Vol.  49.]  LLANDOVERY  AND  ASSOCIATED  ROCKS  OF  COR  WEN. 
435 
corner  of  the  map.  All  these  throw  downwards  to  the  north,  and, 
with  the  exception  of  that  marked  on  the  Geological  Survey  map,  which 
Fig.  8. — Dispositioyi  of  the  Grit  on  the  tiuo  faces  of  the 
Nant  Caivrdclu  Fault. 
N.  17°  W.  S.  17°  E. 
b  —  Corwen  Grit. 
The  heavy  shading  shows  the 
disposition  of  the  grit  on  the  Horizontal  scale :  9  inches  =  1  mile, 
western  face,  and  the  light  shading  Vertical  scale  :  1  inch  =  535  feet, 
its  disposition  on  the  eastern  face, 
of  the  fault. 
we  have  not  closely  examined,  they  all  come  to  an  end  against  the  Nant 
Cawrddu  and  N ant  Llechog  Faults, showing  that  thelatter  are  the  newer. 
IV.  Description  of  the  Strata. 
The  details  which  have  alread)"  been  given  show  clearly  that  in 
this  area  at  least  the  grit-bed  forms  a  well-marked  and  definite 
horizon.  Below  it  we  have  bluish  slates,  "which  are  generally 
rather  irregularly  cleaved.  Above  it  is  a  series  of  slates  which  are 
generally  greyish  in  colour,  and  which  are  characterized  by  the 
presence  of  numerous  thin  bands  of  grit,  most  important  near  the 
base.  In  hand-specimens  it  is  not  always  possible  to  distinguish 
the  lower  slates  Horn  the  upper  ones,  but  in  the  field  there  is  little 
difficulty.  The  slates  below  the  grit  are  invariably  fossiliferous,  the 
fossils  being  of  Middle  Bala  age  ;  those  above  the  grit  are  extremely 
unfossiliferous,  and  only  at  two  points  have  we  found  fossils  which 
may  possibly  belong  to  these  slates.  In  the  sections  in  Nant  Cawrddu 
and  Nant  Llechog,  however,  these  slates  pass  up  into  black  shales, 
which  contain  graptolites  of  the  gregarius- zone,  and  are  therefore  of 
Llandovery  age. 
If  any  line  is  to  be  drawn  between  the  Llandovery  and  Bala  Beds 
in  this  area,  it  must  certainly  be  placed  at  the  base  of  the  grit. 
There  is  no  sign  of  any  unconformity  between  this  and  the  under¬ 
lying  Bala  Beds  ;  but  there  is  no  passage  from  the  one  to  the  other. 
On  the  other  hand,  there  is  a  kind  of  passage  from  the  grit  to  the 
slates  above,  for  the  latter  always  contain  thin  grit-bands,  and  they 
are  most  gritty  towards  the  base.  Therefore  we  refer  the  grit  and  the 
beds  above  it  to  the  Llandovery  Series  and  classify  them  as  follows : — 
Llandovery  ... 
Bala  . 
Pale  Slates. 
Graptolite-shales  (_ gregarius-zone ). 
Grey  Slates,  with  bands  of  grit. 
Corwen  Grit. 
Blue  Slates. 
