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EEY.  J.  F.  BLAKE  ON  FELSITES  AND 
[Aug.  1893, 
We  must  now  proceed  to  examine  the  surface-exposures  of  Moel 
Tryfaen  in  the  light  of  what  we  learn  in  the  adit.  Along  the 
southern  slopes,  the  weathered  blocks  are  so  little  moved  that  we 
can  almost  trace  the  same  succession  there.  Most  of  tbe  upper 
part  is  occupied  by  the  Banded  Slates,  which  are  also  thrown  out  on 
the  other  slope  of  the  hill  between  the  summit  and  the  Moel  Tryfaen 
quarry — though  whether  they  are  in  situ  here  cannot  be  determined. 
Farther  down  are  many  blocks  of  the  coarse  grit,  and  towards  the 
base  some  blocks  of  felsite ;  one  of  these  shows  the  curious  phe¬ 
nomenon  of  enclosing  a  circular  mass  of  coarse  purple  grit,  which 
passes  gradually  into  it  by  an  indefinite  aureole,  the  colour  also 
changing  from  purple  to  light  green — a  state  of  things  which  seems 
to  indicate  that  this  felsite  is  later  than  one  of  the  grits. 
In  the  tramway-cutting  at  the  bottom  we  find  the  junction  of 
the  felsite  with  a  conglomerate.  It  is  now  evident  that  this  cannot 
be  the  same  as  that  observed  at  the  summit  or  in  the  adit,  but 
must  be  another  one  not  seen  elsewhere  ;  it  is  really  of  a  different 
character,  being  more  compact,  with  a  more  gritty  matrix  and 
with  pebbles  more  commonly  of  quartzite. 
And  now,  as  to  the  conglomerate  of  the  summit.  If  this  were 
an  expansion  of  the  thin  bed  No.  2,  the  expansion  would  have  to 
be  abnormally  rapid,  for  the  breadth  of  outcrop  at  the  top  is  55 
yards,  and  this  is  only  about  350  feet  above  the  adit,  so  that  the 
rate  of  expansion  would  be  1  in  2  !  Nor  can  we  find  any  green  grit 
on  the  summit.  Moreover,  on  this  supposition,  the  succession 
observed  in  the  adit  ought  to  be  equally  well  exposed  all  along  the 
western  side  of  the  hill,  but  this  is  not  found  to  be  the  case.  On 
the  northern  slopes,  instead  of  the  vast  thickness  of  Banded  Slate 
seen  at  the  southern  end,  all  is  covered  by  conglomerate  and  grit. 
The  lower  slopes,  indeed,  show  blocks  of  the  same  kind  of  grit  as  that 
which  occurs  in  the  adit,  and  also  a  few  blocks  of  felsite ;  but  above 
this  the  principal  exposures  are  two  lines  of  crags  crossing  the  hill  to 
the  north  of  the  summit-crags.  The  upper  one  of  these  consists  of 
thin  bands  of  coarse  grit  interbedded  with  much  finer  grit — in  the 
way  so  characteristic  of  several  other  examples  of  these  and  of  the 
Bronllwyd  Grit — and  these  most  distinctly  show  a  low  dip  of  not 
more  than  5°  to  the  east  (see  fig.  13). 
Fig.  13. — Crags  of  Grit  on  Moel  Tryfaen ,  showing  dip. 
E.  w. 
The  lower  crags  are  of  conglomerate  like  that  of  the  summit,  and 
they  show  a  lenticle  of  fine  grit  running  almost  horizontally. 
From  these  observations — the  great  mass  of  the  conglomerate  as 
compared  with  anything  in  the  adit,  its  peculiar  character  in  being 
full  of  large  slate-pebbles,  the  nearly  horizontal  bedding  of  the 
