Yol.  49.] 
LLANDOVERY  AND  ASSOCIATED  ROCKS  OF  CORWEN. 
437 
there  ever  were  an)T  they  seem  to  hare  been  obliterated.  Prof. 
Hughes  found  a  badly-preserved  form  which  he  referred  to  Favosites 
cdveolaris ;  and  in  fragments  of  a  similar  rock  on  Cyrn-y-brain  he 
obtained  Petraict  subduplicata,  P.  crenulatci ,  and  MeristeTla  crassa. 
In  a  calcareous  grit  occupying  a  similar  stratigraphical  position  at 
Glyn  Ceiriog,  and  overlain  by  similar  grey  slates  with  grit-bands,  we 
have  found  Strejotelasma ?  equisulcatum ,  M‘Coy,  Linclstrcemia  subdu- 
■ plicata ,  M4Coy,  Favosites  fibrosa ,  Goldf.,  Nebulijpora  lens,  M‘Coy, 
Heliolites  megastoma,  M‘Coy,  Orthis  biforata,  Schloth.  ?,  MeristeTla 
sp.,  Murchisonia ,  Pleurotomaria,  etc.  These  fossils,  together  with 
the  fact  that  at  Corwen  itself  the  grit  occurs  immediately  above 
Pala  Beds,  and  a  little  below  beds  with  a  Birkhill  fauna  ( gregarius - 
zone  =  the  upper  zone  of  the  Lower  Birkhill),  leave  no  doubt  that 
the  Corwen  Grit  is  of  Llandovery  age. 
(c)  Grey  Slates. — The  slates  which  are  found  above  the  grit  are 
generally  greyish  in  colour,  but  sometimes  blue,  like  the  Bala  Beds. 
They  are  softer  and  more  finely  cleaved  than  these,  and  are  dis¬ 
tinguished  by  the  presence  of  small  bands  of  grit  4  to  1  inch  in 
thickness.  Towards  the  base  these  slates  become  more  gritty,  and 
in  Hant  Llechog  they  may  almost  be  said  to  pass  down  into  the  grit. 
They  usually  weather  deeply  along  the  cleavage  and  along  the 
bedding-planes,  and  this  gives  to  large  masses  of  them  a  very  striking 
and  characteristic  appearance. 
In  general  they  are  quite  unfossiliferous,  and  thus  contrast 
strongly  with  the  Bala  Beds.  In  a  little  rill,  however,  just  above 
the  main  grit-band,  due  south  of  T}'-isaf,  we  found  fragments  con¬ 
taining  fossils.  So  far  as  we  could  see,  these  must  have  come  from 
the  grey  slates,  and  in  them  we  obtained  (badly  preserved)  Leptazna 
sericea ,  Sow.,  and  Orthis  elegantula ,  Dalm.  On  a  ridge  of  these 
slates,  some  500  yards  farther  south-south-west,  we  found  Calymene 
Blumenbachii ,  Brongn.,  and  some  badly-preserved  brachiopods,  in¬ 
cluding  Leptaena  sericea  (?)  and  RhynchoneTba  sp. 
(d)  Graptolite- Shales. — These  have  been  found  in  two  places 
only,  namely  in  Nant  Cawrddu  and  Hant  Llechog.  Their  relation 
to  the  neighbouring  beds  has  been  already  described,  and  in  each 
case  they  overlie  the  grey  slates. 
In  Yant  Cawrddu  they  have  recently  been  excavated  just  west  of 
the  stream,  apparently  to  form  a  reservoir,  and  thus  we  obtained  a 
clear  section.  They  are  here  black,  glistening  slates  with  paler 
bands,  and  they  dip  20c  due  north.  They  contain  numerous  grains 
and  small  nodules  of  pyrites,  and  also  large  numbers  of  graptolites, 
which  are,  however,  strongly  compressed  and  not  very  perfectly 
preserved.  Mr.  J.  E.  Marr,  E.B.S.,  has  kindly  determined  them 
for  us,  and  he  finds  the  following  species  : — 
Monograptus  tenuis ,  Portl.  Monograptus  leptotheca  ?  Lapw. 
- gregarius  (?).  Diplograptus  sinuatus,  Nick. 
In  ISTant  Llechog  the  graptolite-shales  are  very  similar  in  cha¬ 
racter  ;  but  the  graptolites  are  not  so  strongly  compressed,  and  are 
