  pointed  out,  the  Corwen  Grit  separates  two 
distinct  series:  blue  slates  with  Bala  fossils  below  the  grit,  and  pale 
slates  above  the  grit.  The  general  sequence,  as  determined  by  us,  of 
the  beds  below  the  Tarannon  Shales  is  as  follows  (in  descending  order) : 
Pale  Slates. 
Black  banded  Graptolite-shales  {gregarius- zone). 
Grey  Slates,  with  bands  of  grit. 
Corwen  Grit. 
Blue  Slates,  with  Bala  fossils. 
This  sequence  is  best  seen  in  Yant  Cawrddu  and  Yant  Llechog. 
III.  Structure  of  the  Area. 
The  area  which  will  now  be  described  forms  a  part  of  the  northern 
slopes  of  the  Berwyn  Hills,  and  includes  a  strip  of  country  lying 
south  of  the  Dee  between  the  town  of  Corwen  and  the  slate-quarries 
of  Pen-y-glog. 
1  See  revised  list  in  his  ‘  Classification  of  the  Cambrian  and  Silurian  Bocks,’ 
Cambridge*  1883,  p.  40. 
2  F  2 
