456  REV.  J.  E.  BLAKE  OX  PELSITES  AND  [Allg.  1893, 
dipping,  as  seen  by  the  well-marked  bands,  at  about  30°  towards 
the  felsite,  and  therefore  beneath  the  conglomerate. 
I  regard  this  district  as  affording  the  most  satisfactory  proofs  of 
the  unconformity  of  the  conglomerate  on  the  Purple  Slate.  The 
problem  here,  in  fact,  is  reduced  to  its  simplest  elements ;  we  have 
three  rocks  to  consider:  felsite,  slate,  conglomerate.  By  common 
consent  the  felsite  is  the  oldest,  and  the  order  of  succession  of  the 
other  two  on  the  surface  is  sometimes  slate  and  conglomerate,  some¬ 
times  conglomerate  and  slate ;  and  the  change  from  one  order  to  the 
Pig.  10. — Slate  Quarry  at  Cefn  Du. 
other  takes  place  along  irregular  lines.  This  can  only  be  accounted 
for  in  any  natural  way  by  one  or  other  of  these  two  being  uncon- 
formable.  It  can  scarcely  be  the  slate  which  is  unconformable  on 
the  conglomerate,  for  it  is  invariably  found  either  at  a  lower  level, 
or  else  descending  to  a  much  lower  level  in  the  immediate  neighbour¬ 
hood.  It  must  therefore  be  the  conglomerate  that  is  unconformable 
on  both  of  the  others. 
TI.  The  supposed  Slates  intercalated  in  the  Pelsite. 
Certain  very  slaty-looking  bands  in  the  felsite  of  the  above  dis¬ 
trict  have  been  referred  to  by  Prof.  Bonney  1  as  contemporaneous 
deposits,  helping  to  prove  the  non-intrusive  character  of  the  felsite,  in 
the  following  words  : — “  On  the  western  side  of  Llyn  Padarn  there 
is  a  band  of  slate  intercalated  in  the  felsite.  This  is  a  rather  soft 
black  slate,  not  conspicuously  altered.  We  found  exposures  (probably 
of  the  same  band)  both  by  the  roadside  and  in  the  railway-cutting . . . 
the  material  of  the  slate  can  be  traced  filling  a  crack  in  the  felsite. 
As  I  know  no  such  black  slate  as  forming  any  part  of  the  Cambrian 
succession  in  this  district,  and  have  been  myselt  deceived,  before  I  was 
familiar  with  that  succession,  by  a  similar  slaty-looking  band,  I  have 
carefully  examined  the  rock  thus  indicated,  at  the  eastern  entrance 
to  the  tunnel  and  in  the  road  above.  At  the  latter  spot  its  dis¬ 
position  is  indicated  in  fig.  11  (p.  457). 
Had  the  dark-coloured  rock  been  deposited  on  the  felsite,  the 
1  Quart.  Journ.  Geol.  Soc.  vol.  xxxv.  (1879)  p.  312. 
