Yol.  49.]  LLANDOVERY  AND  ASSOCIATED  ROCKS  OF  CORWEN.  433 
the  stream  as  far  as  a  second  waterfall,  which  closely  resembles  the 
other.  I11  these  upper  black  shales  we  have  found  Monograptus 
convolutus.  The  basin  of  the  second  waterfall  is  hollowed  out  in 
them,  while  the  face  of  the  fall  is  the  face  of  a  fault  which  runs 
17°  W.  of  IS".  The  fault  itself  is  filled  by  a  quartz-vein,  which 
seems  to  have  been  the  occasion  of  certain  abandoned  workings  on 
the  hillside  above.  The  beds  on  the  western  side  of  the  fault, 
over  which  the  water  falls,  are  Bala  Slates,  yielding  Leptoena  sericea , 
Strophomena  rhomb oidal is,  Orthis  elegantiila ,  0 .  testuclinaria ,  etc. 
They  strike  about  E.N.E.-W.S.W.,  and,  dipping  towards  the  S.S.E., 
are  succeeded  by  the  grit,  as  shown  in  fig.  1  (p.  430). 
5.  I Vest  of  the  Nant  Caivrddu  Fault. — To  the  west  of  this  fault 
an  irregular  patch  of  grit  is  shown  upon  the  map.  The  surface  of 
the  ground  is  here  nearly  parallel  to  the  surface  of  the  grit,  and 
the  beds  above  the  grit  have  been  somewhat  irregularly  denuded. 
But  the  structure  is  tolerably  simple  and  is  sufficiently  explained  in 
the  accompanying  two  sections  (figs.  6  and  7). 
Eig.  6. — Section  immediately  west  of  the  Nant  Cawrcldu  Fault. 
New 
N.  29°  W.  Reservoir.  S.  29°  E. 
Horizontal  scale  :  9  inches  =1  mile. 
a  —  Bala  Beds. 
b  =  Corwen  Grit. 
c  =  Llandovery  Slates  and  Shales. 
Vertical  scale  :  1  inch  =535  feet. 
c'  =  Graptolite-shales. 
d  =  Denbighshire  Grits. 
F  =  Fault. 
Towards  the  south,  the  grit  partially  encircles  a  mass  of  Bala 
rocks.  Close  to  the  fault,  and  on  the  northern  side  of  this  mass 
(fig.  6),  it  dips  northward  at  a  low  angle,  and  for  some  little  dis¬ 
tance  outside  the  moorland  boundary-wall  it  forms  the  bed  of  Eant 
Cawrddu.  Eollowing  the  stream  downwards,  we  find  that  the  grit 
is  succeeded  by  the  grey  slates,  and  these  by  black  banded  shales 
with  graptolites  of  the  gregarius- zone,  which  are  best  seen  in  the 
new  reservoir  cut  just  to  the  west  of  the  stream.  The  graptolite- 
shales  are  overlain  by  bluish  slates,  and  the  next  rocks  exposed 
towards  the  north  are  the  Denbighshire  Grits  brought  in  by  the 
fault  shown  on  the  Geological  Survey  map. 
A  little  farther  wTest  the  grit  still  rests  on  the  Bala  Beds  and  dips 
