434 
MESSRS.  P.  LAKE  AXD  T.  T.  GROOM  OK  THE 
[Aug.  1893, 
northward  at  a  low  angle,  but  before  it  disappears  below  the  higher  beds 
to  the  north  it  plunges  suddenly  downward,  almost  or  quite  vertically. 
The  beds  north  of  it  are  here  very  imperfectly  exposed  (fig.  7). 
Fig.  7  . — Section  250  feet  west  of  that  shown  in  fig.  6. 
K.  29°  W.  s.  29°  E. 
6.  Faults :  Nant  Cawrddu. — The  point  of  most  interest,  however, 
in  connexion  with  this  patch  of  grit,  is  the  fault  which  separates 
it  from  the  grit  to  the  east.  An  inspection  of  the  Map  itself  (p.  428) 
is  sufficient  to  show  that  this  fault  is  of  remarkable  character,  inas¬ 
much  as  at  one  point  in  the  middle  of  its  course  it  has  no  throw  at 
all,  and  crosses  the  grit-band  without  apparently  affecting  it. 
If  we  examine  the  walls  of  the  Nant  Cawrddu  gorge  south 
of  the  main  grit-band — a  gorge  which  has  been  hollowed  out  along 
the  line  of  fault — we  find  that  on  both  sides  of  the  fault  the  grit 
has  a  general  low  northerly  dip.  The  throw  of  the  fault  appears 
to  be  downward  on  the  western  side,  but  it  is  extremely  small 
until  we  come  to  the  main  grit-band.  Here,  on  the  eastern  side 
of  the  fault,  the  grit-bed  turns  suddenly  upward  (fig.  2,  p.  430), 
and  on  the  western  side  it  turns  as  suddenly  downward  (figs.  6 
and  7),  so  that  the  throw  increases  enormously  within  a  few  yards. 
On  comparing  figs.  2,  6,  and  7,  which  are  approximately  parallel 
sections  near  the  line  of  fault,  it  will  appear  that  fig.  6,  which  is  close 
to  the  line  of  fault  on  its  western  side,  is  intermediate  in  character 
as  well  as  position.  It  would  seem  that  the  bed  of  rock  tried  to  yield, 
without  breaking,  to  the  upward  bend  of  fig.  2  (p.  430)  and  at  the 
same  time  to  the  downward  bend  of  fig.  7.  Hut  the  strain  at  length 
became  too  great,  and  the  grit  tore  along  the  line  of  fault.  The  dispo¬ 
sition  of  the  grit  on  the  two  faces  of  the  fault  is  shown  in  fig.  8. 
The  other  faults  do  not  offer  much  worthy  of  remark.  As  will 
be  seen  from  the  Map  (p.  428),  there  are  two  series,  the  one  running 
a  little  west  of  north,  and  the  other  a  little  south  of  east. 
The  former  series  includes  the  Nant  Cawrddu  Fault  and  the  two 
faults  at  Nant  Llechog. 
The  latter  includes  the  fault  just  north  of  the  old  Waterworks 
(fig.  2,  p.  430)  ;  the  fault  which  runs  along  the  face  of  the  southern 
bank  of  the  Dee  (figs.  2  and  4,  pp.  430,  431)  ;  the  fault  marked  on 
the  Geological  Survey  map,  which  brings  in  the  patch  of  Denbighshire 
Grits  at  Corwen  (figs.  6  and  7,  pp.  433, 434)  ;  and  probably  the  fault 
which  brings  in  the  grey  slates  at  Bonwm-uchaf,  in  the  north-eastern 
